Talk:Observable universe/Archive 3

Archive 1

Archive 2

Archive 3

Archive 4

Our location within the total universe

We now know the Earth resides near the inner rim of the Orion arm of the Milky Way. In the same way if we can see to (close to) the edge of the observable universe in terms of its age (younger and younger galaxies the further away), how can we not use this information to triangulate ourselves in the universe? (seeing as its Galaxy content is not infinite).

Why are we considered to be at the centre of the total universe? (not the observable universe).

This is a question a non-scientist with an interest in the topic (such as myself) may have upon reading this article.

PS: I'll try to answer myself, please confirm:

Is it because the further away we look we are looking into the "past" of our own observable neighbourhood? Even beyond a time when there were Galaxies and the Universe had just formed we are seeing the "extreme" past of only our local observable "bubble"?

Is that the reason?

Even then, seeing as the absolute universe is not finite (in contents) there is a chance that a particular "observable universe bubble" would overlap the "edge" of the actual matter limit of the total universe and show, in a certain direction, an uneven distribution of redshifted galaxies or an uneven distribution of matter in general, meaning its "observable universe" bubble had intersected the edge of the total universe, no?

What would be the theory that could support an "edge" of the Universe hanging out there in mid-space where anybody could trip over it? Hcobb (talk) 01:32, 9 April 2014 (UTC)

I don't follow you. I meant an edge to the outer limits of cosmic expansion not an edge midway in space. If the universe is finite it has limits. And therefore content very near those limits. Content whose observable bubbles may transcend those limits. Can you rephrase what you meant? — Preceding unsigned comment added by 82.171.62.254 (talk) 14:28, 9 April 2014 (UTC)

How would you have a physical edge to the Universe? What happens if you stick your hand out over that edge? Hcobb (talk) 15:50, 9 April 2014 (UTC)

I think you know exactly what I mean and you are being sarcastic.

If not please read up on Euclidean, spherical and hyperbolic spacetime curvatures in relation to the cosmological constant (assuming ΛCDM).

I am not talking about a dimensional paradox as in your example.

A plane is infinite and flat, a torus is finite but flat and a sphere is boundless, curved, but has finite surface area like a torus.

Could someone who is smarter than me be kind enough to provide an answer as opposed to another question. I'd appreciate it.

— Preceding unsigned comment added by 82.170.200.187 (talk) 23:45, 27 April 2014 (UTC)

I'm not entirely clear what your question is. But to address a couple of things you said: "I meant an edge to the outer limits of cosmic expansion not an edge midway in space." As far as we know, there is no outer edge to cosmic expansion, because the universe is probably infinite. "If the universe is finite it has limits." Even if the universe is finite, it doesn't have an edge - just like the surface of a sphere which you mentioned. So there's no matter which is 'near the edge' of the universe, and no-one's observable universe crosses the edge. Does that answer your question? Olaf Davis (talk) 12:02, 28 April 2014 (UTC)

Thank you, in a way it does and in a way it does not. Even if the universe is finite in volume but infinite in space (like the surface of a sphere) and we could see, (if we could see far enough into the universe) simply echos of structures closer by duplicating themselves ad infinitum, then still an observable bubble would be able to be localised within this total (repeating) volume.

So for me the following question still remains: If one were to construct a coordinate system for the entire universe (which is independent of any structure within it), how would one do it? I assume it would not be in 3D (x, y, z) coordinates as if what you say is true then the "centre" would be arbitrary. So how would one do it and where would "our" position lie within it? This would show, in very simple terms, the relative relation in terms of size and location between our "observable bubble" and the total universe. We would then know "where we are" so to speak. Which is what I named the title of this section.

You're right that the origin of any 3D co-ordinate system would have to be arbitrary. But there's no way around it: there simply isn't any natural co-ordinate system for the universe, any more than there is for the surface of a sphere - if I want to put co-ordinates on a sphere then I have to arbitrarily pick some point to start from. Olaf Davis (talk) 16:22, 18 May 2014 (UTC)

Even on a sphere, if I know the distance I have to travel to get back to where I started, I'll know its size. I will then also know my observable bubble position. What if we consider a coordinate system with at its centre the 10 billion lightyear wide structure discovered by I Horvath late last year? Now we wait for its duplicate to be found 180 degrees away and we're set are we not?

• There is no reason to expect a structure should have a duplicate 180 degrees away; opposite points of the sphere are totally independent.
• Additionally, current evidence indicates that 1) the universe is very probably infinite and flat, not like a sphere, and 2) even if it is a sphere it's so huge that our observable bubble will never reach an appreciable fraction of the way round.
• This talk page is really designed for discussing improvements to the article. A general discussion of the topic is better suited to the reference desk.
• I notice you removed the signature that was added to your last comment, which can make it a bit harder to follow conversations. If you removed it to hide your IP address you should know that it's still readily visible in the history tab of this page. The easiest way to hide your IP is to create an account. Olaf Davis (talk) 10:20, 21 May 2014 (UTC)

Comparing four methods of calculating mass

The documentation on the four methods of computing mass (atoms) is confusing for a couple of reasons. It should be clear that the first and fourth methods include ISM and IGM; the second and third do not. The third method uses a radius of 13.7 billion light years but does not acknowledge that the expanded radius would produce 39 times more mass. I believe the fourth method also implies the smaller radius. The average mass of stars based on their distribution in the Milky Way is E33 gm not 2 x E33 gm. In method two, dark matter and dark energy are mentioned as if they apply only to this method; this should precede the method discussion. And last, calculating atoms rather than just mass adds to the confusion. If others agree with my comments, I would like to revise the write up. — Preceding unsigned comment added by Jimjohnson2222 (talk • contribs) 20:06, 15 March 2012 (UTC)

I saw your post in help desk. I don't know enough of this subject to help you correct the article. If the article isn't protected you can edit it yourself. If you wish others to examine your changes you could let them know on their talk pages in case they aren't watching this article. This talk page is the appropriate place to discuss the article itself though.--Canoe1967 (talk) 20:26, 15 March 2012 (UTC)

Ok, I am ready to edit but the format is not word and looks intimidating. Who can I talk or email to directly? Jim Johnson — Preceding unsigned comment added by Jimjohnson2222 (talk • contribs) 00:51, 23 March 2012 (UTC)

Giant Fuzzy Picture?

Am I the only one for whom the picture at the bottom is really really long (longer than the rest of the article combined) and fuzzy? It looks to me as if the site can't handle the image properly (not surprising, as it is 23,000 pixels wide and 2,000 tall), but I can't be sure it isn't just my computer. 129.22.125.192 (talk) 06:00, 3 April 2012 (UTC)

Yes, I was about to mention that. If no-one can fix it, I will remove it. SchellZ (talk) 12:16, 4 April 2012 (UTC)

Revising sections 4 and 5

Mass of Observable Universe

Subject of my article is an edit. I am submitting major edits to the web site http://en.wikipedia.org/wiki/Observable_universe .

Section “4 Matter Content” is replaced with “4 Mass of Universe” and section “5 Mass” is replaced with “5 Number of Atoms”. Section 4 has subheadings: 1 Critical density and equivalent mass; 2 Extrapolating total mass from the number of stars; 3 Extrapolating based on stellar density; and 4 Estimation based on steady-state universe. The two existing sections are confusing and miss leading in numerous ways. The revision is simpler and accurate. The editing is based on a reference document I recently published. The main reference “The Cosmic Energy Inventory” Astro-Physics, 2004 replaces references 41-47, and 49. One point of possible contention is the number of stars, the existing quotes four sources with up to 100 times more stars than I assume, 1022 (which is actually at the low end of the references). There may exist more small stars than traditional estimates but if so there mass would be considerably less than 1030 kg so I think my number is preferred. Also, if total star mass is 10-100 times greater, it would invalidate the critical density percentages. Thus, the references 43 and 46 should be dropped and 38 was replaced. I have no knowledge of the formatting used to edit articles so I am hoping you can make the attached consistent with what Wikipedia requires. Thanks for your help. Sections below. Jim Johnson, Dallas, cell 214-450-6850

Mass of Universe One can derive an order-of-magnitude estimate of the equivalent mass (ordinary matter, dark matter, and dark energy) in the observable universe four different ways: critical density formula (3.35 x 1054 kg); extrapolating from number of stars (3.7 x 1054 kg); extrapolating from stellar density (3.3 x 1054 kg); and Hoyle’ steady state formula (3.12 x 1054 kg).

[edit] Critical Density and Equivalent  Mass 

Observations of the cosmic microwave background from the Wilkinson Microwave Anisotropy Probe suggest that the spatial curvature of the universe is very close to zero, which in current cosmological models implies that the value of the density parameter must be very close to a certain critical value. The present value of the Hubble parameter is important because it is related to the value of the critical density at the present,pc , by the equation[44] pc = 3H2/8pieG

where G is the gravitational constant. WMAP seven-year results from 2010 estimate the value of the at 70.4 (km/s)/Mpc[17] or 2.28 × 10−18 s−1, which gives a critical density of 9.30 × 10−27 kg/m3. Multiplying this density by the estimated volume of the observable universe (3.60 × 1080 m3) gives a total mass for the observable universe of 3.35 × 1054 kg.[17,42,44]

[edit] Extrapolating total mass from the number of stars The estimate for star mass is dependent on three variables: mass per star; stars per galaxy; and number of galaxies. Based on the mass and spectral class distribution of stars in the Milky Way, the average star mass is one half of the sun’s mass, about 1030 kg. Thus, the simplistic calculation for the entire mass of stars is 1052 kg (1030 kg per star times 1011 stars per galaxy times 1011 galaxies). The number of stars, 1022, is relatively close to the results of the Hubble Space Telescope stellar density study, 9 x 1021 stars (reference next section). [37,40,41,49] When considering ordinary matter (atoms/luminous/baryonic matter), three separate components must be considered: stars; Interstellar Medium (ISM) - gas and dust, mostly hydrogen; and, the Intergalactic Medium (IGM) [45] residing between galaxies - again mostly hydrogen.

The approximate components of ordinary matter and their masses are: stars - 1052 kg (5.9%); ISM - .3 x 1052 kg (1.7%); and, IGM - 15.7 x 1052 kg (92.4%). [38,48].  Thus, the total for ordinary matter is 1.7 x 1053 kg. (References calculating mass are not always clear on which components of matter are included or if dark matter is included. Also, different values may result based on assumptions, for example, 1023 stars or 25% of star mass for ISM or less mass for IGM; however, most estimates for the mass of ordinary matter fall in the range of 1052 kg to 1054 kg as this estimate, 1.7 x 1053 kg, does.)

The next step is to compute mass equivalence for dark matter and dark energy from the value of ordinary matter. The relationship is as follows (percentages are rounded): critical density (100%) = ordinary matter (4.6%) + dark matter (23%) + dark energy (72%) [38]. Thus, total eqivalent mass is approximately 3.7 x 1054 kg (1.7 x 1053/ 4.6%). [38] In the above percentages, neutrino energy was considered part of dark energy to avoid a separate category. Also, the WMAP seven year results estimated ordinary matter at 4.56% of the total mass, [17] almost identical to number quoted.

[[edit] Extrapolation based on stellar density

A third way to calculate the total mass of the observable universe is to extrapolate, as in the previous section, based on stellar density. Assume a mean stellar mass and multiply by an estimate of the number of stars in the observable universe (as in the paper 'On the Expansion of the Universe' from the Mathematical Thinking in Physics section of a former NASA educational site, the Glenn Learning Technologies Project). The paper derives its estimate of the number of stars in the Universe from its value for the volume of the "observable universe"

Note however that this volume is not derived from the 46 billion light year radius given by most authors, but rather from the Hubble volume which is the volume of a sphere with radius equal to the Hubble length (the distance at which galaxies would currently be receding from us at the speed of light), which the paper gives as 13 billion light years. In any case, the paper combines this volume with an estimate of the average stellar density calculated from observations by the Hubble Space Telescope

yielding an estimate of the number of stars in the observable universe of 9 × 1021 stars. Thus, using the same extrapolation as in the previous section, the result for total mass is 3.3 x 1054 kg.

[edit] Estimate based on steady-state universe

Sir Fred Hoyle calculated the mass of an observable steady-state universe using the formula:[50] 4/3 pie p (c/H0)3

which can also be stated as: c3/2GH

or approximately 8 × 1052 kg. Using the volume of the observable universe, which is 39 times larger than the Hubble volume, the result for total mass is 3.12 x 1054 kg - very close to the other estimates. Here H = Hubble constant, ρ = Hoyle's value for the density, G = gravitational constant and c = speed of light.

[edit] Number of atoms If we make the simplifying assumption that all the atoms are hydrogen atoms which have a mass of about 1.67×10−27 kg, then ordinary matter (1.7 x 1053 kg) divided by the atom’s mass gives an estimate of 1080 atoms in the observable universe.[45,47]

I'm afraid I find your format very hard to follow. I'm not sure what you are saying is the original, and what are your proposed changes.

If you want to go to the trouble, you could break things out into pieces you want to change, say what you want to change it to, and why. Or much easier, you could just save the article with your changes and revert yourself (or wait for someone else to do it) pending discussion — then we can use Wikipedia's diff tool. --Trovatore (talk) 21:21, 14 May 2012 (UTC)

References

37 ^ "Astronomers count the stars". BBC News. July 22, 2003. http://news.bbc.co.uk/2/hi/science/nature/3085885.stm. Retrieved 2006-07-18. 38^ Fukugita, Masataka, Peebles, P. J. E.,”The Cosmic Energy Inventory, Astro Physics Review 18 Aug 2004. 39 Delete. 40^ "How many stars?" 41^ How many galaxies in the Universe? says "the Hubble telescope is capable of detecting about 80 billion galaxies. In fact, there must be many more than this, even within the observable universe, since the most common kind of galaxy in our own neighborhood is the faint dwarfs which are difficult enough to see nearby, much less at large cosmological distances." 42^ WMAP- Content of the Universe. Map.gsfc.nasa.gov (2010-04-16). Retrieved on 2011-05-01. 43^ Delete 44^ Bernard F. Schutz (2003). Gravity from the ground up. Cambridge University Press. pp. 361–. ISBN 978-0-521-45506-0. http://books.google.com/books?id=iEZNXvYwyNwC&pg=PA361. Retrieved 1 May 2011. 45^ Matthew Champion, "Re: How many atoms make up the universe?", 1998 46^ Delete 47^ National Solar Observatory. "The Universe". http://www.cs.umass.edu/~immerman/stanford/universe.html. 48^ Houjun Mo; Frank van den Bosch; Simon White (28 June 2010). Galaxy Formation and Evolution. Cambridge University Press. ISBN 978-0-521-85793-2. http://books.google.com/books?id=Zj7fDU3Z4wsC. Retrieved 1 May 2011. 49^ (PDF) On the expansion of the Universe. NASA Glenn Research Centre. http://www.grc.nasa.gov/WWW/K-12/Numbers/Math/documents/ON_the_EXPANSION_of_the_UNIVERSE.pdf.

Metagalaxy

the term "metagalaxy" redirects here but isn't defined in the article. Since it doesn't appear to have a strict definition I'm not sure what to do with it. Should it redirect here or to galaxy filament? Serendi^podous 13:19, 14 July 2012 (UTC)

sorted. Serendi^podous 11:54, 16 July 2012 (UTC)

it's garbage, delete it. 72.228.189.184 (talk) 10:55, 23 July 2012 (UTC)

Unresolved current total containing spherical volume vs. Observable in this article and in Universe

There are few threads in the archives here and one current at Universe on the unresolved total current size of the Universe. From the known time of the origin and the observation of the metric expansion of space it ought to have been possible to give the total spherical volume, not just that observable from the Earth or at least state some relation between them. 72.228.189.184 (talk) 03:18, 1 May 2012 (UTC)

No, as I understand it, this is not known; the universe as a whole may be infinite in extent. --Trovatore (talk) 03:26, 1 May 2012 (UTC)

As it stands now, the article states a contradiction with the concept of the big bang spewing space and stuff into an empty void, which is what common sense would take as the containing infinite Euclidean space. By the statement given, I can head to the edge of the observable universe from the Earth and having reached a position say 45 Gly distant in that direction will still see an observable universe of radius 46 Gly from there, and so on. This contradiction needs explanation. Is steady state being reasserted here? 72.228.189.184 (talk) 03:35, 1 May 2012 (UTC)

No, that's the wrong picture; an infinite universe is (barring developments I haven't heard of) perfectly consistent with the big bang. But there was never an empty void into which stuff spewed. Rather, at any positive amount of time after the big bang, the universe is infinite and of finite density. However, if you follow any two particles' worldlines backwards in time, their distance approaches zero as the comoving time coordinate goes to zero, and density goes to infinity. --Trovatore (talk) 03:40, 1 May 2012 (UTC)

Maybe someone else can comment, your comments seem cant/rote and ignore the common sense of the Euclidean volume shown in the illustration in the article as well as what I've just said. As well, also, as the conceptual coherence of the ensemble of all the comoving particles back to the origin, that you mention (in the context of containment within a Euclidean volume as is shown for OU in the illustration). Seems to imply some kind of Flatlander thinking to equate that with infinity. Are you for example saying that say 12 billion years ago the shown 46 Gly radius sphere was not mostly void, that the circumscribing sphere then was not much smaller? Lycurgus (talk) 03:45, 1 May 2012 (UTC)

What you've just said, which you claim I've ignored, is wrong. I am trying to correct your misunderstanding. --Trovatore (talk) 03:46, 1 May 2012 (UTC)

Actually what you're trying to do is enforce a paradigm, to the extent you understand it. That's a slightly different action. Lycurgus (talk) 03:53, 1 May 2012 (UTC)

I'm pretty sure there are probably some speculative calculations on this somewhere, so given time I will ferret them out update the article if it hasn't been already. To correct my example above it might make sense given some profile of the metric expansion for the OU sphere to have been just as large 12Gya, but clearly at some time the current OU sphere could have contained the entire spacetime. It's the current size of such a containing Euclidean volume that is (sensically) sought. In the last few generations the known size of the universe has expanded dramatically from just the galaxy to it's current immense but not infinite extent. Current consensus concepts don't allow this spacetime to be infinite in the Euclidean sense. 72.228.189.184 (talk) 04:00, 3 May 2012 (UTC)

Why would anybody want to limit the size of Euclidean space? There is no reason to limit space. What we have a a need for is a (concept of a spacial volume that would contain what we think are the physical entities of our universe. And we have estimates as to the number of galaxies (Say 10E^11 galaxies) and of the number of stars per galaxy (Say 10E^11 stars) and as to the spacial density of these items within a Euclidean volume of space. So what we need is a volume of space that can contain those entities in a manner consistent with our observations of viewable astronomical spacial content. If say there is an astronomical spacial content of 1 cubic megaparsec, (= 10E^73.5 cubic cm) for each galaxy, then we need more than 10E^11 times the cube of 10E^24.5 cm, (= 10E^11 times 10E^73.5) (=10E^84.5 cubic centimeters) of space for such a concept. And if we assume it to be a spherical volume of space we have to add 0.3 to the exponent to get a cubical volume of space. So we need 10E^84.8 cubic centimeters of space, which is that space contained by a 10E^28.3 cm cube which is that of a cube of only a dimension of 6300 megaparsecs (=20,500,000,000 light years) on each side. And we should therefor be able to say that a spacial volume cube of say 10,000 megaparsecs (32,600.000,000 light years) on a side should adequately contain our conceptual content of astronomical matter.WFPM (talk) 17:10, 5 May 2012 (UTC) It is further to be noted that this spacial volume varies from Einstein's estimate of a 10,000 megalightyear radius by the cube of the difference between the light tear and the parsec (=3.26^30 or a factor of 34.6 times the volume, which could be corrected for if the spacial volume allotted per galaxy wrer reduced from 1 cubic megaparsec per galaxy to 1 cubic megalightyear per galaxy.WFPM (talk) 03:07, 10 May 2012 (UTC)

No this is wrong. It's based (implicitly) on the observable universe and counts derived therefrom. You do get that Euclidean space is properly infinite but don't seem to understand that it is the volume that currently circumscribes the stuff produced by the big bang (to oolor within lines of current scientific consensus) that we are after and not the observable (OU) treated on the obverse, counts of galaxies or the like based on same. It is the difference between this OU sphere and the sphere of containment shown in black here) that we are after. The current OU sphere is limited by our current ability to physically measure and make inferences upon the empirical measurements. So before Hubble the black sphere shown was contiguous with the galaxy, in pre modern timex with the solar system, and now not by our ability to observe but to reason and calculate based on the metric expansion of physical space (as conceived currently) integrated over its various rates in time and the totality of stuff/energy of all kinds. Einstein's calculations are of little interest here as most of the relevant astrophysics occurred sometime after his death and long after his productive years, not to mention the issue of ad verecundiam. Also a new concept of space is not (at least directly) sought here. Plain ole ancient Euclidean space and a sphere in that model are what we are talking about in both the actual sphere of containment and the OU. They're not different except for r Lycurgus (talk) 20:49, 11 May 2012 (UTC)

This appears to be a novel theory of your own. It has no obvious connection with the standard view. --Trovatore (talk) 21:10, 11 May 2012 (UTC)

It may appear so to you, but it isn't a theory at all. It's an attempt to address a core fact germane to this article, referring only to the ground of the gestalt consisting of "Euclidean sphere" and "Observable Universe" and nothing more. 72.228.189.184 (talk) 03:50, 7 June 2012 (UTC)

But if we exist within a volume of Euclidian space, we might as well pick a desirable position for observation, like at the position of the Hubble telescope, and see what is observable. And I can pick a cubic volume of Euclidian space and assume that I'm in the center of it and show that if I divide it into octants of observation (the volume within one of the diagonal directions that contains a 3 dimensional continuum), and arrange to sequentially view the 8 octant volume directions, I can then arrange to observe all of the volume of the original euclidian volume from my point of view out to the limiting radial distance.WFPM (talk) 00:12, 13 May 2012 (UTC) And I'm assuming that the question as to the observable universe is related to what we can see (observe) from our position within it. I hope that everybody understands that when you look into the corner of a 3-dimensional cubical volume of space, it is possible to see everything that visably moves within it except for motion congruent with the diagonal direction.WFPM (talk) 01:25, 15 May 2012 (UTC):

To address the apparent misunderstanding of 72.228.189.184 and Lycurgus (and maybe WFPM, though I'm not sure if WFPM is agreeing with the first two about the need for a finite spherical "containing volume" for all the matter created in the Big Bang), the standard assumption about how to model the whole universe in Einstein's theory of general relativity is to use some version of the Friedmann–Lemaître–Robertson–Walker metric, which has different possible spatial geometries depending on the matter density (see the top of this page from Ned Wright's cosmology tutorial for the three types of spatial geometry, only one of which is Euclidean). It is assumed in all versions, though, that matter created in the Big Bang is evenly distributed throughout all of space, it's not like a spherical explosion of matter in a preexisting empty space. If space is infinite, then matter is infinite too, according to these models; if space is finite (either because space is "positively curved", like the 3D surface of a 4D sphere, or because it has an unusual topology) then every portion of the finite space is modeled as being filled with the same density of matter (though the density decreases as space expands and the amount of finite space grows, like the 3D surface of a 4D sphere which is growing in radius). Either way, it'd be wrong to think of the matter of the universe as a finite expanding 3D sphere in an infinite Euclidean space which is devoid of matter beyond the boundary of the sphere. Hypnosifl (talk) 14:57, 3 July 2012 (UTC)

The different spacial geometries are all related to the same measured volume of 3 dimensioned physical space with just different rules of measurement. There is no orthogonal 4th dimension to a 3 dimension spacial continuum. There are maybe equally related 4th dimensions, like the diagonal dimension through a 3 dimensioned cube, which foul up the process of measurement. But there is no reason to limit the potential dimensions of a 3 dimensioned spacial continuum. Decarte said that the only space that mattered was where there was matter, which is reasonable. And the concept that we're inside of an unlimited volume of space keeps us from having to worry about the volume unless we find evidence otherwise, like a variation in the red shift phenomena. But we're inside a 3D physical sphere within which the tangible and observable matter and energy are evolving due to some set of variable processes. And our observation data is related to the physical process for the movement of the observable level of matter within the spacial volume. And evidently there is a process of movement of the smallest particles of matter such that we are unable to determine the nature of the matter, but only sense the integral of the energy carried by the matter during a specific period of time.WFPM (talk) 15:16, 6 July 2012 (UTC)

Noting that current text of this article resolves the issue I had in starting this thread. It now refers to an inflation implied size of the total universe with a numerical value given for the difference. It's not necessary or possible to do more, I think. although a formula for a larger sphere by whatever that factor is is straightforward. 72.228.189.184 (talk) 19:31, 24 August 2012 (UTC)

First paragraph in the first section

At the bottom of the paragraph there is a reference to "Gott et al.'s" and the pages in the book (or whatever it is). Right at the end of the paragraph there's this: </ref>

I don't know what it should be so I'm not fixing it. Acoma Magic (talk) 03:08, 29 August 2012 (UTC)

There seem to have been nested "ref" tags, which I don't believe are supported (someone let me know if they're supposed to be, but they seem like a bad idea even if they are). I removed the inside tags so that it's now a single footnote, but some of the explanatory text may have been lost — maybe someone knows how to fix it better. --Trovatore (talk) 04:04, 29 August 2012 (UTC)

== Size ==173.189.74.160 (talk) 16:12, 7 October 2012 (UTC)

Earth's position in the universe diagram

A typo: this diagram has 'Solar Intersteller Neighbourhood' as a labelled location in the Milky Way Galaxy panel. 'Interstellar' is spelled 'ar' not 'er'.

On a semantic point concerning the label 'Solar Interstellar Neighbourhood' on both the same-named panel and in the Milky Way panel, I've always seen it expressed as the 'stellar neighbourhood' (or even 'solar neighbourhood'). One could call it the 'local stellar neighbourhood' but I think 'local' is more or less assumed. I don't think we need 'Inter' to emphasize that we're talking about Sol's place /between/ stars. We talk about Earth's place in the planetary neighbourhood, the Milky Way's place in the galactic neighbourhood, etc., so it seems appropriate to discuss Sol's place in the 'stellar neighbourhood'.

I concur with the opinion this image is too large in its primary form. Perhaps the smaller version should become the only one referenced. Pinkpedaller (talk) 21:46, 20 October 2012 (UTC)

Inscrutable Sentence

"According to the theory of cosmic inflation and its founder, Alan Guth, if it is assumed that inflation began about 10−37 seconds after the Big Bang, then with the plausible assumption that the size of the universe at this time was approximately equal to the speed of light times its age, that would suggest that at present the entire universe's size is at least 10^23 times larger than the size of the observable universe.[11]"

I find this sentence to be completely opaque. Although the conclusion about the size of the universe can be taken as a statement of theory, the justification for it that proceeds it is not enlightening to somebody who does not already know why the conclusion follows. — Preceding unsigned comment added by 74.192.15.216 (talk) 08:07, 29 October 2012 (UTC)

This Might Be A Stupid Question.

Can someone explain whether my view below is out of order, or whether it should be included in some sense in the article? I know a bit of physics, but I'm still disturbed by the approach of this article. Specifically, I don't understand the 46-Gyr view of the observable universe given how I view the relativity of spacetime. It seems to assume things about time that are not true.

There is no "master clock" in spacetime. Two events are either linked in cause-effect order, effect-cause order, or neither, the latter happening when both events occur for some real frame of reference in which an observer in that frame would see them occur simultaneously (i.e., far apart enough in spacetime that they cannot be causally linked). All observations are made cause (thing we look at) to effect (us looking) in that order. Only things in our past light-cone are actually part of our observable universe (or more bluntly, our universe).

Saying something like "where that galaxy over there is now" is absurd, because there is no master reference to "now". If I change my inertial frame of reference, it changes what "now" I would calculate for any event that is too far from me in spacetime to be causally linked. One might argue that if the furthest point in spacetime to which we can observe somehow occupies the same inertial frame of reference as Earth, the two would be comparable in some sense, but this isn't likely the case since the universe is expanding at an increasing rate (and in fact this is used as justification for the larger value). We don't even know whether inflation is perfectly even, so for all I can tell it is possible (though not consistent with current theory) that the region over there isn't expanding at all anymore.

The limit to what we can observe is just barely inside our past cone of causality. In other words, the future of those stars which we see in their "past" form, is not only unknown to us; it's not part of our universe (yet). The plaque which is captioned "[this is] one of the most common misconceptions" is entirely correct about the observable universe being 13-ish Gy away: we don't know what will happen at the edge of the universe until we are able to observe it, so the distance to the edge (which yes, is in our past) is under 14 Gyr.

The article even contradicts itself in this regard when it says observable means "it is possible in principle for light or other signals from the object to reach an observer on Earth". Light which we will see in the future but cannot see yet is not part of our "now", and neither are the objects which emitted the light, necessarily. We don't even know if the particles that compose distant objects will exist in another 13 Gyr. We're not even certain whether inflation is completely uniform. For all we know, the CBR will disappear tomorrow because of an impending Big Rip, in which case you'd have a pretty difficult time arguing the observable universe is larger than 14 Gyr!

To sum up, "observable" means in our causal past, since you cannot observe something that is potentially simultaneous. There's no true simultaneity, and the theoretical edges that stuff we can see might now reach to, if we travel forward through time a distance equal to the distance light had to travel through space, really has nothing meaningful to do with what constitutes our universe today. The temptation to speculate "where is it now" seems to suggest a lack of understanding of relativity. If there were a "here's the boundary", then our change of reference would change its distance in different ways, in both the direction of acceleration and the opposite direction. There really is no exact "where" for things that aren't in our causal past or future because that spacetime is not causally connected to us at this point.

Am I totally off-base with this? If not, how can you say something that hasn't happened yet is part of our current universe, much less call it "observable"? I think if we're measuring "universe at this moment", then the slice has to extend backwards through time, not through some imaginary simultaneity. I don't really see how something entirely outside our causal cone "exists" from our point of view in a way that it can be measured, or assumptions about it can be made apart from assuming all our theories must apply with certainty to stuff 13 Gyr distant from us after yet another 13 Gyr have passed.

Sorry if this was a bit babbling, I'm not down with the lingo in this field. I think my point is (hopefully) clear, if tl;dr, so if someone can help me out I'd greatly appreciate it! TricksterWolf (talk) 03:11, 17 November 2012 (UTC)

See comoving coordinates. I confess that I haven't read your entire question, but I get the sense that that link might help. --Trovatore (talk) 03:16, 17 November 2012 (UTC)

I'll bet it will help, thanks. I'll follow up if it does not. (Uncanny speed with your response, by the way, which alone may make me reconsider that past-cone thing...) TricksterWolf (talk) 03:21, 17 November 2012 (UTC)

Integrating revised content

There is a notice at WT:AST about integrating revised content into this article. The draft revisions are in /workpage provided by Jimjohnson2222 (talk · contribs) -- 65.94.79.6 (talk) 03:48, 2 July 2013 (UTC)

Please see feddback (comments) here. Thanks. --- Steve Quinn (talk) 19:56, 4 July 2013 (UTC)

Jim Johnson's revisions

I reverted your additions to the Observable universe article, because the information was not presented in an encyclopedic manner. Perhaps, if you listed your facts, sources and issues with the article as it stands, I can incorporate them for you. Serendi^podous 20:15, 4 July 2013 (UTC)

Why change Matter Content and Mass?

• A. Matter Content has the following issues/inconsistencies:
  • 1. In method 1, the two methods are calculating mass which is the same objective as the Mass section. Thus is redundant and confusing. Once mass is estimated the number of atoms is straightforward.
  • 2. In method 1, the value of the Hubble constant and the values ordinary matter are outdated (WAMP rather than ESA).
  • 3. In method 2, the mass of an average star is wrong, two times too large.
  • 4. In method 2, the reference to number of galaxies is outdated by NASA's current estimates.
• B. Mass has the following issues/inconsistencies:
  • 1. Introduction is confusing because of mass energy reference and "space time curvature' which does not belong in section.
  • 2. Critical density section is based on outdated values (WAMP rather than ESA).
    • The reference on the fraction of stars is outdated, the best estimate in the Cosmic Energy Inventory article.
    • Using both the visible and observable volumes is confusing and not relevant.
    • The section uses results from method 1 which is duplication.
  • 3. The Stellar density section uses an obscure reference on density rather than the NASA recent references. It does not correct for the Hubble distance (comoving radius). It also uses the wrong mass for an average star and outdated values (WAMP rather than ESA).
  • 4. The Steady-state section uses an outdated value for Hubble constant and does not correct for a comoving radius.

The revised version, corrects all these inconsistencies. Appreciate your thoughts. Jim Johnson 02:18, 5 July 2013 (UTC)

First I should say, please forgive any obvious misapprehensions. I am not a cosmologist or even a scientist. With that out of the way:

A1. I do not see the doubling up as redundant; rather it shows the same conclusion through multiple lines of evidence

A2: Provide a source, and it can be reworked.

A3: First of all, a factor of two is not a very large difference in astronomical terms. Secondly, it depends how you define "average" (obviously, the "average mass" of a star listed in the article is a solar mass, and the Sun is middle-sized, even if it is relatively rare in stellar terms). Finally, the information is sourced. You need to provide your own source to counter it.

A4: Again, if you have a source, list it.

B1: Why? It is raising important caveats that the mass of the universe should include its energy, and that energy is involved in the curvature of spacetime. Seems pretty straightforward to me.

B2: Again, provide a source and it can be included. I'm not sure what you mean by "visible and observable volumes". Surely those are the same? And it isn't duplication if you are trying to determine a separate result.

B3: again these can be fixed using the appropriate sources, rather than completely rewriting the section

B4: Since the steady-state theory is debunked anyway, does it really matter if it doesn't use up to date figures? Personally I don't think it needs to be in the article. Serendi^podous 07:11, 6 July 2013 (UTC)

I appreciate your detail review of my proposal. I am new to the process and format of updating Wikipedia. I know we can find a mutually agreeable improved revision. Address your specific comments:

A1. "I do not see the doubling up as redundant; rather it shows the same conclusion through multiple lines of evidence."

The point is having a focused objective for the first section - an estimate of the number of atoms. Why try to verify the amount of mass in two sections?

A2. All updated sources were in the revision:" 43. ^Paul Davies , The Goldilocks Enigma (First Mariner books, New York, 2006), pp. 43. 44. ^ Michio Kaku, Parallel Worlds (Anchor Books, New York,2006), pp.385. 45. ^ Bernard F. Schutz (2003). Gravity from the ground up. Cambridge University Press. pp. 361–. ISBN 978-0-521-45506-0. Retrieved 1 May 2011. 46. ^ "Astronomers count the stars". BBC News. July 22, 2003. Retrieved 2006-07-18. 47. ^ "trillions-of-earths-could-be-orbiting-300-sextillion-stars" 48. ^ van Dokkum, Pieter G.; Charlie Conroy (2010). "A substantial population of low-mass stars in luminous elliptical galaxies". Nature 468 (7326): 940–942. arXiv:1009.5992. Bibcode:2010Natur.468..940V. doi:10.1038/nature09578. PMID 21124316. 49. ^ "How many stars?" 50. ^ NASA, Hubble News Release STSci - 2004-7. 51. ^ James R Johnson, Comprehending the Cosmos, a Macro view of the Universe, 2nd Edition, April 2013, pp. 36,ISBN 1477649697. 52. ^ NASA, Hubble News Release STSci - 20012-37. 53. ^ James R Johnson, Comprehending the Cosmos, a Macro view of the Universe, 2nd Edition, April 2013, pp. 34,ISBN 1477649697. 54. ^ Fukugita, Masataka, Peebles, P. J. E., "The Cosmic Energy Inventory", Astro Physics Review 18 Aug 2004. 55. ^ Helge Kragh (1999-02-22). Cosmology and Controversy: The Historical Development of Two Theories of the Universe. Princeton University Press. p. 212, Chapter 5. ISBN 0-691-00546-X. 56. Valevx,Dimitar, Estimation of the total mass and energy of the universe, arXiv:1004.1035v [physics. gen-ph] 7 Apr 2010, pp. 4."

A3. A factor of two may not be too significant but it is wrong! See reference list.

A4. See ref.

B1." Why? It is raising important caveats that the mass of the universe should include its energy, and that energy is involved in the curvature of spacetime. Seems pretty straightforward to me."

Although true, it is in another Wikipedia section -Energy and Mass.

B2. "Again, provide a source and it can be included".

See ref. " I'm not sure what you mean by "visible and observable volumes". Surely those are the same? And it isn't duplication if you are trying to determine a separate result" This is exactly my point, the original uses both which is unnecessary since they are so close. I use the Wikipedia comoving radius for observable universe.

B4. " Since the steady-state theory is debunked anyway, does it really matter if it doesn't use up to date figures? Personally I don't think it needs to be in the article "

Although the Steady-state theory is false, the equation for mass is not. See my ref. In summary, I think my revision is much improved over the current and hope you agree. I can easily remove the rhetorical questions if they are not appropriate. Again, thanks for the feedback. Jim Johnson 15:10, 6 July 2013 (UTC)

Your sources are primarily popular science books and news releases. On Wikipedia, scientific articles are preferred. Serendi^podous 21:02, 6 July 2013 (UTC)

Are you trying to help or be an obstacle? I reused 6 of the original references although 2 were "news" and I debated using them.

The eight added were: 1 from Astro-physics journal, 2 books from Physicist (one, Paul Davies, is used in the previous section!), 2 NASA Releases which are as reliable as anything, 1 arXiv article, and 2 from my recent book.

If you are not going to acknowledge my logic and give this a chance , can you refer this to another editor? Jim Johnson 21:33, 6 July 2013 (UTC)

You want to radically redraft an already established article on a very important topic. So yes, my job right now is to slow you down. These things take time. I would prefer it if I was not currently the only one dealing with this, because I'm not interested in being "the Jerk". So I think I'm going to move this conversation to the article's talk page. Serendi^podous 05:17, 7 July 2013 (UTC)

Thanks, that should work. Do you want me to remove the rhetorical questions first? Also, because the sequence of methods is changed in the revision, it is difficult to directly compare; actually, I used the same equation, with updated values, for Critical Density and Steady State. I replaced the Stellar Density which relied on an obscure reference with more current NASA current observations and renamed it Extrapolating from the Number of Stars. All volume calculations are now based on the same comoving distance. I agree this is an important Wiki article and spent considerable time developing the modifications. My research is this area spans a few years and I hope this contribution will prove valuable. Jim Johnson 12:19, 7 July 2013 (UTC).

• I recommend that Jim Johnson add references and inline citations to the content of the talk:Observable universe/workpage, along with other text that he wishes to add or revise -- rather than generally listing references that he is using. . For some reason, there appears to be avoidance pertaining to doing this. I think this would make it easier to correlate statements and revisions to the sources. Also, I am not convinced that WMAP data is out of date in comparison to the ESA. ---- Steve Quinn (talk) 18:36, 7 July 2013 (UTC)

• Jim Johnson said he doesn't know Wikimarkup, so that'd be hard for him to add inline referencing. -- 76.65.128.222 (talk) 07:01, 9 July 2013 (UTC)

Request for comment

An informed opinion is required on the facts and figures provided by User:JimJohnson2222, and whether or not they are superior to the sources already provided in this article. Serendi^podous 13:01, 7 July 2013 (UTC)

• Neither sufficient “figures” nor a single diff provided by Serendipodous, neither in this talk section nor in the previous one. Apparently it does not matter for him how accessible will be the dispute for third party users whom he is inviting. Then his point is, similarly, not interesting for me. Incnis Mrsi (talk) 14:50, 7 July 2013 (UTC)

A diff would be impossible to read, as he essentially rewrote sections of the article from the ground up. Here's the article as he wrote it. Jim, do you think you could list the specific updated figures you changed (along with the originals of course, and the sources you got yours from) so that other people don't have to read entire paragraphs of text to see what you did? Serendi^podous 15:35, 7 July 2013 (UTC)

Please be more specific as to what you need feedback on. Thanks. --JustBerry (talk) 00:13, 20 July 2013 (UTC)

Diffs as follows

The folloiing are changes made by User:JimJohnson2222 : first edit, second edit, third edit, which was then reverted by User:Serendipodous : fourth edit. ---- Steve Quinn (talk) 03:18, 9 July 2013 (UTC)

Continue discussion below

• Serendipodous, ok, I will summarize the important changes most of which were addressed in our previous dialog.

A. References:

WMAP reference was replaced with Planck ESA results(used existing reference # 2 which someone updated for reference in the text earlier ); thus, replaced #49 with #2.

The seven added were: one from Astro-Physics journal - a famous article; two books from Physicist (one author, Paul Davies, is previous referenced #19); two NASA Releases which are as reliable as anything; one arXiv article; and, two from my recent book. Other references deleted were #47 (replaced with NASA ref),#48 (not needed), ,and #50 (replaced with NASA rer). B. Content:

I used the same equations, with updated values, for Critical Density and Steady State. I replaced the Stellar Density which relied on an obscure reference( #50)with more current NASA observations and renamed it Extrapolating from the Number of Stars. All volume calculations were modified to be based on the same comoving radius.

Method 1 was merged with Critical Density. Method 2 was merged with Extrapolating from Number of Stars. Used correct value for average star mass and correct value for the fraction of star mass (Astro -Physics article). This list of changes may still be confusing without someone spending significant time studying the two versions.

My suggestion is for others to read my version and ask if it provides a consistent, logical explanation/documentation.

As stated before, I can remove the rhetorical questions if appropriate. Jim Johnson 17:40, 7 July 2013 (UTC)

• The rhetorical questions are not the only problem. I recommend that Jim Johnson add references and inline citations to the content of the talk:Observable universe/workpage, along with other text that he wishes to add or revise -- rather than generally listing references that he is using. . For some reason, there appears to be avoidance pertaining to doing this. I think this would make it simpler to correlate statements and revisions to the sources. Also, I am not convinced that WMAP data is out of date in comparison to the ESA. ---Steve Quinn (talk) 18:47, 7 July 2013 (UTC)

Steve, you are referencing the wrong version, the one I submitted before understanding how to include references in the text. Go to the one Serendipodous deleted which he referenced above. If you do not think the ESA data is current why was it allowed in the other update earlier in the section? I am using the same reference! Do you have any factual concerns? If not, I think it is time to update.

For one thing, as explained above, it is difficult to make sense of all that you wish to change by reading WP:Diffs. I would appreciate if you created a block of text with references and in-line citations so that it can be properly critiqued. Some of what I saw in the previous section were comments referring to sources that you intend to use. I think that if you are not willing to do this then, no, it is not time to update. There is no reason for me to want to change the article. As far as I am concerned the article is great just the way it is. Also, there is no requirement that I agree to changes in this article. So, the onus is not on me. ---- Steve Quinn (talk) 23:04, 7 July 2013 (UTC)

OK, never mind. I see how this was set up to look at your refs, in the other section. The back and forth between you and User Serendipodous was not clear to me. I will have a look. Part of the problem seems there was some jumping on around on talk pages. ---- Steve Quinn (talk) 23:17, 7 July 2013 (UTC)·

Syeve and Serendipodous, If you can,please view my 8 minute video, it may provide some credibility. Thanks.

http://www.youtube.com/watch?v=K8V8Iy9Tozk Jim J Jim Johnson 18:48, 8 July 2013 (UTC)

Jim, thanks for the video. It is very good, it is well thought out, and the issues and your conclusions are presented very succinctly. Also, it helps to clarify what you are attempting to do. I have to honest - it's probably going to take me a couple of weeks to come to some yeas or nays pertaining to your proposed contributions to this article. I just want to make sure that all your assertions are supported, even if it's the science section of the New York Times. Also, I'll be looking to see if other sources share those conclusions. In the meantime, it sounds like there is an interest in editing other articles. Why not go ahead and make small edits to other articles that are of interest, while waiting. I would also like to see if other editors will get involved, here. --- Steve Quinn (talk) 02:08, 9 July 2013 (UTC)

Jim, I say I'll be looking to see if other sources share those conclusions because on Wikipedia there are two central policies: WP:SYN and WP:NOR. ---- Steve Quinn (talk) 02:23, 9 July 2013 (UTC)

Also, I found an interesting PDF document here. The author's name seems familiar, but I am not sure, Hm-m-m-m-m-m-m... Maybe I've seen this name on Wikipedia. Hm-m-m-m-m-m... :>) Also, I see your book on Amazon.com. Good job, Jim! ----Steve Quinn (talk) 02:36, 9 July 2013 (UTC)

Steve, thanks for your comments on the video. This particular subject intrigued me because there are no good references (including the existing Wikipedia article). I spent considerable effort discussing/questioning physicist on my assumptions and calculations. I looked over the two Wikipedia policies and think the revision passes. Yes, I am the James R Johnson. As far as other updates, the only thing planned is to assist Dr Dimitar in updating the Large Number Hypothesis after his work is published in a journal. As far as the revision, it is more logical for section 4 to follow section 5. Because I did not initially understand how to update the references, the original order was maintained. Do you agree? I noticed that in one reference the authors did not print due to my typo. I can correct. Also, should the rhetorical questions be removed as we discussed? Look forward to hearing from you. Jim Johnson 20:12, 9 July 2013 (UTC)

FWIW - ran across a reference which may (or may not) be relevant to the present discussion => total mass in universe = 10^58 grams? - in any case - Enjoy! :) Drbogdan (talk) 22:14, 9 July 2013 (UTC)

Well considering that it's probable that the universe is infinite, it is probable that it would have an infinite mass.... StringTheory11 (t • c) 03:58, 15 July 2013 (UTC)

Steve, please go to workspace for updated version. It should be ready to go. Jim Johnson 22:02, 16 July 2013 (UTC) — Preceding unsigned comment added by Jimjohnson2222 (talk • contribs)

• Comment from a user brought here by the RFC bot. Is this RFC withdrawn, or still active? If the latter, it does not seem well-formed. -- Scray (talk) 14:48, 25 July 2013 (UTC)

Considerable research and effort was epent on revising this material. Can you explain your comment? Jim Johnson 19:44, 25 July 2013 (UTC) — Preceding unsigned comment added by Jimjohnson2222 (talk • contribs)

Your post's lack of indentation leaves me wondering whether it is a response to my comment, but it seems that you are. My comment was not about your proposed edits, but the RFC itself (which started this section of the talk page). Does that make my comment more clear? -- Scray (talk) 23:11, 25 July 2013 (UTC)

I just corrected an error in workspace revision,superinposed numbers. All number should balance now. Jim Johnson 22:32, 26 July 2013 (UTC) — Preceding unsigned comment added by Jimjohnson2222 (talk • contribs)

Hello Jim. I'll do my best to run through this. First, the next time please don't remove large blocks of text from the article as is what happened here [1]. There is a difference between simply adding text and removing text and then adding it. I recommend just creating a new section, providing your assertions are accurate.

Also, with the link I provided please avoid general assertions such as "...mass of the universe is often quoted as...". Using the word "often" needs to be backed up by sources. The only way to use this is if a source actually says "often", otherwise it is synthesis.

So, you are just presenting two methods for the article? Is this correct?

If this is the case then I request that new sections are added without removing text. First, I feel more comfortable with this kind of change to the article. Second, I can compare the sections you add with the sections you are contending.

My first instinct is to leave your addition and the sections you don't agree with. I can see that it is possible to label your additions as updates or something like that. If you are willing to do it like this, then I don't have a problem with what you want to do.

As discussed before, use declarative statements in place of rhetorical questions. Furthermore, please make sure your sections are well sourced, and not a reference to the a citation in a section below. A reference can be used as a citation in the body of the text more than once. If you don't know how to do this, I can show you how.

Also, I recommend staying away from puffery (peacock words) and so called weasel words. At the same time, don't worry too much, because I can come behind and copy edit. However, you deserve the experience of writing your text into the article.

One more thing, I think it would be alright to post a link to your video in the external links section.

So far this was easy. Let's see how it goes. ---- Steve Quinn (talk) 05:18, 27 July 2013 (UTC)

Hello again. Sorry. I just looked at your worksheet. I should have looked at it sooner. It is certainly well written enough to include in the article as is. I will request that someone from WikiProject Astronomy or WP:Physics check the math after you add this content to the article. Go ahead and use the main headings and sub-headings as is, if you want. ---- Steve Quinn (talk) 05:28, 27 July 2013 (UTC)

Steve, thanks for the go ahead. I should update tonight (7-27). Please take a look tomorrow to verify it updated ok.

Because there is so much reshuffeling in the order, I think deleting the exsiting and adding the two new sections is the best approach. I will add the video to external links and reference the ultra-deep field phrase so it ref Wikipedia. Thanks, 75.16.184.9 (talk) 01:07, 28 July 2013 (UTC) Jim Johnson 01:09, 28 July 2013 (UTC) — Preceding unsigned comment added by Jimjohnson2222 (talk • contribs)

Steve, I made the changes but overlooked the "explain why changed" box because I was concentrating on the format. Hope that is not a problem. Jim Johnson 01:50, 28 July 2013 (UTC) — Preceding unsigned comment added by Jimjohnson2222 (talk • contribs)

"Obviously there is no way to know exactly the number of stars..." Is it obvious?

Is it really obvious that there is no way to know the exact number of stars? Do we want to label the answer to such an interesting question as 'obvious'? I worry it discourages inquisitive thought.

Would it be better to say either:

a) "There is currently no method for counting the exact number of stars," or
b) "It is believed that it will always be impossible to count the exact number of stars"

(we'd pick the one that best reflects the consensus of the scientific community)

Why (to me) the statement was not obvious (and indeed I still don't know if I believe it)

My initial response would be to say: 'count them'. And then I'd try to imagine the difficulties of counting them. Indeed, the only reasons I (a non-physicist) can think of for not being able to know the exact number of stars is that there are somehow more than it would be possible to observe in a limited time-frame, or maybe stars are always being born and dying so that the number of stars is constantly changing at a rate faster than we can keep up with. Or maybe we can never be sure there aren't stars that we can't see. It's not obvious to me which of these reasons (if any) is the explanation for the claimed impossibility of knowing the number of stars the stars. So I stand by my point that the statement is not obvious. Happy to hear responses either way, Best, 77.100.95.190 (talk) 21:56, 7 September 2013 (UTC)Matt

All I have put is There is currently no way to know exactly the number of stars and removed the Obviously as unencyclopedic. We cannot tell what the future may hold, see WP:CRYSTAL♫ SqueakBox talk contribs 22:22, 7 September 2013 (UTC)

I t is obvious that an accurate star counts do not exist. The reasons quoted are logical. Also, we are including the observable universe with 100-150 billion galaxies. It is obvious that an accurate count of galaxies is not possible much less stars within each galaxy. With such large ballpark numbers, only estimates are possible. Jim Johnson 23:08, 7 September 2013 (UTC) — Preceding unsigned comment added by Jimjohnson2222 (talk • contribs)

Move discussion in progress

There is a move discussion in progress on Talk:Age of the universe which affects this page. Please participate on that page and not in this talk page section. Thank you. —RMCD bot 22:15, 3 October 2013 (UTC)

When did/will the practically visible universe peak?

It states that we will eventually see 2.36 times more universe, but not really, cause more of it will be redshifted out of detectability. When? — Preceding unsigned comment added by 12.196.0.56 (talk) 20:01, 30 October 2013 (UTC)

The section on mass of ordinary matter--why "of universe"?

"Mass of ordinary matter[edit]The mass of the universe is often quoted..."

Shouldn't it be written as "The mass of the OBSERVABLE universe is often quoted..."? The article title is Observable universe for one thing, and the article often indicates that the actual universe may be much larger, by an unknown amount,so that the quoted estimate might not be close at all for the "universe," although it may well be a good estimate for the observable universe.76.218.104.210 (talk) 12:42, 21 January 2014 (UTC)

The assumption is that the universe is generally uniform and the spot we're at ain't that special. Hcobb (talk) 04:24, 23 January 2014 (UTC)

I don't see what that has to do with anything. The observable universe is (possibly much much much) smaller than the whole universe. In fact the whole universe may well be infinite. That has nothing to do with whether we're in a "special" place in it. I think the OP is right here. --Trovatore (talk) 04:36, 23 January 2014 (UTC)

Cosmological principle Hcobb (talk) 05:28, 23 January 2014 (UTC)

You seem to have missed the point. The cosmological principle has nothing whatsoever to do with the point raised by the anonymous editor. Assuming the cosmological principle holds, the situation would be the same from anywhere in the universe, and from anywhere in the universe, the observable universe would be (possibly much much much) smaller than the whole universe. --Trovatore (talk) 05:56, 23 January 2014 (UTC)

I made the edit, then undid it, because I actually don't know what the ref says or means. It's possible that this author thinks he has an estimate on the mass of the universe, as opposed to the mass of the observable universe. I'm pretty sure that such an estimate could not possibly be consensus in the astronomical community, however.

That entire section of the article says "universe" in several places where it probably means "observable universe". But I don't have the refs, and all I can say is "probably". Ideally this needs attention from an expert. --Trovatore (talk) 06:32, 23 January 2014 (UTC)

Something wrong here

Quote The age of the universe is about 13.75 billion years, but due to the expansion of space humans are observing objects that were originally much closer but are now considerably farther away (as defined in terms of cosmological proper distance, which is equal to the comoving distance at the present time) than a static 13.75 billion light-years distance.[2] The diameter of the observable universe is estimated at about 28 billion parsecs (93 billion light-years

If the big bang was one singular explosion point that spewed out all the matter in the universe, than knowing that the universe is 13.7 billion years old, how can the universe have a diameter of about 93 billions light years ? 1) If from the big bang it expanded , then it should have had a maximum speed equal to c. 2) If matter was coming out in all directions, or at least two opposite directions, then at most the greatest distance the universe could have expanded is 2 X c, that's 2 X 13.7 billion light years. 3) So the universe should only be 27.4 billion light years in diameter. 4) Or we accept that it expanded at a pace faster than the speed of light. — Preceding unsigned comment added by 90.208.172.45 (talk) 11:13, 15 January 2013 (UTC)

It seems to be so after all: However, due to Hubble's law regions sufficiently distant from us are expanding away from us much faster than the speed of light (special relativity prevents nearby objects in the same local region from moving faster than the speed of light with respect to each other, but there is no such constraint for distant objects when the space between them is expanding)

Does that mean object can only move faster than light when there is no nearby objects? And what is 'nearby' in this context? 82.141.95.124 (talk) 11:06, 21 January 2013 (UTC)

Basically you're talking about the distances between clusters of galaxies. Within those clusters gravity "pulls together" space to prevent large degrees of expansion. Between those clusters there's so little mass that the expansion of space is only affected by the intrinsic expansion. Imagine a balloon (the universe) with pieces of tape (the gravity in a galactic cluster) on it. While the whole balloon can expand, the tape will prevent it from doing so locally so that distances between the tape (along the surface of the ballon) expand faster while the areas under the tape get "left behind." The difference in those expansion rates are what allows remote galaxies to be moving faster than light away from us. Within the tape area, the speed of light is still the maximum. GaidinBDJ (talk) 14:06, 9 April 2014 (UTC)

Cosmic web observed, notable enough to split?

http://www.universityherald.com/articles/6939/20140120/universes-cosmic-web-visualized-for-first-time-thanks-to-distant-quasars-natural-flashlight.htm

Now that it has been observed to exist, should Cosmic web get its own article? Hcobb (talk) 19:02, 20 January 2014 (UTC)

It's notable, but that's not a reason for an article; a reason for an article would be if there was enough information on it to sustain an entire page. Articles one sentence long aren't really all that informative. Serendi^podous 08:22, 21 May 2014 (UTC)

Inconsistent dates under "Large Scale Structures"

The text there reads:

Prior to 1989, it was commonly assumed that virialized galaxy clusters were the largest structures in existence, and that they were distributed more or less uniformly throughout the universe in every direction. However, since the early 1980s, more and more structures have been discovered. In 1983, Adrian Webster identified the Webster LQG, a large quasar group consisting of 5 quasars.

It doesn't make sense that "prior to 1989" (meaning 1988 and before) galaxy clusters were the largest structures known, while in "the early 1980s" larger structures were discovered. To me, "the early 1980s" came before "1989". So whats going on here?

Fresheneesz (talk) 21:36, 11 August 2014 (UTC)

Size - epoch of decouping?

In the Size section of the article, the "redshift" is given as z, and the scale is given as a(t) = 1⁄(z + 1). Previously, the article was updated with the 9-year WMAP data, but the 7-year data reference was kept, so it Drbogdan, rightly confused by the discrepancy, changed the value to match the citation. In discovering this error, I updated the citation, but the number I found there was 1091.64, not 1092.64 as it was before Drbogdan's edits. Additionally, in both the 7- and 9- year data sets, the relevant quantity (one called "epoch of photon decoupling" the other called "redshift of photon decoupling"), is called z_*, not z. I'm thinking that the previous update had simply anticipated the need to add 1 for the scale factor and jumped the gun a bit, which explains the off-by-one error, but can any astrophysics types please weigh in on this? What is the commonly accepted nomenclature for z? Should we update it to be z_* to be consistent with the references? Is it possible that this z_* value from the reference already includes the +1, and the actual value for the "z" is 1091.64? It also seems the original author used z-1, not z+1 - I don't have the cited textbooks, can anyone verify that z+1 is the correct formula? 0x0077BE [^talk/_contrib] 20:19, 11 September 2014 (UTC)

@0x0077BE - Thank you *very much* for posting - yes, a clarification, if possible, would be welcome of course - in any case - Enjoy! :) Drbogdan (talk) 20:32, 11 September 2014 (UTC)

z_* is not defined in the reference, but I'm willing to bet that it's a redshift z, not z+1, and the subscript * means that it's the redshift of the epoch of photon decoupling (as defined in the referenced table in Bennett et al 2013). "Epoch of photon decoupling" and "Redshift of photon decoupling" are intended to mean the same thing. (It's very slightly sloppy language to refer to a redshift as an epoch, but that phrasing would be universally understood by an astronomer -- I wouldn't even think of it as sloppy until I read your question!)

I've never seen anywhere in the literature use a modified z symbol to mean z+1; as you say, it would be terribly confusing.

I don't think using the nomenclature z_* is appropriate or necessary in the Wikipedia article.

Re 1090.64 vs 1091.64: I'd guess (without checking the history) that what happened is someone originally entered the WMAP-only value (1090.97, from that same table in Bennett et al 2013), and then someone else changed it to the concordance number 1091.64 but failed to change the 0 to a 1. (These differences are only marginally outside the 1-sigma uncertainties anyway, so not worth worrying about. But 1091.64 is clearly the best number to use based on the cited paper.) Anyway, with this minor correction to the numerical value, everything looks fine. (I will make some tweaks to that paragraph in the article.) —Alex (ASHill | talk | contribs) 21:54, 11 September 2014 (UTC)

Great, thanks for the clarification! I think that, looking closely, what happened was that the original person just swapped the two numbers by accident, leading to my confusion. They had z = 1092.64 and a(t) == 1⁄1091.64, when it should have been z = 1091.64 and a(t)=1⁄1092.64 using the citation and the formula. It seems like everything has now fallen into place. 0x0077BE [^talk/_contrib] 13:45, 12 September 2014 (UTC)

Scale factor (cosmology) has the factor 1/(1+z) for the Friedmann–Lemaître–Robertson–Walker metric. The formula is is sourced by a couple of books, neither of which I have. --Mark viking (talk) 22:05, 11 September 2014 (UTC)

The scale factor as written is correct; no doubt about that. —Alex (ASHill | talk | contribs) 23:55, 11 September 2014 (UTC)

Excellent. Thanks again for the help guys! 0x0077BE [^talk/_contrib] 13:45, 12 September 2014 (UTC)

Section on Misconceptions

The Misconceptions section in the article is, IMO, both confusing and out-of-date, and needs to be (partially) deleted, I think. Here's my reasoning--

1. The 78 billion light year sub-section is listed as a misconception, but doesn't explain why it is so. This is why its confusing to me as I read.

2. The 2003 paper referring to the 78 billion light year misconception http://arxiv.org/abs/astro-ph/0310233 has been updated by the authors in 2012 http://arxiv.org/abs/1206.2939. The lower bound is now 85 billion light years. The next sections, 156 billion Light year and 180 billion light year are related to the 78 and should be dropped as well.

I was just concerned about updating the article to show the latest data on patterns of repeating circles. The latest data--that was my concern. I did do a poor job of revising since I keep orphaning a ref :), and would appreciate any help. Chasrob (talk) 13:53, 7 July 2012 (UTC)

For #1, the text does explain why it's a misconception, when it says "This is a lower bound for the diameter of the whole universe (not just the observable part)". So, it's a misconception to say that the observable universe has been shown to be 78 billion light years wide. For #2, we could include the more recent lower bound, but the more recent paper wasn't widely misreported as the size of the observable universe as with the 78 billion light year figure. Hypnosifl (talk) 17:25, 8 July 2012 (UTC)

Its not the lower bound for the whole universe as well; so you're right, it is a misconception.Chasrob (talk) 00:09, 9 July 2012 (UTC)

It was the lower bound at the time the Cornish paper was published; estimates that as far as I know were made correctly given the data at the time but have been slightly altered by newer data cannot really be called "misconceptions" IMO. Hypnosifl (talk) 14:13, 9 July 2012 (UTC)

I don't see any basis for supposing that there is a misconception with respect to the 78 billion light year sub-section. Yes, it would be a misconception if people thought that this referred to the size of the observable universe, but there is no reason to suppose that anybody thinks that. John M Baker (talk) 21:28, 2 August 2012 (UTC)

This concerns a different item in that section: While I'm fine with having a radius of 13.8 billion light-years listed as a misconception regarding the present era radius of the observable universe, the statement that "Distances obtained as the speed of light multiplied by a cosmological time interval have no direct physical significance" feels much too negative. 13.8 billion light-years is the distance that was travelled through expanding space by any light that is just now reaching us from the edge of the observable universe. That's physically significant, and I rather strongly feel that it should be said in the article in place of the statement I quoted.50.174.178.168 (talk) 05:22, 16 June 2015 (UTC)

I agree. Do it! —Alex (Ashill | talk | contribs) 05:33, 16 June 2015 (UTC)

Matter content — number of atoms

"The chemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the Universe was only 10–17 million years old. According to the panspermia hypothesis, microscopic life—distributed by meteoroids, asteroids and other small Solar System bodies—may exist throughout the Universe. Though life is confirmed only on the Earth, many think that extraterrestrial life is not only plausible, but probable or inevitable."

That is an interesting statement with lots of good links to read, but I fail to see why it has anything to do with how many atoms are in the universe. I might speculate that the author there was suggesting the large number of atoms creates some sort of probability, but that's the only assumption I can make. So, If someone could either add more explanation in that section as to why it relates, or just delete it, I think either would be better.

I'm not going to delete it for now because I don't find it particularly harmful, just confusing flow. AutumnWind 17:48, 20 July 2015 (UTC) — Preceding unsigned comment added by Autumn Wind (talk • contribs)

formula for the radius of the visible universe

I would ask for attaching to an article in the section Size: The formula for the radius of the visible universe, based on the age of the universe.

The comoving distance from Earth to the edge of the observable universe: R = pi * c * t when t = 13.8 bln y. R= 43.4 bln ly

Formula was derived in FRW metric, for positively curved universe (spherical). The resulting radius is 5% shorter, the possible reasons for this: 1) data error, 2) Spherical model of the universe is too big an approximation.

According to the formula the universe is expanding at fixed rate contrary to observed an increasing rate.

I can indicate the method for confirming the hypothesis that observed effect depends on the position of the observer in the universe and is caused by the gravitational time dilation effect: If the observed acceleration depends on the local curvature of the universe, the results of observation from various directions should vary randomly.

MironN (talk) 12:07, 11 June 2015 (UTC)

---

perhaps add the standard SI version of any of the radius/diameter/future distances, using the standard SI prefixes?

e.g.: 8.8×1026 metres would be 880 yottameters.  ? etc etc.

For me, easier to understand are light years, but of course I use SI units when checking the correctness of the formula. [s] [m / s] = [m]

If it will help you in understanding the problem, multiply the years by the seconds, and then by the speed of light.

I'm trying to solve Janusz Drożdżyński paradox. During the work, I found an interesting correlation between the radius of the visible universe and the age of the universe. MironN (talk) 13:28, 18 August 2015 (UTC)

The fact that the ratio of the universe's radius to its age is close to π·c is coincidence as far as I know. The edge of the observable universe is related to the time at which the universe became transparent, which depends on complicated non-gravitational physics, so there's no reason to expect a simple number like π to fall out of it.

Janusz Drożdżyński appears to be a crackpot (at least as far as relativity is concerned; he's a chemist). If you're referring to the paradox he presented in "Evidence for an Invalidity of the Principle of Relativity", the solution is that he didn't take aberration into account. -- BenRG (talk) 00:22, 19 August 2015 (UTC)

Thank you for your comment. Writing abberation you had in mind relativistic beaming? (in rocket light source is moving) In both cases (abberation and relativistic beaming) observer and the light source are in different frames of reference. In Drozdzynski they are in the same frame of reference (example of the rocket). I went a different route, I assumed that the perceived 3-dimensional space is the projection of a 4-dimensional spacetime. A time when the universe was opaque is negligibly small in terms of the age of the observed universe. I have not guessed this relation with the number Pi. First, I brought the relationship, then introduced the experimental data, and I was surprised by correspondence. MironN (talk) 11:55, 16 September 2015 (UTC)

Use of "universe" to mean "observable universe"

The article currently claims that 'Both popular and professional research articles in cosmology often use the term "universe" to mean "observable universe".' I don't think this is right. This usage is simply a solecism.--76.169.116.244 (talk) 19:35, 1 August 2015 (UTC)

The article doesn't say it's right, just that it happens :-) Oh, looking at it now, it does say it can be "justified", which is maybe a little too essayish, especially without a source. In principle the statement that the term is used that way should also have a source, but I would be inclined to bend the rules just slightly on that one on the grounds of usefulness (lots of people read things about, say, "the number of atoms in the universe" and take them at face value, not realizing that the word is being used to mean "observable universe", and that it's an open question whether the number of atoms in the whole universe is even finite).

So, what shall we do? Other opinions? I don't want to go in and just start hacking stuff out, but it probably does need to be better sourced and less opinionated. --Trovatore (talk) 20:04, 1 August 2015 (UTC)

I'm pretty sure that the statements are both true and could be found in a source, though I certainly agree that we should find one. I've added citation needed tags. I think that the statements are useful as a description of common usage, though I also agree that it's not particularly good or precise usage. Descriptions of usage serve a clarifying and encyclopedic purpose, I think. —Alex (Ashill | talk | contribs) 13:24, 16 September 2015 (UTC)

Note that this source says "The terms "universe" and "observable universe" are sometimes used interchangeably by cosmologists, because by definition, regions outside what we can see are causally disconnected from us. We cannot observe, much less influence, these regions, and the inverse applies as well." That certainly supports the uncited statement, though it's not clear to me if that source is reliable, and it might even be based on the Wikipedia article. —Alex (Ashill | talk | contribs) 13:33, 16 September 2015 (UTC)

Hatnote

Paradoctor's second version, currently in the article, is certainly much better. It says:

"Known Universe" redirects here. For the fictional setting frequently confused with it, see Known Space.

and that strikes me as just the right thing to say, given that Known Universe in fact redirects to this article.

But should it? It doesn't make a lot of sense to me. There's an awful lot of the observable universe that is not "known" in any particularly meaningful sense. Maybe it should go to RfD? --Trovatore (talk) 20:17, 17 October 2015 (UTC)

I see what you mean, but there's an awful lot of our own planet that is not "known" in any particularly meaningful sense. I think most people interpret "known" to mean "known about" in the sense that we know that it exists even if we don't know much about it. Personally, I think the redirect is appropriate, but we'll see what others think. Dbfirs 20:38, 17 October 2015 (UTC)

Well, to me the "known" locution sounds like something from the Age of Exploration. This part of the world we know about; elsewhere be dragons, or something. That isn't very appropriate to this concept, which has a definition based on physical ontology rather than human epistemology. We can know more of the universe, but we can't make the observable universe any bigger.

But of course it doesn't really matter that much whether the phrase makes sense or not; what matters is whether it's a name that people use for the concept, and whether searches on it get people where they want to go. I don't really know whether that's the case or not. --Trovatore (talk) 20:48, 17 October 2015 (UTC)

Populist sources such as the UK Daily Mirror (not that I'd regard it as a reliable source) and physics-astronomy.com and Cornell University seem to use "known" in the sense of "observable" when referring to the universe, but I haven't found a really good reference yet. Dbfirs 21:23, 17 October 2015 (UTC)

(edit conflict)

Certainly not, since "Known Universe" is a synonym for "Known Space". The only question left here is whether

1. there should be a path from "Known Universe" leading here, and
2. if so, whether "observable universe" is the WP:PRIMARYTOPIC.

Browsing through the 10000+ hit list for "known universe" shows that the term is at least occasionally used by astronomers, so that takes care of the former question. Also, [2], [3] are among the easy catches from the general population of web pages.

"niven's known universe" yields only 1800 hits on the general web, but unlike the Scholar results, there is little noise, so there seems to be no clear primary topic. Together with two other uses for the term that cropped up, this implies a disambiguation page. Will be open for business in a few minutes. Paradoctor (talk) 21:53, 17 October 2015 (UTC)

Thanks, I think the disambig page is an improvement. I was a bit worried that the hatnote would contribute to the misconception that the "observable universe" depends on our powers of observation, so that for example it would have been very small before the invention of telescopes. --Trovatore (talk) 22:31, 17 October 2015 (UTC)

Since it appears not possible to falsify the notion that our experience of "reality" is the result of a bottomless hierarchy of simulations, I wouldn't worry about people's misconceptions. Paradoctor (talk) 23:08, 17 October 2015 (UTC)

Capitalization?

Apparently astronomical proper nouns should be capitalized according to the IAU style manual: thus the Earth, the Solar System, etc., but not when referring to collections of objects, thus the planets, the stars, etc. Hence should it be the Universe when talking about the one Universe we live in, but universes and universe when talking about the Multiverse (which presumably should also be capitalized)? As an example, 'the Earth is one of the planets in the Solar System' - compare with 'our Universe is one universe in the Multiverse'. Aarghdvaark (talk) 23:25, 11 April 2012 (UTC)

I disagree. Wikipedia follows common usage, which is universe without capitalisation. An editor has recently capitalised every "universe" in this article, and I reverted this because it introduced redirects and a distortion of a direct quotation. Dbfirs 21:28, 25 November 2015 (UTC)

The editor appears not to have noticed that the linked article from which the Brian Greene quote is taken uses lower case. Dbfirs 18:37, 26 November 2015 (UTC)

GN-z11

A new discovered galaxy - GN-z11 - is the most far away object in the Universe. Please update the article. 46.70.188.72 (talk) 22:28, 5 March 2016 (UTC)

Rewording

In the Misconceptions section there are two sentences which read: "This reasoning would only make sense if the flat, static Minkowski spacetime conception under special relativity were correct. In the real universe, spacetime is curved in a way that corresponds to the expansion of space, as evidenced by Hubble's law." This seems unnecessarily complicated. Can't it just say "This reasoning would only make sense if the universe wasn't expanding, but it is." Richard75 (talk) 19:22, 9 March 2016 (UTC)

"Distances obtained as the speed of light multiplied by a cosmological time interval have no direct physical significance" This distance is actually the "Schwarzschild radius" of the universe. — Preceding unsigned comment added by Mendi80 (talk • contribs) 01:54, 19 March 2016 (UTC)

Possible OR in section "mass of ordinary matter"

This section appears to contain synthesis. Individual parts are reliably sourced (e.g. the fraction of ordinary matter vs. dark matter vs. dark energy, the number of stars, etc) but the calculations that lead to the result are not. That is, none of the sources actually contains the assertion that extrapolating from the number of stars leads to a mass of the observable universe of ___. I'm tagging one sentence that most illustrates the problem with . If anyone knows any source that does perform the calculations in the section explicitly, please share. Banedon (talk) 01:09, 20 March 2016 (UTC)

I'm currently looking at Introduction to Cosmology by Barbara Ryden, which is to my knowledge a classic textbook on undergraduate cosmology. Barbara spends section 8.1 discussing visible matter, and calculates a density parameter of stars as ~0.004. However she also says that this number is not a precisely determined one because of uncertainty in the number of low-mass, low-luminosity stars. She then spends some time discussing non-stellar baryonic matter, before eventually concluding that the best current limits on the baryon density of the universe comes from big bang nucleosynthesis. Reading between the lines I'm inclined to conclude that adding up the mass of visible matter is not sufficiently precise to come to any conclusion about the total baryonic density; in fact if Davies (or whoever) actually performed the calculations in the article I get the feeling that it's a case of knowing the answer you want to get, and tweaking the numbers to get there. Like, just look at the assumptions that go into the calculation. "There is no way to know exactly the number of stars, but from current literature, the range of 10²² to 10²⁴ is normally quoted." If the number of stars is uncertain to a factor of two orders of magnitude, how can any of the calculated results be reliable? I'm tagging the section for OR again. Banedon (talk) 01:25, 23 March 2016 (UTC)

I've gone ahead and deleted the entire section. From further reading I noted a "missing baryon problem" (that term's Google-able) whereby totaling up the mass of all the known baryons aren't enough to reproduce the density parameter of baryons from BBN and CMB constraints. If that's so, the section must not only be OR, it's probably wrong as well. Banedon (talk) 13:54, 5 April 2016 (UTC)

Mass of ordinary matter - intro and "steady state"

The introduction to the [Mass of ordinary matter] section stated "The calculations obviously assume a finite universe." That isn't true, it is valid for both finite and infinite but only calculates the values for the observable portion. I have updated the text accordingly.

The article also says "Two calculations substantiate this quoted value for the mass of ordinary matter in the Universe: Estimates based on critical density and estimates based on steady-state.". How does calculation based on the long-discredited steady-state model "substantiate" the number? This seems completely misleading, as if there were some possibility that the steady-state model were still viable. George Dishman (talk) 13:09, 27 April 2016 (UTC)

Agreed - I'm just going to remove the subsection, and rewrite the section accordingly should I find the time. Banedon (talk) 02:02, 16 June 2016 (UTC)

" and the observed fraction of the universe always increases"

This phrase is found in the section "Particle Horizon", and IMHO means something entirely different from what somebody intended it to mean. Therefore it should be rewritten.

Explanation: The observable volume of the universe always increases, which I think we all agree with. But if a fraction of something increases, it grows proportionally more than the whole. So, if you observe 10% of a thousand liters of water, you see a hundred liters. If there is three thousand liters tomorrow, and you observe twice the amount (two hundred liters), you now only see 6.6% of the water. In other words, your fraction has decreased.

The erroneous wording suggests that the percentage always increases. You would end up seeing the entire universe, ultimately. I believe it is trivial to show that the "total volume" of the universe grows faster than the observable volume. (Alas, I don't feel qualified to try it here.) 88.114.128.29 (talk) 19:50, 23 December 2012 (UTC)

The percentage does always increase. But that does not mean "You would end up seeing the entire universe, ultimately." Something can be both monotonically increasing and also have an upper bound. Assuming money was infinitely divisible, I can give you $1 today and every day give you half of what I gave you the day before, forever, then the total amount of money I gave you would increase every day, but would never reach $2. The locations of galaxies are essentially fixed over time in comoving coordinates, so the comoving distance is proportional to the "percentage" of the universe (if it were finite). And the comoving distance to the particle horizon is always increasing over time. However, assuming the universe keeps expanding the way it does now, it will increase slower and slower at some point and there will be a comoving distance that the particle horizon will never reach. --73.241.96.130 (talk) 06:19, 21 June 2016 (UTC)

Sizes inconsistent with inflation and horizon problem?

The article says that the edge of the observable universe (which is the particle horizon) is about 46.5 billion light-years away, which is only a little more than the distance to the particles from which the CMBR was emitted (the optical horizon), which are about 45.7 billion light years away.

However, this seems to me to be inconsistent with inflation, or any other solution to the horizon problem which explain how the particles that emitted CMBR from opposite directions in our sky were causally connected. If the particles at the opposite ends of our optical horizon were causally connected at the time of emission of the CMBR, then that would mean light had time to travel from one side to the other since the beginning of the universe until that time, or in other words, the particle horizon at that time was greater than the distance between those two points. In terms of comoving coordinates, that means the comoving distance of the particle horizon at the time of emission of the CMBR was at least the comoving distance of the diameter of our visible universe, i.e. twice the comoving distance of our optical horizon now. Furthermore, it is obvious that (the comoving distance of our particle horizon now) >= (the comoving distance of our optical horizon now) + (the comoving distance of the particle horizon at the time of emission of the CMBR), and substituting in the above observation, this is >= 3 * (the comoving distance of our optical horizon now). The numbers that our particle horizon is only a little bit larger than our optical horizon is wildly inconsistent with this. --73.241.96.130 (talk) 06:03, 6 July 2016 (UTC)

You probably misunderstood something. It is extremely unlikley that the brightest physisists of the world made such a silly error! You'd better to read specialized liturature. By the way, for such questions Wikipedia:Reference desk/Science is a better place. Ruslik_Zero 19:32, 6 July 2016 (UTC)

The problem is here, "If the particles at the opposite ends of our optical horizon were causally connected at the time of emission of the CMBR". As you can see from the inflation article, inflation is theorized to have occurred very soon after the Big Bang and ended very shortly afterwards as well - hence the opposite ends of our optical horizon were not causally connected at the time of emission of the CMBR. Banedon (talk) 00:49, 7 July 2016 (UTC)

It doesn't matter how early inflation happened. Saying it is causally connected means that light had at least enough time to travel from one place to the other between the beginning of the universe and that time. That's the only thing that I am assuming. --73.241.96.130 (talk) 03:40, 8 July 2016 (UTC)

Can you explain where you got "i.e. twice the comoving distance of our optical horizon now" and "(the comoving distance of our particle horizon now) >= (the comoving distance of our optical horizon now) + (the comoving distance of the particle horizon at the time of emission of the CMBR)"? Banedon (talk) 05:56, 8 July 2016 (UTC)

Yes. The "twice the comoving distance of our optical horizon now" is based on the assumption that light had enough time to travel from our optical horizon in one direction to our optical horizon in the other direction, in the time between the beginning of the universe until CMBR was emitted. The comoving distance that was travelled was the diameter of our visible universe, i.e. twice the radius of our visible universe, i.e. twice our optical horizon.

(the comoving distance of our particle horizon now) >= (the comoving distance of our optical horizon now) + (the comoving distance of the particle horizon at the time of emission of the CMBR) is because, if light from a point X was able to travel, between the beginning of the universe until CMBR was emitted, to a point where CMBR was emitted, and light (CMBR) was able to travel from there to us between that time and now, that means light from point X was able to travel to us between the beginning of the universe and now.

After I wrote this I realize that having light be able to travel from one point to the other might be too strong a condition. There is a slightly looser, and I think indisputably necessary, condition for causal connection -- that light from some place was able to travel to both of them since the beginning of the universe, i.e. their past light cones overlap. The numbers will be a little different but it will still show the problem with the numbers. In this case, the comoving distance of particle horizon at the time of emission of CMBR would be at least the comoving distance of our optical horizon now (not twice), since the light must cover half the comoving distance from one point to the other. This will still mean that our particle horizon now must be at least twice (1+1) our optical horizon now, which is still way off from the sizes given.

On page 5 of this PDF (page 33 on the page) it shows a diagram that clearly illustrates waht I mean: for CMBR in opposite directions to have received information from the same source, it requires that the conformal time from the beginning of the universe to now must be at least twice the conformal time from CMBR emission to now, and thus our particle horizon now must be at least twice our optical horizon now. --73.241.96.130 (talk) 02:33, 9 July 2016 (UTC)

@Parejkoj: as an expert in this topic.

From first impressions I'm guessing that you are interpreting the particle horizon as a "diameter" but the optical horizon as a "radius", which is where the factor of two is coming from. Easy thing to do would be to evaluate the integral for optical and particle horizons explicitly to verify the values in the article, but I don't have the software to do that right now. Banedon (talk) 04:42, 9 July 2016 (UTC)

Nope, both are radiuses. What I am guessing is that the sizes they gave are based only on the FLRW metric, without considering inflation, so it is basically the horizon of things since the end of inflation. If that is the case, then they should state so clearly, because it is still possible in principle to observe things from during and before inflation at a greater distance. We don't have a good idea of how much the universe expanded during and before inflation, so we are not yet able to calculate the actual particle horizon, but we know that a lower bound must be twice the optical horizon to solve the horizon problem. --73.241.96.130 (talk) 18:48, 9 July 2016 (UTC)

Accurately calculated mass

The introduction includes (and currently ends with) the following sentence, which is based on a controversial non-mainstream theory, and the reference is on a site which is of questionable scientific validity:

The total mass of the universe can be accurately calculated with the speed of light c, gravitational constant G and the age of the universe to be 1.8×10⁵³ kg with the relation of c³=GM/t.[14]

I think that at the very least, it should be made clear that this is a result from a non-mainstream theory, and I'd be inclined to say that something this controversial doesn't really belong in this article at all. Jonathan A Scott (talk) 16:58, 29 November 2016 (UTC)

I agree and changed the section. Banedon (talk) 00:47, 30 November 2016 (UTC)

Beyond the observable universe

Why isn't there a section discussing what might credibly exist in the regions of the universe beyond which we can observe? Abyssal (talk) 04:38, 20 December 2016 (UTC)

Because it's not within the realm of science. Anything theory of what lies outside the observable universe is not (directly at least) falsifiable, since it cannot be observed. I am not convinced such a section should be included (or what to write there, if it is included). Banedon (talk) 06:01, 20 December 2016 (UTC)

I agree that this would be entirely speculation and would cause controversy because everyone would have their own opinion (including those of us who believe we would be writing about nothing). I suppose we could mention well-referenced speculation by WP:Reliable sources if there are any. Dbfirs 10:44, 20 December 2016 (UTC)

Well, if you're talking about "who won the recent election just beyond the particle horizon", sure, that's pure speculation. But the models that work for our part of the universe also make predictions for the part of it beyond the observable universe, at least in the large (namely, that it should look roughly like our part of the universe) and there's no clear reason to think they work less well there than here. To suppose that this is "talking about nothing" sounds like a radical version of positivism that is not really a useful approach to understanding reality.

I would expect this point to have been made in reliable sources. Whether it's worth stating explicitly is a different question, of course. --Trovatore (talk) 08:48, 26 December 2016 (UTC)

Citation Errors

Referring to link citation 22 links to a Guth's "Inflationary Universe" book, it should be noted that even thought this wiki article correctly cites Guth's figures as depicted in the footnote for figure 10.6., the size comparison he made was NOT specific to actual universe volume and should read 3x10^23 times the RADIUS in both wiki and Guth's footnote. This is apt to the comparison that Guth was attempting.

Referring to link citation 24, this wiki article should read 10^1010122Mpc not "times the observable universe." Units 'Mpc' is what is actually indicated in the abstract of the cited paper. (The figure is listed again in the body of the paper sans unit measure type.) Pending the review of other editors, (I may be missing something) I will edit in accordance with the cited publication. — Preceding unsigned comment added by John11235813 (talk • contribs) 05:49, 26 December 2016 (UTC)

So sure, if that's what it says in the source, then by all means we should report it correctly. Just the same, I'll take the liberty of noting that when you're talking about 10^1010122, it doesn't matter much what the units are. Megaparsecs or observable-universe-radii or femtometers, it's pretty much all the same. --Trovatore (talk) 08:53, 26 December 2016 (UTC)

At those scales it would be trivial indeed, I was just wanting to have another set of eyes or two check both sources and confirm. I would just as soon have the article reflect the sources as best as possible despite the triviality issue. :) — Preceding unsigned comment added by John11235813 (talk • contribs) 00:18, 28 December 2016 (UTC)

End of Greatness?

Numerous articles by Francesco Sylos Labini state that galaxy distribution is inhomogeneous at the largest scales, and thus inconsistent with the assumption of homogeneity encoded in the Cosmological Principle. I've been looking for other articles in response that contradict these findings, but I haven't found much yet. This may merely reflect my unfamiliarity with the subject. See, e.g. Francesco Sylos Labini & Luciano Pietronero (2010) "The complex universe: recent observations and theoretical challenges" in Journal of Statistical Mechanics: Theory and Experiment 2010, IOP Publishing, P11029.

Are Sylos Labini's (and others') findings reliable and noteworthy? They are not mentioned here or at the Cosmological Principle article. Any help appreciated, thanks. Fuzzypeg★ 01:23, 12 September 2012 (UTC)

The section immediately preceding "End of Greatness" lists numerous structures greater than the 300 million light years above which EOG section claims everything is homogenous. Currently the last mentioned structure is 8 billion light years, which is 27 times the maximum structure scale of 300 million light years claimed in EOG section. That's a direct SELF CONTRADICTION. And it's a pretty darned stupid one. 2001:464C:C287:0:31B3:7E3E:DA09:5707 (talk) 04:22, 8 May 2017 (UTC) Alf P. Steinbach

"Mass of ordinary matter" in body + intro

Surely this should be stated to include energy as well?

if so, that's an easy fix, but what do we do about the figure for number of atoms, which could vary as the proportion of atomic matter vs energy varies without affecting overall energy density? FT2 ^{(Talk | email)} 06:24, 5 September 2017 (UTC)

This should not be changed because the definition of ordinary matter is clear and does not contain energy. Jim Johnson 19:40, 5 September 2017 (UTC)

Need a physics expert to verify the article.

Moved from Wikipedia:Village pump (miscellaneous)/Archive 56 § Need a physics expert to verify the article.

 – ―Mandruss ☎ 22:51, 13 October 2017 (UTC)

Article Observable universe features a claim: "higher estimates implying that the universe is at least 10^1010122 times larger than the observable universe". I have checked the given reference but a referred scientic paper talks about megaparsecs as units (one megaparsec is around 3 and ¼ million light years) in its annotation rather than times thr observable universe. Also, it's not exactly clear if the paper's author meant about parsecs of length (of radii or diameter) or cubic parsecs of volume but it didn't seem to be a comparison to observable universe's magnitude (a ball of ~45.7 billion light years in diameter, ~399.8 trillion cubic light years in volume). Can someone recheck the original PDF and say if it's a mistake.Fixmaster (talk) 22:25, 13 October 2017 (UTC)

I don't think we need a physics expert for this, per se. I'm not sure what PDF is being referred to, but when you're talking about 10^1010122, you really almost don't care about the units. If the units are off by a factor of, say, a googol, you won't be able to see the difference in the 122:

10^1010122·(1 googol) =

10^1010122·(10¹⁰⁰) =

10^{(1010122+100)}

I could carry this calculation out further, but hopefully you already see the point — 100 is completely negligible next to 10¹⁰¹²². By the time you propagate it up to the 122, you'll have to go out a huge number of digits to see the difference. --Trovatore (talk) 05:28, 14 October 2017 (UTC)

Mistake in critical density

In the article I came upon this sentence:

"Critical density is the energy density where the expansion of the Universe is poised between continued expansion and collapse." This however is false - it is only correct for a universe without dark energy. Because of dark energy for example a universe with density higher than the critical can expand forever. So I recommend this line be removed. — Preceding unsigned comment added by 77.70.29.135 (talk) 20:54, 10 November 2017 (UTC)

Content in Wikipedia is based on reliable sources. If you have a source to drive this change please cite it. Thanks. Jytdog (talk) 16:36, 11 November 2017 (UTC)

The objection is correct, I'll fix the problem. Banedon (talk) 00:16, 13 November 2017 (UTC)

External links modified (January 2018)

Hello fellow Wikipedians,

I have just modified 2 external links on Observable universe. Please take a moment to review my edit. If you have any questions, or need the bot to ignore the links, or the page altogether, please visit this simple FaQ for additional information. I made the following changes:

• Added archive https://web.archive.org/web/20110929142755/http://cosmos.phy.tufts.edu/~zirbel/ast21/sciam/IsSpaceFinite.pdf to http://cosmos.phy.tufts.edu/~zirbel/ast21/sciam/IsSpaceFinite.pdf
• Added archive https://archive.is/20120802035723/http://www.etsu.edu/physics/etsuobs/starprty/120598bg/section7.htm to http://www.etsu.edu/physics/etsuobs/starprty/120598bg/section7.htm

When you have finished reviewing my changes, you may follow the instructions on the template below to fix any issues with the URLs.

This message was posted before February 2018. After February 2018, "External links modified" talk page sections are no longer generated or monitored by InternetArchiveBot. No special action is required regarding these talk page notices, other than regular verification using the archive tool instructions below. Editors have permission to delete these "External links modified" talk page sections if they want to de-clutter talk pages, but see the RfC before doing mass systematic removals. This message is updated dynamically through the template {{source check}} (last update: 18 January 2022).

• If you have discovered URLs which were erroneously considered dead by the bot, you can report them with this tool.
• If you found an error with any archives or the URLs themselves, you can fix them with this tool.

Cheers.—InternetArchiveBot (Report bug) 00:33, 22 January 2018 (UTC)

Size of Universe

Under this statement under 'Cosmology and general relativity' ; "Size of universe: The diameter of the observable universe is about 93 billion light-years, but what is the size of the whole universe? Does a multiverse exist?"

I don't quite understand what this question is about. Is this questioning if the Universe finite or infinite or actually asking about its size. Universe#Size and region does not say 93 but 91 million light-year, 46 billion light years to 'edge'. Also this says "The size of the Universe is somewhat difficult to define." All here is not cited.

However, on this link[4] under Observable universe#Size says "93 billion ly (light years) (It is not however known if the size of the entire universe is finite or infinite.)"

Another is Observable universe#Misconceptions on its size, assuming "Many secondary sources have reported a wide variety of incorrect figures for the size of the visible universe. Some of these figures are listed below, with brief descriptions of possible reasons for misconceptions about them."

It seems this size/question seemingly all hinges on this single and likely dubious web cite.

(I have also posted this on Observable universe) Arianewiki1 (talk) 00:52, 6 February 2018 (UTC)

Extraterrestrial objects receding faster than light

@Ruslik0: Please explain why this is "crap". [5] Banedon (talk) 20:53, 25 February 2018 (UTC)

Because none such objects exist. Ruslik_Zero 21:00, 25 February 2018 (UTC)

Well, it depends on what one means, I think. Here's my vague understanding of what it means: Suppose f(t) is the distance between galaxy A and galaxy B, in the sense that t is a time coordinate in an inertial coordinate system in galaxy A, and then you find the distance between the event points "galaxy A at time t" and "galaxy B at the same comoving time that galaxy A has at time t". The notion of "distance" also needs to be appropriately specified, but I believe this can be done — the idea is that you're summing up small distances in frames of reference that are at rest with respect to the Hubble flow.

Then, I think, df(t)/dt can be greater than c.

The notion appears often enough that it probably ought to be discussed, together with caveats. We need high-quality sources and expert editors for this one. --Trovatore (talk) 21:08, 25 February 2018 (UTC)

"None such objects exist" is incorrect. They can not only exist, they can potentially be observed (for a while at least). Read the source and state what you don't understand, because this is a standard concept. Here's another source that discusses it: [6] Banedon (talk) 21:45, 25 February 2018 (UTC)

I've reverted since it's been a few days with no response, and this really is a standard concept. See also sources 73-76 in the dark energy article. Banedon (talk) 04:23, 28 February 2018 (UTC)

@FlightTime: if you're going to revert can you say why on the talk page? This is getting ridiculous. Banedon (talk) 04:56, 28 February 2018 (UTC)

@Banedon: Because you do not have consensus or a source for that matter. - FlightTime (open channel) 04:59, 28 February 2018 (UTC)

Banedon does have sources, but they don't say exactly what the disputed text says. In fact they partially contradict it; as Banedon him(?)self says, it doesn't follow from the recession velocity being greater than c that you can never detect the object.

This topic is treated in considerable detail at faster-than-light#Universal expansion. I think it might well be reasonable to mention it here as well. But it needs to be done carefully. There doesn't seem to be any simple relationship between faster-than-light recession and being in the observable universe, or being eventually detectable. This really needs expert attention. --Trovatore (talk) 05:32, 28 February 2018 (UTC)

@FlightTime: There are sources, it was in the reverted text and I listed at least five on this page. Trovatore is right though that the sentence is actually slightly inaccurate. I will fix the inaccuracy. Banedon (talk) 08:48, 28 February 2018 (UTC)

The subject is already mentioned in the section "The Universe versus the observable universe", where it says

However, due to Hubble's law, regions sufficiently distant from the Earth are expanding away from it faster than the speed of light....

It seems to me that the content is already present in the article (before Banedon's edits). The question is what (if anything) should be said in the lead section.

Lead sections are supposed to summarize the body, and this material is reasonably important, so I do think something should probably appear in the lead. However the lead of this article is already substantially too long, so the wording should be chosen carefully, getting the most important facts across with no wasted words. (Also the lead needs to be trimmed down in general.)

So I'm not sure Banedon's addition:

It is also possible some extraterrestrial objects are moving away from Earth faster than the speed of light, in which case will eventually not be detectable from Earth

is really ideal. It is not just "possible"; it is apparently established that some objects are receding faster than light. And it does not seem to be clear whether that is the dividing line to whether they will be eventually detectable from Earth, as that depends on whether the expansion will accelerate. Also the meaning of "move away faster than light" is not completely specified. And I'm not sure this is the appropriate place to say this, anyway, as the text up to this point is talking about light emitted in the past that is reaching us now, whereas this text is talking about light emitted "now" ("now" meaning at the same cosmological age, I guess) and whether it could reach us in the future.

So something should probably be said, but I'm not sure it's this. --Trovatore (talk) 09:09, 28 February 2018 (UTC)

This is a misconception that any object can move faster than light. Any apparent speed as measured by its redshift will always remain subluminal. The expansion of metrics does not equal any physical speed. The effect that Banedon was trying to insert into the lead again has nothing to do with the superluminal expansion of the metrics. After all in a flat matter dominated universe the metric expands exactly 3 times faster than speed of light but any object will remain observable indefinitely. The effect is more subtle than that. It is related to the exponential future expansion of the metrics. This is actually described in the first two paragraphs of the first section. I also should say that it is not good to use some blog-like source which looks like a salad of words to me. Ruslik_Zero 18:53, 28 February 2018 (UTC)

Ah, so that is the point you're sticking on. I thought it was but you hadn't made it completely explicit.

The disputed text doesn't say "moving faster than light". It says "moving away faster than light". No object can move faster than light, in an inertial reference frame. But there is no inertial reference frame that includes both us and the distant galaxies. The proper distance between us and those galaxies (at the same comoving time) is increasing faster than the speed of light. I think that situation is reasonably summarized by saying they're moving away faster than the speed of light. However there is definitely potential for confusion if not carefully explained. --Trovatore (talk) 19:11, 28 February 2018 (UTC)

Ok, I updated the lead and removed some bloat from it. Again the limited temporal visibility of all objects is related to the exponential expansion of the metrics not just to the superluminal expansion. This is basically sort of an event horizon (see De_Sitter_space). Ruslik_Zero 19:29, 28 February 2018 (UTC)

Observable universe misconception in the article

Throughout the article a misconception is regularly pointed out about the observable and not observable Universe in the sense that we do not see the entire Universe.

We always see the entire Universe, but not the complete history of each region as the information from that region did not or cannot reach the Earth.

However, even those regions whose history we cannot see are present. We see them at least as they were at the time of the Big Bang, as cosmic background radiation. In that sense, no galaxy ever disappears just because the Universe is expanding, for example.

This misconception is very common, but it is still a misconception nevertheless.

95.34.4.122 (talk) 16:07, 14 July 2018 (UTC)

Farthest objects

In the introductory section I find "Because no signals can travel faster than light, any object farther away from us than light could travel in the age of the universe (estimated as of 2015 around 13.799±0.021 billion years[5]) simply cannot be detected, as they have not reached us yet." If it took 13 billion years for light to reach us from some object, that object would have to be 13 billion light years away at the moment it emitted that light, which means the universe had a radius of at least 13 billion light years 13 billion years ago. Do we know that? Is there another explanation of why it took so long for the light to reach us? Some explanation seems to be needed. Marty39 (talk) 16:59, 7 August 2018 (UTC)

Misconceptions on its size

I may be missing something, but the section on misconceptions on its size seems to contradict the first sentence of the article.

"The observable universe is a spherical region of the Universe comprising all matter that can be observed from Earth at the present time, because electromagnetic radiation from these objects has had time to reach Earth since the beginning of the cosmological expansion."

This section seems to be discussing how big the observable universe actually is today, as compared to how big the it appears to be is as observed from earth at the the present time. To me, this seems rather irrelevant since the most distant signal we can receive appears to be at a distance of 13.8 Billion Ly. This means that distant objects affect us as they were at the time when the light we see was emitted.

The reason I have been saying appears is because: 1. Yes we were closer when the light we see now was emitted and 2. Yes we are farther away now;

but because of the the universe expanding in-front of the moving signals they appear at a distance greater than the 1 and less than 2.

The reason I have been saying signals is because light isn't the only thing that can reach us from distant objects.

I am not a subject matter expert so I won't change anything. But also if I am misunderstanding something please expand the article by explaining why discussing comoving distance of an object we cannot presently observe is relevant in an article about the observable universe.

Thank You. — Preceding unsigned comment added by 64.98.246.9 (talk) 14:18, 5 November 2018 (UTC)

Use of the term 'exponential'

I would like to question the use of the term 'exponential' in the sentence 'Since the expansion of the universe is known to accelerate and will become exponential in the future...'. If the expansion is exponential, it is happening now as well, albeit a bit more slowly. Exponential growth can start slowly. That is, unless there's something known about the expansion being fundamentally different now as opposed to the future.

Jim Kruse (talk) 19:33, 26 October 2018 (UTC)

The expansion is exactly exponential when dark energy is 100% of the energy density. Right now dark energy is 70%, but in the future it will be >99% and the expansion will be extremely close to exponential. More precisely, exponential growth is characterized by d(log(size))/dt being constant; right now d(log(size))/dt is decreasing and will asymptotically approach a value about 20% smaller than its current value. We live in a strange cosmic era where expansion is accelerating but not yet nearly-exponential. Patallurgist (talk) 19:33, 6 November 2018 (UTC)

Would it be more correct to say the the expansion is '...known to be accelerating', rather than '...known to accelerate'? The current wording seems to suggest surges & lapses in rate of expansion, which I suspect is NOT what is understood to be happening? UnderEducatedGeezer (talk) 07:32, 13 November 2018 (UTC)

Cosmic horizon

The article says "For example, the current distance to this horizon is about 16 billion light-years, meaning that a signal from an event happening at present can eventually reach the Earth in the future if the event is less than 16 billion light-years away, but the signal will never reach the Earth if the event is more than 16 billion light-years away." and cites Misconceptions about the Big Bang (here's the PDF link). But the very article it cites says " Imagine a light beam that is farther than the Hubble distance of 14 billion light-years and trying to travel in our direction. It is moving toward us at the speed of light with respect to its local space, but its local space is receding from us faster than the speed of light. Although the light beam is traveling toward us at the maximum speed possible, it cannot keep up with the stretching of space. It is a bit like a child trying to run the wrong way on a moving sidewalk. Photons at the Hubble distance are like the Red Queen and Alice, running as fast as they can just to stay in the same place. One might conclude that the light beyond the Hubble distance would never reach us and that its source would be forever undetectable. But the Hubble distance is not fixed, because the Hubble constant, on which it depends, changes with time. In particular, the constant is proportional to the rate of increase in the distance between two galaxies, divided by that distance. (Any two galaxies can be used for this calculation.) In models of the universe that fit the observational data, the denominator increases faster than the numerator, so the Hubble constant decreases. In this way, the Hubble distance gets larger. As it does, light that was initially just outside the Hubble distance and receding from us can come within the Hubble distance. The photons then find themselves in a region of space that is receding slower than the speed of light. Thereafter they can approach us." The source seems to contradict what is written in the article. Am I wrong? --Mati Roy (talk) 12:36, 16 February 2019 (UTC)

If I am understanding you correctly.... I don't believe the Hubble distance grows fast enough to make a previously un-observable photon observable at some point in the future. For example if a photon is sitting right on that limit "not moving" the intervening space expands at an accelerating rate such that it matches the expansion only for a instant then starts falling away. — Preceding unsigned comment added by 64.98.246.9 (talk) 19:48, 26 February 2019 (UTC)

The following is consistent with the 16 billion light year (16 Gly) claim, the quoted article, and (hopefully) this Wiki article: A photon that is 14 Gly away (at the Hubble distance) and aimed towards us maintains its distance, not getting any closer. As the Hubble distance grows, that photon will eventually be inside the Hubble distance and will be able to make progress towards us (getting closer and eventually reaching us).

In contrast, a photon that is 16 Gly away (at the cosmic horizon) and aimed towards us is currently becoming more distant (losing the Red Queen's race). The Hubble distance will increase, eventually surpassing 16 Gly, and will chase after but never quite catch this second photon, so it will never begin to approach us (let alone reach us).

Now consider a third photon, currently 15 Gly away, between the Hubble distance and the cosmic horizon. This photon is currently becoming more distant, because it is beyond the Hubble distance, but the Hubble distance will eventually pass it; after that point, this third photon will get closer to us and eventually reach us.

To summarize, the Hubble distance (14 Gly) is the point beyond which a photon aimed at us cannot *currently* get any closer to us. The cosmic horizon (16 Gly) is the point beyond which a photon will *never* be able to get closer to us or reach us. In between, photons will get further from us for some period of time, before eventually approaching and reaching us.

(I'm not sure what "previously un-observable" means -- either an event can be seen by us, or it can't. There is always some time-delay involved in seeing anything, because light has to cross some distance.) Patallurgist (talk) 07:33, 27 February 2019 (UTC)

Regarding the very first line

The article states in the very first line that the observable universe is what we the humans see from Earth. You do not need to be on Earth to see images of the universe. We have taken a lot of images of the universe from the moon, Mars and all space stations. And by the way, we were never able to see our own planet, but only when we left it. — Preceding unsigned comment added by 185.156.63.5 (talk) 11:26, 9 March 2019 (UTC)

Very good point, thank you. Space-based telescopes and exploratory probes have enlarged the view of the universe, and I'll add that in. Nice work. Randy Kryn (talk) 11:32, 9 March 2019 (UTC)

Clarification about mass

I might be not understanding something here but the value of the mass of the ordinary matter of the observable universe doesn't seem consistent across the article. The table states "Mass (ordinary matter): 4.5e51 kg" while it is mentioned in the text that the mass is 1.5e53 kg - which are almost two orders of magnitude apart... Both values appear to based on similar reasoning too. Also note that in the "Schwarzschild radius" wikipedia article the total mass is given as 8.8e52 kg, and surely the ordinary mass can't outweigh the total mass?

Could someone please explain this disparity? — Preceding unsigned comment added by 109.64.57.38 (talk) 18:15, 27 April 2019 (UTC)

Curvature of spacetime

To explain my revert just now: There is a difference between spatial curvature (usually denoted k) and spacetime curvature (usually denoted R).

Friedmann–Lemaître–Robertson–Walker_metric#Curvature gives the formula relating these; spacetime curvature R has a contribution 6k/a(t)^2 from spatial curvature k, but it also has two terms that come just from the spatial expansion over time, a(t).

Spacetime curvature is coordinate-independent. Spatial curvature depends on the "slicing" (into constant-time slices) imposed by coordinates; for example de Sitter is spatially flat in one slicing and spatially curved in another, but always has uniform positive spacetime curvature.Patallurgist (talk) 22:02, 13 October 2019 (UTC)

Number of galaxies

The article says "There are at least 2 trillion galaxies in the observable universe."

However, the linked article in the NYT says that this number was derived mathematically. Quote from NYT "Because not even the Hubble or large Earth-based telescopes can see the oldest, faintest galaxies, they also did some mathematical work to come up with two trillion."

Can somebody with more knowledge than me check this and remove the word "observable" if necessary?

Wikipediun2000 (talk) 08:26, 25 January 2020 (UTC)

It's fine. Observable Universe means the parts of the Universe that are amenable to observation in principle, given the finite velocity of light. A figure given for the total number of galaxies in the observable Universe will be an estimate based on assumptions about the number of galaxies per unit volume of space, how that number may change over time because of merging together of galaxies, and the volume of the observable Universe. TowardsTheLight (talk) 10:37, 25 January 2020 (UTC)

Actually reading further (paragraph two of the summary) they do state "Integrating the number densities phgrT, we calculate that there are $({2.0}_{-0.6}^{+0.7})\times {10}^{12}$ galaxies in the universe up to z = 8, which in principle could be observed." so my original comment can be ignored. Wikipediun2000 (talk) 18:52, 25 January 2020 (UTC)

Contradicting information/ "End of Greatness"

The "End of Greatness" section states that there is no observed structure at scales greater than ~300 million light years, yet elsewhere in the article structures as large as 2-4 billion light years are listed. Is the "End of Greatness" claim based on obsolete data..? Firejuggler86 (talk) 05:24, 21 March 2020 (UTC)

The Hubble volume is also called the "Minkowskian subluminal future causal cone mapped on 3D space" or just "causal cone"

Add more data and make comparisons.

I you don't like something I wrote methodically correct me. — Preceding unsigned comment added by 2A02:587:4110:D3CB:214B:C286:7FD5:63E (talk) 06:53, 11 July 2020 (UTC)

Please sign all your talk page messages with four tildes (~~~~) — See Help:Using talk pages. Thanks.

For the time being I have undone the edit per wp:unsourced. I did the same here. Please find and include a wp:reliable source for such content. Thanks and cheers. - DVdm (talk) 08:27, 12 July 2020 (UTC)

The universe versus the observable universe

This statement is posted as a fact when in fact it is unprovable. It is equally possible that the entire universe is infinite and space-time is everywhere. There is no expansion other than the expansion of the observable universe due to time, the speed of light and the motion of the solar system in space. The speed of light is not faster outside the observable universe, it is same speed everywhere.

However, owing to Hubble's law, regions sufficiently distant from the Earth are expanding away from it faster than the speed of light (special relativity prevents nearby objects in the same local region from moving faster than the speed of light with respect to each other, but there is no such constraint for distant objects when the space between them is expanding; see uses of the proper distance for a discussion) and furthermore the expansion rate appears to be accelerating owing to dark energy.

--Ametrica (talk) 03:16, 2 October 2020 (UTC)

Could you be more explicit about which "statement" you are talking about? I don't see anything in the section you mentioned that contradicts the possibility that "the entire universe is infinite and space-time is everywhere". (In fact I don't really follow what it would mean for spacetime not to be everywhere, but one thing at a time.) --Trovatore (talk) 06:12, 2 October 2020 (UTC)

This statement for example at the beginning of the 4-th paragraph:

As the universe's expansion is accelerating, all currently observable.....

If it is referring to the observable universe and not the entire universe, it should state observable in the sentence. Of course if the observable universe's is accelerating, then the expansion would then have to be travelling faster than light. I'm also sure red and blue shifts don't universally indicate something is moving away or towards our observation point at a high speed, it can also be an effect of us moving as well, since our solar system is also in motion around the milky way at 230 km/s and who knows what speed the milky way is moving that will also affect the shifts in the wavelengths of light. Also consider that the father out we look, the farther back in time we are looking. We are seeing things the way they were, not the way they are.

Also, if we can see to the edge of the observable universe, we should see light as it was at creation, not as it is now. Yes? No?

--Ametrica (talk) 17:45, 13 October 2020 (UTC)