Science desk
< August 24	<< Jul \| August \| Sep >>	August 26 >

Welcome to the Wikipedia Science Reference Desk Archives
The page you are currently viewing is an archive page. While you can leave answers for any questions shown below, please ask new questions on one of the current reference desk pages.

August 25

Total value of precious metals

What's the total value of all silver, gold, and platinum in Earth (in USD)? --75.10.48.39 (talk) 00:29, 25 August 2011 (UTC)[reply]

According to this place, there were an estimated 800 million ounces of silver as of 2009 or 2008 at let's say $40 an ounce, so about $32 billion. Gold reserve says there were 165,000 tonnes of gold mined in total as of 2009; at the current price of roughly $1750, that's $9.2 trillion, again according to that article. I'm having trouble finding the figures for platinum. Clarityfiend (talk) 03:03, 25 August 2011 (UTC)[reply]

Something seems very wrong with those silver figures. If there are only 800 million ounce divided by some 7 billion people, then there's just over a tenth of an ounce for every person on Earth. Silver's not that rare. Many people have silverware sets, for example. I think what you have there is the yearly production of silver. Here's a source which claims the total silver supply is about 43 billion ounces: [1]. StuRat (talk) 06:18, 25 August 2011 (UTC)[reply]

Our article on silver mining gives an estimate for 2010 total silver production as 735 million ounces, so yes - the 800 million ounce figure is an annual production number rather than a grand total. TenOfAllTrades(talk) 12:14, 25 August 2011 (UTC)[reply]

This is the mined metals. Based on the earlier question #Total amount of platinum by the same IP, the question may include the unmined. There is no meaningful answer to the total value in that case. If the current price was multiplied by the total amount then the result would be a meaningless number far greater than the total assets of all humans. But prices in a competitive market depend on supply and demand. If we imagine that all the metals were available then value would depend on who had possession of what. If all people had an equal share then there would be no reason for anybody to buy. If one company controlled everything then they couldn't get people to carry it all away if they gave it away for free, but they could earn a fortune by setting a significant price and only sell a small part. PrimeHunter (talk) 03:45, 25 August 2011 (UTC)[reply]

http://www.gold-eagle.com/editorials_05/zurbuchen011506.html gives an estimate based on multiple sources and comes up with approximately 45 billion toz of silver mined in human history, at a rough value of $1.8 trillion. Combined with $9.2 trillion for gold and about $325 billion for platinum (based on 180 million toz I referenced in the earlier question by this OP), the total is $11.3 trillion. Googlemeister (talk) 14:10, 25 August 2011 (UTC)[reply]

It should be noted that these dollar values are just for the raw metal. Something like Tutankhamun's gold mask or the 1794 silver Flowing Hair dollar coin would be worth far more. Clarityfiend (talk) 18:47, 25 August 2011 (UTC)[reply]

Perhaps, but I doubt that the metals that happen to be in antiquities would make much of an impact when we are talking about an amount this size. I mean 11 trillion is roughly on the order of the US national debt. Googlemeister (talk) 20:23, 25 August 2011 (UTC)[reply]

... and, of course, the figures above are all for the metals that have been recovered (mined) from Earth. The amount still there is at least thousands of times the largest figure mentioned above, possibly millions of times, but if there were an easy way to obtain the metals then they would not be valuable (as explained by PrimeHunter above). Dbfirs 08:26, 27 August 2011 (UTC)[reply]

And it would be impossible to price those since if they were available, the current price for these metals would crash. Googlemeister (talk) 14:05, 29 August 2011 (UTC)[reply]

What makes the difference in weight gain/loss?

I did some figuring of nutritional information of what I ate today. It came out roughly like this:

1800 Calories
640 Calories from Fat
71 Fat - Total (g)
18 Saturated Fat (g)
2 Trans Fat (g)
145 Cholesterol (mg)
5050 Sodium (mg)
243 Total Carbohydrates (g)
9 Dietary Fiber (g)
88 Sugars (g)
56 Proteins (g)

Given that this is below the 2000 calories that's the daily value, roughly at the upper end of daily value of fat calories, and spot on for protein (according to this), but given that it is off the charts for carbohydrates and cholesterol, if I kept up this type of diet (assuming that there was no deficiency in vitamins or minerals) for an extended period of time, 1. would I be more likely to gain weight or lose weight, and 2. why? Note that I don't necessarily want the response specific to me...I'd be happy if it were more along the lines of if a person went an extended period of time on a diet like this and assuming they had no vitamin and mineral deficiencies (say if they took a multivitamin), would they be more likely to gain or lose weight and why? Thanks, Ks0stm ^(T•C•G) 01:15, 25 August 2011 (UTC)[reply]

I am not going to be popular with anyone else for saying this, but this is clearly the sort of question you need to ask your doctor or similar professional about. We in no way know your medical history, current physical condition, or anything else about you, and so there is no way to tell if the diet you outline above is appropriate for you, or what its effects would be on you. If it isn't you you are asking about, it is impossible to answer for ANY individual, and so saying "you will lose weight" or "you will not" is irresponsible for anyone to answer. Any individual who wishes to know the correct answer should have the diet, and their own condition, looked at by a professional and not assholes like me. --Jayron32 02:38, 25 August 2011 (UTC)[reply]

It is totally impossible to give a generic answer. The answer depends on whether you are male or female, how much you presently weigh, how old you are, and how active you are. Some people can eat 1500 calories per day and gain weight, others can eat 4000 calories per day and lose weight. Those are extremes, but with zero information there is no way to rule out that those extremes apply to you. Looie496 (talk) 02:40, 25 August 2011 (UTC)[reply]

243 g/day of carbohydrates isn't really "off the charts". The 130 g/day listed for carbohydrates in the DRI is a recommended minimum, not a maximum. Protein and carbohydrates both have about 4 cal/g, so if someone were to only eat the 130 g/day of carbohydrates and 56 g/day of protein listed in the DRI for an adult male, and limit fat to no more than 35% of calories as the DRI recommends, the person would be eating at most 1145 cal/day, which wouldn't be sufficient for most adults to maintain a healthy weight.

Under your assumption of "if I kept up this type of diet", the only nutritional information you listed above that's pertinent to your question as to whether you'd be likely to gain or lose weight if you ate similarly every day is pretty much just the calories. If you consume more calories each day that you expend, you'll gain weight, and if you consume fewer calories each day than you expend, you'll lose weight. (Saying "digest" rather than "consume" in that sentence would make it more accurate, but that's pretty much a minor quibble.) Diet fads come and go as to whether emphasizing high protein, low fat or low carbohydrates sells the most diet books, but the scientific reality is that whether you'll lose weight or not basically just depends on how many calories you eat, not how those calories are divided among the three macronutrients.[2] This is basically a statement of conservation of energy, since a calorie is a unit of energy. To a very high degree of accuracy, within any given period of time, energy that enters your body, which is mainly in the form of chemical energy stored in food, minus the energy that leaves your body in one form or another, must equal the difference in how much energy is stored within your body. Your body creates fat as a way of storing excess energy.

As pointed out above, you have given insufficient information above to determine if you will gain or lose weight by eating 1800 cal/day. The missing information is how many calories you expend each day, which depends primarily on your gender, age, weight, height and activity level. The Wikipedia article discussing this matter is Energy balance (biology), which refers you to this rather rigorous paper on the topic[3] as well as the commonly used and fairly easy-to-use Harris-Benedict equation. There are also numerous calculators on the internet for calculating your energy requirements, such as this one[4], but those unfortunately tend not to explain whether they're using Harris-Benedict or what. Red Act (talk) 04:27, 25 August 2011 (UTC)[reply]

Actually, you gave exactly the information I was looking for. I didn't really need it related to me...I was just using what I ate today as an example of the type of situation I was inquiring about, and you gave me all I needed to answer my questions in your second paragraph. Sorry my question was so strangely asked...it was a very hard question for me to phrase. Thanks for taking the time to answer. Ks0stm ^(T•C•G) 05:20, 25 August 2011 (UTC)[reply]

Just out of curiosity, did you have any brazil nuts? 208.54.86.157 (talk) 07:35, 25 August 2011 (UTC)[reply]

And I can tell you that I would gain weight rapidly on such a diet. The carbs in particular seem to pack on the weight for me. That much sodium would also make by blood pressure spike. StuRat (talk) 06:04, 25 August 2011 (UTC)[reply]

I see you are getting one third (33%) of your daily calorific intake from fats. This can't be healthy, whatever diet you're on! I suspect you will lose weight a lot quicker if you bring this down to around 20% as recommended by this site.--TammyMoet (talk) 14:54, 25 August 2011 (UTC)[reply]

You'll lose weight a lot faster by exercising instead of dieting. Muscles always burn energy, even if they aren't used. So, the more muscles you build, the faster you can lose weight. Count Iblis (talk) 00:18, 26 August 2011 (UTC)[reply]

But then you'll be hungrier and eat more. There's no amount of exercise which can't be undone by a bad enough diet. So, diet is definitely important, and exercise is nice, too. StuRat (talk) 03:27, 26 August 2011 (UTC)[reply]

I agree, but if you do a lot of exercise, you won't be able to overeat easily. E.g. I run for about 35 minutes at a pretty fast pace 5 times per week, and I eat quite a lot (more than 3000 Kcal per day). But my weight is very stable, the weekly average has been constant to within 1 kg for over 3 years now (between 62 and 63 kg). To gain weight, I would have to eat a lot more, but I'm already eating huge amounts of foods all day long. The fact that my weight is stable over long periods of time, means that my metabolic rate is compensating for excess energy intake or excess energy use (eating just 7.3 kcal more per day would lead to 1 kg weight gain over 3 years if te metabolic rate would not increase). So, to gain weight I would have to eat so much more that the feedback mechanism that keeps by weight constant would fail, and I don't think I'm near the boundary of that region.

According to a BBC Horizon documentary that I saw a few years ago, people who have a stable weight, only gain 5 kg to 10 kg of weight if they double their energy intake. So, it could be that I would need to eat 6000 kcal per day to get to 70 kg. But I'm quite sure that if I were to stop exercising and wait until my fitness levels dropped, then I could gain weight far more easily. I would become a couch potato who only needs 2000 Kcal. And eating a bit more would already lead to weight increase, because with physical inactivity and less mucle tissue, the feedback mechanism that keep the weight constant become less effective. Count Iblis (talk) 04:09, 26 August 2011 (UTC)[reply]

Molar concentration

Hi there. I hope this will be simple for someone with a chem background.. This is NOT a homework question, it's in relation to a discussion I'm having regarding this document. It states concentrations of sodium chlorite (molar mass 90g/mol) of more than 100 .mu.mol/l can be cytotoxic. Up until now I've only come across concentrations in ppm or %. I'm trying to sub my values into the example but I'm getting screwed up with the powers.. 100 mu.mol/L is 0.0001mol/L(I think) * 1L * 90g/mol = 0.009g .. 0.009g in 1 L of water? That doesn't sound right.. Vespine (talk) 01:20, 25 August 2011 (UTC)[reply]

No, your math is fine. 100 micromolar = .000100 mol/L *90 g/mol = .00900 g/L. In terms of % by mass, you can convert the denominator to grams (at this low concentration, the density should be the density of water, so 1L = 1000 grams), this would be .009/1000 = .000009 or .0009% or 9 ppm. --Jayron32 02:32, 25 August 2011 (UTC)[reply]

That's great, thanks for that. Vespine (talk) 03:41, 25 August 2011 (UTC)[reply]

Resolved

Nuts in Chinese shops

I've noticed that peanuts, pistachios, and sunflower seeds taste differently when purchased from Chinatown shops. They're obviously not raw, but also differ vastly from anything from American retailers. What's the process they use? I don't tend to buy anything but plain nuts in shells. 66.108.223.179 (talk) 03:43, 25 August 2011 (UTC)[reply]

Probably a difference in seasoning? --Jayron32 04:01, 25 August 2011 (UTC)[reply]

Maybe monosodium glutamate. Sean.hoyland - talk 05:40, 25 August 2011 (UTC)[reply]

I buy sunflower seeds in a chinese shop which are called "stewed", these are the ones. They're still in the shell and you couldn't tell they've ever been wet, I don't even know if "stewed" is really the process used to prepare them, or if it's a "lost in translation" kind of thing, but I like them.. Vespine (talk) 06:49, 25 August 2011 (UTC)[reply]

Perhaps they are boiled. I've had boiled peanuts, and they are very different, more like beans in texture. StuRat (talk) 06:59, 25 August 2011 (UTC)[reply]

But aren't they all boiled? :/ I thought that was the only way you can cook peanuts in their shells - boil them in water and salt (or some other seasoning) and either eat them outright (which I prefer) or dry them again. And yeah, it may be a difference in the kind of seasoning used when boiling them, you can add anything from beer to sugar to chilli in the water in the process. I wonder if microwaving them works... -- Obsidi♠n Soul 10:17, 25 August 2011 (UTC)[reply]

I believe most peanuts are blanched or roasted, which leaves them crispier than prolonged boiling. StuRat (talk) 03:32, 26 August 2011 (UTC)[reply]

We used to grow peanuts, so I'm pretty familiar with boiled peanuts (and still prefer them to any dried peanuts), not so much with roasted though. Raw peanuts are anything but crispy. :P They taste vaguely vegetative and rubbery for one, and remain so even after drying (imagine raw potatoes), parents would admonish us not to eat too much raw peanuts or we'd get a stomachache. Blanching might work for unshelled peanuts but not for shelled ones. As it would not penetrate the thick peanut shells. Apparently, though, they don't boil roasted peanuts in a shell, they soak it in brine then roast it.-- Obsidi♠n Soul 04:00, 26 August 2011 (UTC)[reply]

So how do boiled peanuts taste ? StuRat (talk) 04:13, 26 August 2011 (UTC)[reply]

LOL, that's hard to describe. The texture is soft like you mentioned, like beans, and it's salty of course. The remaining water helps the flavor, like soup.-- Obsidi♠n Soul 04:26, 26 August 2011 (UTC)[reply]

OK. Mine were boiled without salt, and were definitely quite different from peanuts produced by the usual preparation methods. StuRat (talk) 04:30, 26 August 2011 (UTC)[reply]

The real answer may be quite simple. Most food crops grown today are verities that have been developed so that the whole field ripens at once and give high yields. Decades of modern plant breeding practice has resulted in the produce losing their original flavours - leaving them tasting bland. This is very evident when comparing the older non-commercial verities of vegetables grown by gardeners. Your China town probably imports direct from China like my China town in London. Look at the packaging as to their place of origin. You may well be tasting the original flavours.--Aspro (talk) 12:39, 25 August 2011 (UTC)[reply]

That it could be. I've visited different Chinese cities, and their nuts seem to taste the same. It's not the seasoning or blanching. 66.108.223.179 (talk) 02:12, 28 August 2011 (UTC)[reply]

Heirloom plant seems to refer to a "…cultivar that was commonly grown during earlier periods in human history…" This may be what the above posts are referencing. I know little about this but I recalled encountering the term "Heirloom plant". Bus stop (talk) 03:07, 28 August 2011 (UTC)[reply]

Facial resemblance between parents & children

I have noticed that the daughter of a mother would look almost identical to what her mother looked like during her younger days. Is this the general rule? Is there a similar rule for other family members (fathers, sons)? — Preceding unsigned comment added by 98.116.187.230 (talk) 04:28, 25 August 2011 (UTC)[reply]

Umm, heredity?? I don't think it's really a general "rule" at all about being "almost identical". You probably just notice it more when the effect is strong, and don't notice it when the effect is weak. You probably have 25% chance of looking more like your father, 25% looking more like your mother, and 50% chance looking more in between.. I look quite different to my brother, there's only 16 months age difference between us, I look quite like my father when he was younger, my brother looks like more like my mum's father when he was younger. Vespine (talk) 05:01, 25 August 2011 (UTC)[reply]

I'm not sure Vespine's numbers should be strictly understood; the complex melange of genetics that goes into a persons appearence can't be simplified that well, but the spirit is true: people may look more like either parent, may look like a mix of both, or may not resemble either that closely. It isn't necessarily that a daughter will always look more like her mother; she may look like more like her father, and likewise for sons and mothers. My brother favors my mother very strongly (same hair, facial bone structure) in some ways, and my father (slight build) in others. I am the exact opposite: I facially resemble my father, but physically more resemble my mother's family. But that doesn't mean that my family is necessarily typical; if there is a typical. People will, on average, tend to resemble one or both of their biological parents, but it isn't a 100% guarantee sort of thing. --Jayron32 05:07, 25 August 2011 (UTC)[reply]

Sorry yes, I'm not a geneticist or anything, I was just making a pretty wild generalization. I completely agree with Jayron's above assessment. Vespine (talk) 05:32, 25 August 2011 (UTC)[reply]

I'd expect this to be something that is very prone to confirmation bias as Vespine suggests above given that our brains seem to be hard wired to do facial matching. Sean.hoyland - talk 06:08, 25 August 2011 (UTC)[reply]

Yes to confirmation bias. In my own case, I'm looking more like my mother as I get older: when I was a child I looked like my paternal grandmother. However, I look almost identical to my mother's grandmother, whose image I have in a picture nearly 100 years old! I'd be interested to know if the OP, to confirm his case or otherwise, has tried to guess mothers from daughters and vice versa! --TammyMoet (talk) 12:29, 25 August 2011 (UTC)[reply]

I recall reading an article in Discover magazine some years ago that research had indicated that infants tend to look more like the father than the mother. The basic test was having people match up babies with fathers and babies with mothers. They went on to suggest that this was an evolutionary adaptation to reduce infanticide, though that always seemed like a tenuous connection to me. Surely it would be better to leave things ambiguous so that "dad" doesn't see anything in the face of the child. See, for example, our article on concealed ovulation where keeping the potential fathers guessing is explained as being adaptive. Matt Deres (talk) 10:38, 26 August 2011 (UTC)[reply]

Cockroaches!

Do cockroaches themselves create (or worsen?) unsanitary conditions, or are they simply a highly visible indicator that an environment itself is unsanitary? For example, we have two kitchens of equal dirtiness. Kitchen A is our control. We introduce a cockroach population into Kitchen B. Does the presence of the cockroaches themselves actually worsen the situation? The cockroach article's section about Roles as Pests pretty much just lists odor as a real, identifiable problem.

This series of thoughts occurred to me yesterday when my wife spotted a cockroach in our kitchen and insisted I head out to the store to buy some baited traps to kill them. I was trying to decide if removing the cockroaches actually solved (or at all helped) the real problem - the (evidently) dirty kitchen. The Masked Booby (talk) 04:44, 25 August 2011 (UTC)[reply]

Anything that crawls around in filth spreads filth. And they can carry dangerous microbes. Beeblebrox (talk) 04:58, 25 August 2011 (UTC)[reply]

(edit conflict) It's probably a dynamic situation; i.e. the filth attracts the roaches which themselves contribute to the filth. After all, roaches both shit and die; known sources of filth of their own rights. Coackroach turds and corpses sound like a clear increase of filth to me. --Jayron32 05:00, 25 August 2011 (UTC)[reply]

turds and corpses. I'm not sure why, but this made me lol.. Best science ref desk comment for a while.. Vespine (talk) 05:37, 25 August 2011 (UTC)[reply]

Cleaning the kitchen, getting rid of clutter, and making sure they can't access food sources (the food you eat and rubbish) will probably help more. And no of course they don't cause the dirt, they can actually be found even in the cleanest of homes, as long as they have ready access to food and water. Cockroaches are also not dirty per se, but they can carry dangerous bacteria to your food especially if their immediate environment is already dirty in the first place. So yeah, clean the room in addition to placing baits.-- Obsidi♠n Soul 05:41, 25 August 2011 (UTC)[reply]

Note that keeping food away from them might not be as easy as it sounds. If they can burrow into cardboard boxes, for example, that means all food in boxes must be moved somewhere inaccessible, like in the fridge, in sealed glass, metal, or plastic containers, etc.

Garbage disposals can also hold small bits of food that roaches might like, and you'll need a tightly sealed trash can or to flush anything they might find edible. However, when desperate, they might even eat things like paper and hair. StuRat (talk) 05:55, 25 August 2011 (UTC)[reply]

It is silly and specist to posit that cockroaches "love filth" and want to crawl around in "filth." The Mama roach will consume or will lay her eggs near any source of food, which might include toothpaste on a toothbrush, sugar cookies left for Santa by the Christmas tree, or any fresh food we would introduce into our mouths or serve to our families in the belief it was completely clean and wholesome. Edison (talk) 01:35, 26 August 2011 (UTC)[reply]

Where did anybody claim that 'cockroaches "love filth" and want to crawl around in "filth.' [my added emphases]? I agree that such antropomorphic motivatory terms would be inappropriate, but I don't see any suggestions of them above. "Filth" is merely a descriptive term, useful in context for its implications for us, of a subset of the substances that cockroaches regard as food or environment, but which we don't. {The poster formerly known as 87.81.2301.95} 90.197.66.127 (talk) 05:58, 26 August 2011 (UTC)[reply]

"Anything that crawls around in filth.." and "The filth attracts the roaches"implies a preference for "filth" over good wholesome food. The choice of words implies the roach would ignore Mom's fresh apple pie or the sandwich you just bought at the deli and go for the spilled milk, stale crumbs, cat throwup, and garbage. Edison (talk) 18:21, 26 August 2011 (UTC)[reply]

What is the EXACT name of the green birds on the photo?

On this photo taken in Barcelona, Spain:

File:Doves in Barcelona.jpg

two kinds of city birds are seen:

— city doves, or Columba livia

— something like monk parakeets from True parrots

However the Myiopsitta monachus are placed into the Category:Birds of Uruguay, and the article explicitly says that Myiopsitta monachus is a group endemic to South America (while Barcelona is in Europe).

I would appreciate anybody to help to name these „parrots” correctly, which is necessary for the further renaming of the file and its correct categorization. Thanks in advance, Cherurbino (talk) 16:10, 25 August 2011 (UTC)[reply]

The article also says that they have been introduced widely in other areas, and specifically: "They are a common sight in Barcelona parks, often as numerous as pigeons." Ghmyrtle (talk) 16:21, 25 August 2011 (UTC)[reply]

I also noted an article List of non-native birds of Great Britain where the introduced birds are listed for Britain. However they list another breed, a Ring necked Parakeets — while those in Barcelona look slightly different. Cherurbino (talk) 16:23, 25 August 2011 (UTC)[reply]

The birds in Britain are a different species, Rose-ringed Parakeet (Psittacula krameri). Ghmyrtle (talk) 16:28, 25 August 2011 (UTC)[reply]

Thank you, Ghmyrtle — I think it's time to take a decision: I shall rely upon the article you've mentioned. So, let them be Monk Parakeets! Cherurbino (talk) 16:27, 25 August 2011 (UTC)[reply]

space

Would it be fair to say that space is expanding because space is what a Black hole ejects every time it consumes a bit of matter? --DeeperQA (talk) 21:16, 25 August 2011 (UTC)[reply]

No, I can't think of any way in which that sentence makes sense. Dragons flight (talk) 21:44, 25 August 2011 (UTC)[reply]

I think it's an intriguing idea. The 2D rubber sheet analogy will have more surface area when massive objects are depressing it. It's hard for me to think about this clearly now and I'll try again later. 76.254.20.205 (talk) 22:18, 25 August 2011 (UTC)[reply]

No, the OP's statement makes no physical sense. The universe expansion has nothing to do with black holes except for the fact that both are solutions of Einstein's equations, but they are completely independent solutions. Dauto (talk) 23:23, 25 August 2011 (UTC)[reply]

So then what is it that you (or Einstein) are saying that space is expanding in which does make physical sense? --DeeperQA (talk) 23:44, 25 August 2011 (UTC)[reply]

I'm not sure I understand the question. Space is not expanding in anything. It's just expanding - that is the distance between objects in space increases over time. That expansion is observed for distances in a cosmological scale such as the distance between us and far away galaxies. Dauto (talk) 23:56, 25 August 2011 (UTC)[reply]

Check out Metric expansion of space, which immediately explains that space is not expanding in anything. Staecker (talk) 00:26, 26 August 2011 (UTC)[reply]

It says "It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space. In other words, the universe is not expanding "into" anything outside of itself."

If I hear correctly what you and the article are saying then the Universe was not part of the Big Bang or effected by it but only space and time were effected by the Big Bang and the Universe existed prior to the Big Bang. --DeeperQA (talk) 05:01, 27 August 2011 (UTC)[reply]

I think I'd include matter as part of the Big Bang. If the Universe comprises more than space, time and matter, then what else is it? Dbfirs 08:06, 27 August 2011 (UTC)[reply]

Nobody knows why the big bang caused the metric expansion of space, or why it is still happening today, or what would reverse it. I'm not convinced it's indistinguishable from isotropic gravitational pull from outside the observable universe. 76.254.20.205 (talk) 07:09, 28 August 2011 (UTC)[reply]

There are several flaws with that theory. Firstly, it would violate the Copernican principle, since the pull could only be isotropic when viewed from one point in space. There is no single "observable universe", there is a different observable universe for each point. Secondly, there is no need for gravity to explain the expansion. It is expanding today because it was expanding yesterday and nothing has happened since then to stop it. A gravitational pull like the one you describe could potentially explain the acceleration we observe in the rate of expansion, but, as I say, it would violate the Copernican principle, so it isn't likely to be a very good explanation. Thirdly, if it is outside the observable universe then it can't have any effect on us. Gravity travels at the speed of light so the gravity from something outside the observable universe can't have reached us yet. That's the whole point of the concept of an observable universe. --Tango (talk) 13:18, 28 August 2011 (UTC)[reply]