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Support for this

What observations and axioms support this. Hackwrench 18:08, 1 November 2005 (UTC)[reply]

My understanding, and perhaps someone can correct/expand as needed, is that the cosmological principle is itself taken as an axiom, but there is experimental evidence as well. The best evidence for isotropy is the [cmb] which is isotropic to one part in 10,000. I think for homogeneity, we can look at other galaxies/clusters and see that the physics there appears to be the same, and galaxies on average are the same. Threepounds 06:54, 26 November 2005 (UTC)[reply]

This is true and there is more to it. The distribution of matter (galaxies and clusters) , dark matter, dark energy all seem homogeneous and isotropic. In fact, we can put some limit on the inhomogeneity and anisotropy of the universe on the large scales and we find out that it is agreeing very well with the cosmological principle. But we cannot ignore the tiny possibility that the cosmological principle is not 100% true, but we might never detect the slight variations. There is also a lot of theoretical arguments in favor of the cosmological principle. I don't want to go into the details but let's say that it is very hard to invent a successful model of our universe in which the cosmological principle is not respected. Basically, all work on cosmology (with FRW cosmological models at least) uses equations which were derived assuming the cosmological principle. So I'd say there is very good support in favor of the Cosmological Principle. (F.G. - McGill - Physics) —Preceding unsigned comment added by 74.56.206.181 (talk) 15:50, 18 November 2007

This vs. Anthropic

OK, I'll bite: Under what logic does the Cosmological principle disprove the Anthropic principle? Do sources exist for such claims? ----Jasonuhl 21:26, 13 October 2005 (UTC)[reply]

I agree that this needs to be expanded. I think more needs to be said or this part removed. It seems to be that the Cosmological principle is a statement about averages over cosmological distances (as the article indicates) while humanity exists on an entirely negligible length-scale. It is not obvious to me how the two should be compared or where a contradiction exists. Threepounds 06:42, 26 November 2005 (UTC)[reply]

I've removed this sentence. The link between this and the Anthropic principle seems tenuous at best, and a claim that one disproves the other without any attempt at explanation adds nothing but confusion. ----Jasonuhl 23:42, 21 February 2006 (UTC)[reply]

Milne's Formalization of the Cosmological Principle

I found a link in Timeline of cosmology that said Milne "formalized" this cosmological principle. While I agree that he did, I don't think that the gist of his argument is well presented here. For instance, Milne did not like the idea of a finite universe, nor did he like the idea of expanding space.

Milne says in Relativity, Gravitation, and World Structure "I am well aware that some mathematicians believe that such difficulties are at once swept awy if we use the concept of 'curved space.' I have examined such attempts at explanation with the greatest care, and I have found that in all cases the proposed explanations break down at some point. Two-dimensional analogies with hypothetical inhabitants on the surface of a sphere fail as soon as we recall that a survey of the astronomical universe is made by taking a photograph with a telescope and camera, and that, for a telescope of arbitrarily large light-gathering power, either the number of nebulae that can be counted is finite and therefore contains one faintest and so presumably most distant member, or it is infinite, in which case either the same nebula is photographed as an infinite number of separate spots or the total number of actual nebulae in existence is infinite. The latter will be our eventual conclusion. Here I am only concerned to argue that the phrase 'curvature of space' used in connexion with astronomical photographs merely involves a mist of mysticism. Such photographs can always be interpreted in flat space, and then the assumption of a finite number of density-maxima inevitably leads to some kind of accessible edge of the universe."

In this book, at least, Milne eventually concluded that the universe was not finite in terms of number of nebulae. He did not conclude that space was curved. JDoolin 16:27, 27 August 2006 (UTC)[reply]

I never read that book but I wanted to point out one thing. If you assume no big bang and an eternal universe, then you can assume that a large enough telescope could possibly measure an infinite number of nebulae. But if there is a big bang, then you cannot see further than the light travel-time since the big bang so you will always get a finite number of nebulae no matter how big your telescope is. This explanation had a name I cannot recall and was figured out long ago. Anyway, I guess this was already obvious and that this book is contemplating the possiblity of no big-bang. —Preceding unsigned comment added by 74.56.206.181 (talk) 15:57, 18 November 2007

I guess you mean Olbers' paradox. You don't even need a large telescope because the number of galaxies increases with distance as fast as the light dims. Cosmo0 (talk) 22:56, 18 November 2007 (UTC)[reply]

In layman's terms

Guys, this doesn't read at all well for someone not well versed in cosmology/physics. I imagine it is quite complex but is it possible to be expresed in more simple terms, or at least to explain the concept a little better? Some parts of it read almost like the article was created with the intention of being as obscure as possible. :-) Diliff | (Talk) (Contribs) 23:47, 21 August 2007 (UTC)[reply]

Words like "homogeneous" and "isotropic" probably must stay. ;) ◙◙◙ I M Kmarinas86 U O 2¢ ◙◙◙ 18:22, 24 August 2007 (UTC)[reply]

Unforunately, Diliff, many articles are obscuranist like this. I've added a line to the intro that, hopefully, makes things a little clearer.--Michael C. Price ^talk 18:27, 24 August 2007 (UTC)[reply]

"The universerve is the same everywhere" does not make anything clearer. It just states you assume the reader to be an imbicil.192.114.175.2 12:06, 6 September 2007 (UTC)[reply]

I agree (well, imbecile is taking it a bit far). That's what the links to the articles on homogeneity and isotropy are for. Cosmo0 10:27, 23 September 2007 (UTC)[reply]

Merge from Perfect Cosmological Principle

I propose that Perfect Cosmological Principle should be merged into this article as a new section, since it's a simple extension of the subject discussed here and there's not much more to add to the other article that wouldn't be repeating this article. Cosmo0 21:20, 22 September 2007 (UTC)[reply]

Opposed. These are radically different concepts. --Michael C. Price ^talk 07:54, 23 September 2007 (UTC)[reply]

In what sense are they radically different? The perfect principle is just an extension of the other (the dictionary of astronomy on my bookshelf defines it as precisely that). Everything that can be said about the cosmological principle also applies to the perfect cosmological principle. Cosmo0 10:24, 23 September 2007 (UTC)[reply]

One is compatible with big bag, the other only with a steady-state cosmogony. That's pretty different.--Michael C. Price ^talk 13:22, 23 September 2007 (UTC)[reply]

True, although you could argue that even that isn't a big difference compared to the basic assumption of spatial homogeneity & isotropy. Anyway, I'm not dead set on this merger - I was just going through the astronomy stubs and I didn't think that the perfect CP article was likely to progress much beyond a stub without significant repetition of this one. But if you can think of some ideas to expand it then that's great. I suggest we leave it a week or two to give others a chance to comment and if there's no consensus in favour of merging (or the consensus is against) I'll remove the tags. Cosmo0 13:33, 23 September 2007 (UTC)[reply]

These two concepts should not be merged or confused. The Cosmological Principle is what we commonly refer to and is consistent with the standard model of cosmology (big bang, inflation, dark energy, etc...). The Perfect Cosmological Principle is a weird concept which contradicts this standard model. For instance, there cannot be a big bang if the universe never changes in time. The Perfect Cosmological Principle is somewhat along the lines of Einstein's original view of a static universe. Even if some strange theories might be able to fit in the Perfect Cosmological Principle, it remains an unproved principle unlike the Cosmological Principle which seems to apply very well to our universe. I hope it is now clear that these two concepts should not be merged together and it should even be emphasized that they are very different. I hope this helps. Sorry this is my first post on Wikipedia so I don't know the format and stuff. ( F.G. - McGill - Physics )—Preceding unsigned comment added by 74.56.206.181 (talk) 15:40, 18 November 2007

The fact that one is consistent with observations and the other manifestly inconsistent doesn't make them "very different" as principles. Both are rooted in a much more basic principle, often called the principle of mediocrity: that we do not occupy a special place in the Universe. The fact that the perfect cosmological principle is almost certainly an incorrect application of that principle is why I don't think it deserves its own page.

The concensus appears to be against me so I'll withdraw my suggestion and remove the tags.

Cosmo0 (talk) 23:06, 18 November 2007 (UTC)[reply]

"smooth (i.e.: not fractal)"

I don't get it. An isotropic fractal is an oxymoron then, or am I missing something? (it's not unlikely that I am) --Extremophile 19:25, 8 October 2007 (UTC)[reply]

I'm no expert, but as I understand it, fractal means that something looks the same viewed at different scales - it doesn't say anything about how it appears when viewed from different directions, so I imagine an isotropic fractal is possible. But a fractal that is smooth on large scales but not on small scales isn't, by definition. Cosmo0 21:18, 8 October 2007 (UTC)[reply]

Is the cosmological prinicple still true?

This is a big holein the sky! Although you can't see it with the naked eye it actually covers almost 3 degrees of the sky, and to put that into perspective the full Moon covers about half a degree! Until recently no-one was sure how big or far away this void was but the latest calculations suggest it is 900 million light-years across and 8,000 million light-years away. So can we still say the universe is homogeneous and isotropic? Is the Big bang theory still true?

The void: Imprint of another universe? 24 November 2007 Marcus Chown Magazine issue 2631 IN AUGUST, radio astronomers announced that they had found an enormous hole in the universe. Nearly a billion light years across, the void lies in the constellation Eridanus and has far fewer stars, gas and galaxies than usual. It is bigger than anyone imagined possible and is beyond the present understanding of cosmology. What could cause such a gaping hole? One team of physicists has a breathtaking explanation: "It is the unmistakable imprint of another universe beyond the edge of our own," says Laura Mersini-Houghton of the University of North Carolina at Chapel Hill.

It is a staggering claim. If Mersini-Houghton's team is right, the giant void is the first experimental evidence for another universe. It would also vindicate string theory, our most promising understanding of how the universe works at its most fundamental level. And it would do away with the anthropic arguments that have plagued string theorists in ...