Cosmology: Is Universe Finite or Unbounded?

[da_willem]

I am confused about the currents status of knowledge in cosmology, I always thought that cosmologists believed the universe to be finite but unbounded. Now I read that 'current cosmological theories state that the universe is infinitely big' (http://curious.astro.cornell.edu/question.php?number=575) , how is this possible in light of the big bang theory?

 

[marcus]

I am confused about the currents status of knowledge in cosmology, I always thought that cosmologists believed the universe to be finite but unbounded.

I think you were mistaken about that. AFAIK there has never been a consensus among cosmologists on the "finite but unbounded" issue

Now I read that 'current cosmological theories state that the universe is infinitely big' (http://curious.astro.cornell.edu/question.php?number=575) ,

the information at the cornell website is false or misleading. The fact is that SOME current theory assumes spatially infinite.

the prevaling cosmological model (LCDM, standing for LambdaCDM) can be spatially infinite or it can be spatially finite and "nearly flat"----with a slight positive curvature.

even more to the point, the observational data is CONSISTENT WITH EITHER spatial finite or infinite!

the decisive parameter is called "Omega_total" and if you set Omega exactly equal to 1, and plug that into LCDM, you get zero largescale spatial curvature, spatially infinite, version.
Mathematically this is the simplest to deal with, so it is very attractive to set Omega exactly = 1.
But if you set Omega = 1.01, for instance, then you get a spatially finite version of LCDM, with very slight positive curvature, WHICH FITS THE CURRENT OBSERVATIONAL DATA ABOUT EQUALLY WELL.

Ned Wright, a prominent mainstream cosmologist, posted a paper in january this year where he said the "best fit" to all available sets of relevant gave an Omega of 1.011. BUT he stressed that because of uncertainty, the data was CONSISTENT with the spatially flat infinite picture with Omega = 1.
Here is preprint of Ned Wright's january 2007 paper (to be published in Astrophysical Journal)
http://arxiv.org/abs/astro-ph/0701584

It is common practice to set Omega = 1 in doing analysis, but Wright and several others have warned against doing this. At present one can't be sure whether it is 1 or something like 1.01, so safer to leave it open.

how is this possible in light of the big bang theory?

Big bang theory works equally well with spatial finite and spatial infinite.
There is a popular misconception that BB theory says the universe began "as a point" or as something finite and pointlike like maybe a garbanzo bean or a peanut. But that is not what it says. BB theory could have an infinite-sized region of initial singularity----that is one possibility.

A singularity is where a theory breaks down and stops giving meaningful results----it doesn't have to be a "single" point even though the word SOUNDS like "single". A theory can fail at an extensive region.

Maybe the singularity was infinite, but all that we can now observe came from something as big as a peanut. The part we can observe is finite, so finite comes from finite----but the whole thing could be spatially infinite and have a spatially infinite singularity in its past.

In any case improvements on General Relativity that folks are now working on seem to be going in the direction of getting rid of the initial singularity---so the theory does not break down and probes back further into the past BEFORE where the classical GR breaks down.

I've seen an announcement that Springer press is bringing out a book in 2008 called "Beyond the Big Bang" edited by R. Vaas, with essays by several of the researchers involved in this. In February this year there was a three week workshop at the Santa Barbara ITP (institute of theoretical physics) about this kind of thing (improving GR so it doesn't break down, so you can go back before----a smooth 'bounce' instead of a bang)

 

[da_willem]

BB theory could have an infinite-sized region of initial singularity----that is one possibility.

A singularity is where a theory breaks down and stops giving meaningful results----it doesn't have to be a "single" point even though the word SOUNDS like "single". A theory can fail at an extensive region.

Maybe the singularity was infinite, but all that we can now observe came from something as big as a peanut. The part we can observe is finite, so finite comes from finite----but the whole thing could be spatially infinite and have a spatially infinite singularity in its past.

Ah, that answers my questions, thx!