Just read decoding the universe : question

[revelshade]

just read "decoding the universe" : question

I am a "fan" of science. I probably read a dozen or so popular science books a year, and I think I have a decent layman's grasp of the current state of the art (so to speak); but apparently I have fallen behind in my understanding of modern cosmology, or maybe I misunderstood it to begin with.
In the final chapter of "decoding the universe" Seife basically says that most physicists consider the universe to be infinite. Not just immeasurably huge but literally infinite. He paints a picture of an infinite cosmos containing an infinite number of "hubble bubbles" isolated from the rest of infinity by the limited distance light could have traveled since the big bang. Since there are a very large but finite number of ways the contents of each "pocket universe" can be arranged, it follows that there are an infinite number of every possible one.
Is this idea really as mainstream as Seife makes it sound? Or is he talking about a smaller subset of information-centric theorists? How do you get to an infinite universe in the finite amount of time since the big bang? What am I missing here?

 

[EL]

Hi revelshade, and welcome to PF!
Recent observations, such as the WMAP 3rd year data, points at a flat, or even a slightly open, (observable) universe. What's outside the observable universe is of course not possible to measure, but it may be tempting to simply extrapolate the pattern we observe (so that we can sleep good at night? ).
Anyway, i think you could find the following thread interesting regarding your concerns about an infinite universe:
https://www.physicsforums.com/showthread.php?t=125698

 

[Jorrie]

Mainstream theory and modelling do allow an infinite universe - although we have no direct evidence for it. I think most cosmologists agree that the universe must be much larger than our "Hubble bubble", but probably not infinite (e.g. PJE Peebles: Principles of Physical Cosmology).

As you stated, the problem is the finite time since the Big Bang. In my "engineer's view", if we consider just the 4-space of standard cosmology, then for the universe to be infinite today, it must have started as infinitely large in the first place! Perhaps string-theory and/or quantum gravity will eventually prove otherwise.

 

[SpaceTiger]

In the final chapter of "decoding the universe" Seife basically says that most physicists consider the universe to be infinite. Not just immeasurably huge but literally infinite. He paints a picture of an infinite cosmos containing an infinite number of "hubble bubbles" isolated from the rest of infinity by the limited distance light could have traveled since the big bang. Since there are a very large but finite number of ways the contents of each "pocket universe" can be arranged, it follows that there are an infinite number of every possible one.
Is this idea really as mainstream as Seife makes it sound?

I don't really think there is or should be a mainstream opinion on this issue, but the "standard" inflationary model (the process that leads all of those "hubble bubbles") has been infinite in most incarnations. However, they usually rest upon the assumption of a trivial topology, an assumption that myself and many others consider to be unjustified. See Andre Linde's paper here:

http://arxiv.org/abs/hep-th/0408164"

and my post here:

https://www.physicsforums.com/showpost.php?p=1017343&postcount=49"

There is some evidence that inflation is correct, but very little that can speak to the specific inflationary model and tell us whether or not the universe is infinite.

 

[SpaceTiger]

I think most cosmologists agree that the universe must be much larger than our "Hubble bubble", but probably not infinite (e.g. PJE Peebles: Principles of Physical Cosmology).

I'm not actually sure what the majority view is, but if it does lean toward an infinite universe, it's likely just because our current "naive" models make simplifying assumptions that have that result. Generally, the mainstream models of the universe (or of anything else, for that matter) are only as complicated as we can test. A non-trivial topology would be an extra complication that's beyond near-future testability, so the models don't include it. That doesn't, however, mean that this complication is not possible.

 

[PhysicsDilettante]

Space Tiger -

Can you elaborate a little on what is meant by trivial and non-trivial topology? Thanks.

 

[SpaceTiger]

Can you elaborate a little on what is meant by trivial and non-trivial topology?

Here's a nice description:

http://astro.uchicago.edu/home/web/olinto/courses/A18200/nbower.htm"

Basically, you can think of it as describing the global "shape" of the universe (as opposed to the local curvature). If one believes the standard Big Bang theory to arbitrary distances, then they're stuck with one of three topologies, two of which are infinite.