I Discover the Universe from Unique Vantage Points | Your Ultimate Guide

[kimbyd]

On the other hand consider that out of all of the infinite values it COULD take on, it is very nearly exactly flat. What an amazing coincidence if it isn't actually flat.

EDIT: Actually, "infinite values" is not an accurate description of what it could be since if inflation is true (and there is every indication that it is) then the range of values is much more limited.

I don't think this follows. There are basically three possibilities:
1) There is some kind of physical process that drives the universe towards flatness to an extreme degree. Cosmic inflation is an example of one such process.
2) Most universes are actually close to flat and we're just not thinking about it correctly (see https://arxiv.org/abs/1406.3057 for an argument along these lines).
3) Some kind of symmetry exists which demands the universe be exactly flat.

So far, lines of argument (1) and (2) have more potential than (3) in my opinion, as nobody has come up with a symmetry along the lines demanded by (3). Also I believe that the near scale invariance of the primordial power spectrum guarantees some small deviation from flatness within the observable universe.

 

[anorlunda]

That's potentially a stretch. The universe being actually flat requires infinite fine tuning. Specifically, it requires an exact balance between the rate of expansion and the average density.

Excuse this late post. This morning, I was viewing an Alan Guth lecture on Inflationary Cosmology. He touched on this point, and argues that because galaxies formed, Ω must be one to 15 decimal places. That is indeed pretty fine tuning.

Guth argues further that inflation would flatten any curvature that existed pre-inflation.

The slides are from
https://ocw.mit.edu/courses/8-286-the-early-universe-fall-2013/resources/mit8_286f13_lec01/

 

[pbuk]

by the time you got back to "the same place" (after probably many tens of trillions of years) it would be "the same place" only in the most technical sense and would not look anywhere near the same as when you left.

That's not how expansion works (as we understand it). Light emitted now from Earth will only ever reach objects that are less than about 16.5bn light years away - inside the cosmic event horizon. So either we can already see what it used to look like "here" (we can see things that are now almost 47bn light years away), or we never will, no matter how fast we travel or for how long.

(there is a third alternative which is that the expansion of the universe stops accelerating but we don't have a model that predicts that).

 

[KobiashiBooBoo]

As a simple thought experiment you can conceive of observers in the most distant galaxy from us, looking at us "now" and seeing us as a dim red-shifted galaxy.

SO - take a galaxy halfway between us and the most distant galaxy we can observe. THEIR cosmological horizon will overlap both.

Extrapolate this and it goes on forever...

?

 

[phinds]

@KobiashiBooBoo, Yes (assuming the universe is infinite or has a "wraparound" topology if finite). So, why the question marks?

 

[KobiashiBooBoo]

So for each observer - the physics are the same, not changing?

 

[Ibix]

So for each observer - the physics are the same, not changing?

As far as we know.

 

[phinds]

So for each observer - the physics are the same, not changing?

Exactly. The exact details of what they see will vary but they will see the same number of galaxies and so forth.