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DR

@ZachWeinersmith To elaborate further, one of the big selling points of inflation is that it explains why the universe looks so close to being spatially flat today, despite the fact that you would expect a non-zero curvature to be very much more likely. The way inflation takes care of this is to say that, whatever the initial (or primordial) curvature in the universe (positive or negative), space will expand so rapidly that a small region of space that is initially flat soon becomes the entire observable universe.

The added benefit of this is that it also explains another issue, which is why the universe looks so uniform/the temperature of the CMB is basically the same everywhere. Without inflation, it means that regions that could never have been in contact with each other (via speed of light signals) coordinated to look the same. With inflation, the universe looks the same because it was initially some very small region (in good thermal contact with itself) that expanded ~e^60 times

30 Mar 2023 at 14:36 | Wall-to-wall | Open on universeodon.com
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DR

@ZachWeinersmith Just to clarify a couple of points

- We model spacetimes and spatial slices as manifolds, so that at small enough scales, they look like flat spacetime and space respectively

- The uniformity issue arises because we take the universe to be finite in age, so without inflation there just hasn't been enough time for antipodal points in the celestial sphere to show the same CMB temperature -- at the beginning of the universe, they would have been causally disconnected and no signal could have reached one from the other by today

@ZachWeinersmith Just to clarify a couple of points

- We model spacetimes and spatial slices as manifolds, so that at small enough scales, they look like flat spacetime and space respectively

- The uniformity issue arises because we take the universe to be finite in age, so without inflation there just hasn't been enough time for antipodal points in the celestial sphere to show the same CMB temperature -- at the beginning of the universe, they would have been causally disconnected and no signal could...

30 Mar 2023 at 14:45 | Open on universeodon.com
DR

@ZachWeinersmith

Just one more:

Going back to the Earth analogy, imagine you were at the top of a mountain, standing on a flat rectangular carpet with a fancy design. You are high enough up that you can see the curvature of the Earth. The effect of inflationary expansion is to basically stretch out your rectangular carpet to the horizon, so it looks like the whole world is just flat-carpet land, and the carpet pattern is everywhere you look. The curvature of the Earth, all the landmarks in the distance are far far beyond what you can see.

@ZachWeinersmith

Just one more:

Going back to the Earth analogy, imagine you were at the top of a mountain, standing on a flat rectangular carpet with a fancy design. You are high enough up that you can see the curvature of the Earth. The effect of inflationary expansion is to basically stretch out your rectangular carpet to the horizon, so it looks like the whole world is just flat-carpet land, and the carpet pattern is everywhere you look. The curvature of the Earth, all the landmarks in the distance...

30 Mar 2023 at 14:55 | Open on universeodon.com
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