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The CMB and large-scale structure observations show that our universe was very homogenous in the past. This leads to the horizon problem, and there are two ways of getting around it.

  • One way is to simply postulate that the universe was perfectly homogenous at some $t\approx 10^{-40}$ seconds.
  • Another way is by postulating inflation, which says that there was a period where different regions had enough time to thermally equilibrate with each other and then the universe blew up by a factor of $\sim 10^{26}$ (in terms of the scale factor $a$) in $\Delta t \approx 10^{-33}$ seconds.

Both of these involve homogeneity and inflation involves thermal equilibrium. Either way, it seems like the big bang conditions give a high-entropy state of the universe rather than a low-entropy state like people keep saying. So then why do people say that the big bang involves low-entropy? Doesn't this contradict the past hypothesis? What is the right way to think about this?

Maximal Ideal
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  • See https://www.youtube.com/watch?v=GvV2Xzh11r8 , and see if you can answer your own question (which is allowed on PSE): Penrose has formulated an eternal cosmological model whose temporal iterations are based on a low entropy initial condition, which had been the usual assumption prior to work by Guth and Vilenkin, that implied a high entropy initial condition and resulted, via asymptotically-exponential spatial expansion, in a multiverse of causally-separated "local" (or "pocket", or "bubble") universes. The Guth-Vilenkin model includes no specs before the BB, except for "test particles". – Edouard Apr 01 '22 at 21:57
  • With Penrose, you get no beginning & no end; with Guth & Vilenkin, you get a beginning, no end, & a variety of spatial configurations that may be maximal. Predictable monotony, vs. religion &/or sci-fi. Safety vs. thrills: Which would a civilization that has (so far) produced two world wars be inclined to prefer? – Edouard Apr 01 '22 at 22:05
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    See Immediately after the Big Bang, was the universe in a state of extremely low or extremely high entropy? – benrg Apr 02 '22 at 00:56
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    The gravitational entropy referred to by John Rennie, in his answer to the question to which Benrg has referred, is the main difference between Penrose's single-universe model and the inflationary ones that are based on a scalar field, and perhaps on inflationary cosmological models in general. (There's one, by Nikodem Poplawski and described in 2010-2021 preprints found by his name on Arxiv, that involves rotation: I don't believe scalar fields rotate.) – Edouard Apr 02 '22 at 03:57
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    @Edouard I see. I'll have to process this when I find the time, but I highly appreciate the info. – Maximal Ideal Apr 02 '22 at 04:09

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