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The claim that the young universe was in a low-entropy state seems at odds with

  • maximal entropy being thermal equilibrium, and
  • the young universe being in thermal equilibrium.

I've looked at some other answers and they're too technical for me, but I think I've understood the reason to be basically this:

"Entropy was lower because the universe was smaller."

Is this right?

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  • $\begingroup$ Considering the cooling was enabled by the expansion of the universe, I think there's a good case for it. $\endgroup$ – Alan Rominger Sep 4 '12 at 17:59
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Is this right?

No. I think you've arrived at this because you're not considering the gravitational degrees of freedom. Sheldon Goldstein puts it this way:

[T]he attractive nature of the gravitational interaction is such that gravitating matter tends to clump, clumped states having larger entropy. . . . For an ordinary gas, increasing entropy tends to make the distribution more uniform. For a system of gravitating bodies the reverse is true. High entropy is achieved by gravitational clumping — and the highest of all, by collapse to a black hole.

So, gravitationally speaking, the young universe had very low entropy because the distribution of mass-energy was nearly uniform rather than clumped.

Now, the question of why this was the case is much more difficult to answer.

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  • $\begingroup$ Can you cite the source, please? Intuitively I would expect clumped states to have LESS entropy, what about gravity makes this the reverse of the non-gravitational case? (And aren't all cases gravitational?) Can we derive this result, or is it an axiom we've inserted in order to preserve the doctrine of always-increasing-entropy? $\endgroup$ – spraff Sep 11 '12 at 10:09
  • $\begingroup$ This is basically right, but I believe there are also arguments to the effect that a maximum-entropy state would have a lot of energy in the form of gravitational waves. Unfortunately I can't find a reference, but this talk may be helpful: newton.ac.uk/webseminars/pg+ws/2005/gmr/gmrw04/1107/penrose $\endgroup$ – Ben Crowell Apr 29 '13 at 22:12
  • $\begingroup$ @spraff: Here is a reference that supports the answer, although it doesn't discuss gravitational waves: arxiv.org/abs/gr-qc/0507094 $\endgroup$ – Ben Crowell Apr 29 '13 at 22:13
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At the beginning there was a single waveform expanding to the present , very high temperature /energy, and no space ie. one single wave state- low entrophy.

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    $\begingroup$ -1 Your answer makes a statement without any supporting evidence or links or details. $\endgroup$ – Brandon Enright Apr 29 '13 at 21:55

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