In order for the universe to be as it is, containing stars and galaxies, it is a requirement that the early universe was at a relatively low entropy state.

Why did the big bang produce this low entropy universe?

  • $\begingroup$ Low relative to which other universe we know of? Or are you rather asking "what set the entropy of the early universe?" $\endgroup$ Jun 14 '17 at 9:09
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    $\begingroup$ @AtmosphericPrisonEscape I meant low relative to the present, I guess that answer would explain "what set the entropy of the early universe" $\endgroup$
    – Mike H
    Jun 14 '17 at 9:52
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    $\begingroup$ Who says the early universe had lower entropy? The Higgs field hadn't even collapsed yet. $\endgroup$ Jun 14 '17 at 12:53
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    $\begingroup$ @CarlWitthoft One would naively assume the second law of thermodynamics implies this should be the case. $\endgroup$
    – zephyr
    Jun 14 '17 at 13:45
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    $\begingroup$ @MikeH its a consequence of the fact that more likely states with gravity are clustered not uniform. Most entropic arguments used for the universe seem to miss this point for some reason. Isothermal uniform is very unlikely. $\endgroup$
    – chris
    Jun 18 '17 at 13:34

The second law of thermodynamics states:

the total entropy of an isolated system can only increase over time

If applied to our universe, one can conclude that the entropy of our universe must necessarily increase over time (or, improbably, stay constant), resulting in the final heat-death of our universe. Just as Georges Lemaître saw that the Universe was expanding and surmised that it must have been smaller in the past (implying a Big Bang), we can also surmise that if the entropy of our Universe is increasing to a maximum entropy wherein the Universe experiences a heat death, it must necessarily have been lower in the past. This begs the question, why was it lower? How did the universe come to be in a state with such low entropy such that stars and galaxies formed. Why did it not spring into existence, already in the heat death stage?

The answer to this is, I don't know. And I'm not sure anyone could know or give a solid answer, backed up by facts and observational evidence. At best, there are some guesses and hypotheses which I can go over a bit.

First off, let me point out that the second law of thermodynamics may not in fact apply to our Universe as a whole. The problem with the laws of physics is that they often apply in localized cases, say when we're talking about a science lab or a galaxy. But when you try to apply them to the universe as a whole, there's just too many unknowns and usually the rules change. For example, is our universe an isolated system? Nobody can answer that because nobody can view the entire Universe. We're limited to only observing the observable Universe so already we can't definitively say that the Universe adheres to one of the primary pre-conditions of the second law.

Not to mention, you have the difficulty that it may be impossible for all, absolute laws to apply to the Universe as a whole, when we already know some of them don't. The laws of physics all rely on each other. If you change one, you affect all the others. We know (or at least some believe) that the law of conservation of energy does not necessarily hold for the Universe as a whole. At any local point in space and for any localized event, it does, but not when applied to the entire Universe. So already the laws of physics are messed up and that will have unknown consequences for other laws of physics when applied to the Universe as a whole.

Energy in our universe is not conserved because it is expanding. This expansion is constantly dumping new energy into our universe, effectively making our universe not "isolated" even if it may be based on the dictionary definition of the word. As such, it's entirely possible for the entropy to be driven down by this energy input via expansion. The initial inflationary epoch, which likely violated energy conservation globally, could have resulted in our low, initial entropy state.

But as I said, a lot of this is guessing and hypotheses. I don't think there's a good answer out there to this good question, and if there is, I'd really like to hear it.

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    $\begingroup$ As I understand it, the second law of thermodynamics is essentially a restatement of statistics. I can see how some "absolute laws" may not apply to all parts of the universe (if some universal constants are not actually always constant, perhaps) but it seems unlikely that statistics would break down? $\endgroup$
    – Mike H
    Jun 16 '17 at 4:54
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    $\begingroup$ @MikeH My point isn't that statistics breaks down, but rather the assumptions that must be true for the 2nd law to apply, are not true for the Universe. For example, the entropy of Earth is increasing, but that doesn't mean statistics breaks down when applied to the Earth, it means the assumptions of the 2nd law aren't true, i.e., the Earth is not an isolated system. Similarly, the Universe isn't an isolated system because there is an energy input in the form of expansion, meaning that the statics behind the 2nd law are still true, but the assumptions for it to apply are not true. $\endgroup$
    – zephyr
    Jun 16 '17 at 12:33
  • $\begingroup$ Answer this question >>>> Win Nobel Prize. $\endgroup$ Jun 25 '17 at 21:05
  • $\begingroup$ You can also point to the fact that we can choose to set our zero for this scale. "Low entropy" can just mean "lower than it is now". $\endgroup$
    – Phiteros
    Dec 13 '18 at 6:34

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