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Non-physicist asking.

Assuming the origin of the universe of your choice: If entropy only increases, at what point was it at its lowest value? Why? Was this value ever zero?

chasly - supports Monica
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  • I wouldn't be surprised to find there's not a general consensus as to how to calculate entropy prior to the first inflationary period ( $10^{-37} to 10^{-32} s$ approximately) – Carl Witthoft Sep 10 '20 at 12:07
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    @CarlWitthoft I would argue that there's not a general consensus on how to calculate basically anything prior to the first inflationary period! It's not really surprising that the period where most of known physics is inapplicable has a lot of question marks. – probably_someone Sep 10 '20 at 16:17
  • A possible related answer: https://physics.stackexchange.com/a/193582/110499 – Thomas Moore Sep 10 '20 at 18:09

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The statement about increasing entropy applies to an isolated system. One should be cautious of speaking of "the whole universe" because it is not clear if we know how to reason correctly about that. But we can talk about the entropy of some very large part of the universe, such as the region known as "the observable universe" and that is isolated to good approximation. The entropy of the observable universe had its lowest value right at the start, at the Big Bang. But at a sufficiently early time the conditions are such that we have not built up a good understanding of what mathematical and physical ideas apply. So we can't say much about entropy at the very earliest moments. We have to say "don't know" or "understanding that is a work in progress". What one can say is that the entropy at a time, density and temperature when the existing Standard Model of physics is trustworthy was lower than it was at later times. And indeed it was remarkably low. The universe was somehow placed in a very special state by whatever happened at the Big Bang and the immediately ensuing period.

Andrew Steane
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  • Can you explain precisely why the observable universe actually is "isolated to good approximation"? At first glance, there appear to be two competing effects: more and more distance becomes "observable" as time passes, but space is also expanding. I imagine that the expansion of space "wins" these days, but 1) that probably wasn't always true, and 2) I don't know enough cosmology to confirm that. – probably_someone Sep 10 '20 at 16:13
  • @probably_someone I don't claim the observable universe is completely isolated but I think there is not much exchange of heat at the boundary and the only other way would be a long-range gravitational effect which I suppose is small since the universe is quite homogeneous on the largest scales. But I admit this is a bit vague; I have not studied this issue at length. Certainly the standard cosmology adopts an adiabatic model. – Andrew Steane Sep 10 '20 at 20:02