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We know that the Universe evolved from a very low entropy state in the early universe to very high entropy state of today's universe.

  1. What is the quantitative definition of the net entropy of the present universe?

  2. How can we measure the net entropy or its change in the universe today?

I want some concept and formulas related to that and I'm not talking about the early universe. The question What is the entropy of the universe today? discusses theoretical calculation of the entropy but not experimental measurements of it. An experimentalist's or observers perspective might also be illuminating for me.

John Rennie
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Solidification
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  • @JohnRennie the question is addressing the problem of measuring entropy, which was not touched by the previous Q&A. – GiorgioP-DoomsdayClockIsAt-90 Feb 20 '19 at 07:46
  • Do you mean the observable universe? –  Mar 06 '19 at 17:24
  • Yes. The obs. universe – Solidification Mar 08 '19 at 09:35
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    Since the linked answer establishes that most of the current universe's entropy comes from supermassive black holes, I am wondering what do you mean by “experimental measurements”? Experimental check of the Bekenstein bound, accretion rates and mass estimates of SMBH population? – A.V.S. Mar 09 '19 at 15:34
  • @A.V.S. I want the formula. The entropy formula for black holes and the universe are different. Are they not? – Solidification Mar 12 '19 at 08:08
  • The entropy for the universe is the sum of entropies for all SMBHs in it plus a small contribution of everything else. Some parts of “everything else” could be calculated with high precision (such as CMB entropy), for some parts (such as dark matter entropy) there are only speculations, but the uncertainties still mainly come from what we don't know about SMBHs. – A.V.S. Mar 12 '19 at 08:45
  • f you see this lecture (initial 5-6 mins), https://www.youtube.com/watch?v=gdFldkitkJA Prof. Susskind is saying that in many contexts when people talk about the entropy of the Universe, they mean the number of CMB photons. He is not saying anything about black holes. Can you make some comment on this? thanks! – Solidification Mar 27 '20 at 04:47

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