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.

  • $\begingroup$ @JohnRennie the question is addressing the problem of measuring entropy, which was not touched by the previous Q&A. $\endgroup$ – GiorgioP Feb 20 at 7:46
  • $\begingroup$ Do you mean the observable universe? $\endgroup$ – Ben Crowell Mar 6 at 17:24
  • $\begingroup$ Yes. The obs. universe $\endgroup$ – mithusengupta123 Mar 8 at 9:35
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    $\begingroup$ 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? $\endgroup$ – A.V.S. Mar 9 at 15:34
  • $\begingroup$ @A.V.S. I want the formula. The entropy formula for black holes and the universe are different. Are they not? $\endgroup$ – mithusengupta123 Mar 12 at 8:08

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