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The big crunch effect on entropy and time

  1. Aug 21, 2014 #1
    Assuming a big crunch theory were to be true (I know it's not too popular a theory these days) I figure a collapsing universe would need to have decreasing entropy. If entropy is decreasing, would the arrow of time flip? Also, because physics is just as valid with time moving backwards, would all the events of the universe "rewind" in a collapsing universe exactly how they played out when time moved forward? Would I live my life again, moving from death to birth? Just curious of anyone else's thoughts or any content that explores this. Thanks!
  2. jcsd
  3. Aug 21, 2014 #2


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    As I understand it, entropy is not an absolute. It is OBSERVER-DEPENDENT. It is a logarithmic measure of how many microstates comprise the given macrostate (which affects the observer).
    So the question is WHOSE entropy?

    A lot of cosmology research these days is concerned with no-singularity bounce cosmologies. Quantum gravity effects at extreme density cause a rebound, so our expanding phase could have been preceded by a prior contracting phase (according to the bounce models being studied.)

    Some people involved: Brandenberger at McGill, Ashtekar at Penn State, Steinhardt at Princeton, Sloan at Oxford…

    There are scores of researchers involved and to my knowledge none of them talk about "time going backwards" or some observer's entropy decreasing.

    It may be because entropy is not defined at Planck scale, or cannot be defined continuously through the bounce. Maybe no single observer can make it through the bounce, so what the macrostates are (and what microstates comprise them) have to be redefined post bounce.

    Anyway nonsingular bounce cosmology is getting a lot of research attention, by smart people who don't seem bothered by pre and post bounce entropy considerations. I'll get link to the "quantum cosmology" research literature from 2009 onwards:


    These are ranked by number of citations received so that the more representative/influential research tends to show up first on the list.

    Loop QC tends to dominate the highly cited end of the list, so if you want to just look at the non-Loop QC here's a link:

    It might also be interesting to check out what you get from a search for "bounce cosmology".
    For some reason you don't get as many hits as you do with "quantum cosmology" even though over half the QC hits involve a bounce. It's just a peculiarity of the keyword tagging/cataloging I guess.
    Last edited: Aug 21, 2014
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