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Entropy in Cosmology

  1. Jul 6, 2013 #1
    So we know that at the moment of the big bang the universe was relatively low in entropy; as the universe has expanded entropy has increased, but what about further down the line? What about when the universe is so cold that it's nothing more than a collection of black holes, and further more what about when those black holes have evaporated? How would we describe the levels of entropy in that phase of the universes life? Wouldn't entropy have decreased since the universe has become less messy and there's fewer configurations in which the constituents of the system can be ordered? I realize the second law of thermodynamics isn't taken quite as seriously as the rest of the laws, but I'm curious about it anyway.
     
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  3. Jul 6, 2013 #2

    mfb

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    Black holes evaporate to a very large number of low-energetic photons - you get a very high entropy as result. Not per volume, as they get thinned out by expansion, but per comoving volume.
     
  4. Jul 6, 2013 #3
    I see. So the matter contained in and radiated from the black holes has been broken down into even smaller components that of which has an even greater number of potential configurations? That makes sense. What about during the black hole era? Is entropy still greater than that of our current era even though matter is contained within the black holes? To the common eye a black hole era would look a lot tidier than the era we live in now, although I'm not sure how you would measure entropy in a universe populated solely with black holes.

    This is probably a question more for Thermodynamics rather than Cosmology but I'll ask it anyway, does it matter on the number of different particles you're looking at in a system, or can it be a system full of the same types of particles and still have higher entropy? If the black holes radiated into photons, couldn't we argue entropy has decreased since they're all photons and it's less likely that other particles would be present in said configurations, such as atoms of hydrogen, helium, oxygen, etc?
     
    Last edited: Jul 6, 2013
  5. Jul 6, 2013 #4

    mfb

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    Right.
    You put in massive particles with ~MeV to ~GeV, and you get mainly photons with something like peV (pico-eV) or less.

    It is expected that the entropy of a black hole is connected to its surface area, and very large. Unfortunately, we don't have them in a lab, so it is hard to verify this.
    http://en.wikipedia.org/wiki/Black_hole_thermodynamics

    Different particle types make it "easier" to get a high entropy, but the total number of particles is more important.
     
  6. Jul 6, 2013 #5
    Great! Thanks for your help!
     
  7. Jul 13, 2013 #6
    Maximum Entropy

    A question kind of following on... in zillions of years as we approach maximum entropy and everything has reached a dark even nothing and by definition NOTHING happens, does that mean time stops?

    my first post so be kind, I'm probably missing something basic...

    nr
     
  8. Jul 13, 2013 #7

    mfb

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    There is still something happening, it is just very boring - a few photons and a few particles fly through nearly empty, ever-expanding* space.

    *probably
     
  9. Jul 13, 2013 #8

    Haelfix

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    It really depends on how long you want to wait and what you mean by zillions.

    At some point, very rare processes that are allowed by quantum mechanics will eventually take place. For instance, you might have another bout of inflation take place. Alternatively, our vacuum might decay. Even later, the classical state of the world might randomly fluctuate into some new configuration.

    Physicists don't like to talk about these events, b/c the time frames considered are astronomical (so large that our normal assumptions and approximations of physical law start breaking down) and it really required detailed knowledge about certain types of physics (quantum gravity, black hole physics, inflationary physics etc) that we simply don't have a hold on.

    So no one knows really, but its safe to say that the understanding will probably change a lot over the next twenty or thirty years.
     
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