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Entropic gravity and cosmology

  1. Feb 6, 2010 #1
    Entropic gravity and cosmology

    The considerable impact of Verlinde’s recent paper in which he suggests that gravity is nothing but an entropic force has familiarised many with this type of force.

    Verlinde wrote:
    The examples he mentions involve constituents in thermal equilibrium with a fluid heat
    bath: a uniform liquid or, in the case of pressure --- also an entropic force --- a uniform
    ideal gas.

    In cosmology, the LCDM model is based on the cosmological principle that we are not
    in any way specially situated in the universe. The model assumes that the universe’s
    uniformly expanding spatial geometry is that of a homogeneous and isotropic everywhere gravitating cosmic fluid whose dynamics are ruled by general relativity. At first glance, therefore, gravity as an entropic force fits well into this scheme, based on a model fluid.

    But the universe is not quite uniform. Wiltshire,
    while discussing the modifications that he argues should be made to the perceived age of the universe and the perceived acceleration of expansion ---- because of inhomogeneities --- notes that:

    I find puzzling the concept of gravity being treated as the same entropic force both in empty voids and deep inside galaxies, and get confused in trying to identifying the nature
    of the heat bath which validates calling gravity an entropic force. Does the CMB perhaps act as a heat bath for gravity? And is there evidence that Newton's law of gravity (which Verlinde derives) is the same deep inside voids as it is deep inside galaxies?
  2. jcsd
  3. Feb 7, 2010 #2
    Re: Entropic gravity and cosmology

    If I remember correctly, Penrose claims that the entropy of the radiation content in the universe is very small compared to that of black holes. If so, the CMB could not be considered a heat bath for gravity.

    Anyway, is a heat bath necessary? After all, you can define entropy e.g. for a closed microcanonical system without a reference to any heat bath.

  4. Feb 7, 2010 #3
    Re: Entropic gravity and cosmology

    About the CMB, yes, I guess I was grasping at a straw. The examples used to illustrate the concept of "entropic force" --- colloids, osmosis, polymers, ideal gases --- all deal with pretty homogeneous systems equipped with "heat baths" , though. I can understand why classifying forces as "entropic" is appropriate here. But our present universe is so heterogeneous in a hierarchical way, ranging all the way from quarks to black holes, that I find the adjective "entropic" quite baffling when used for a force that rules such a mixed bag of stuff. Makes me unkindly suspect that all Verlinde is doing is reinventing wheels.

    Especially when I look at papers like that of Jerzy Kowalski-Glikman (arXiv:1002.1035v1 [hep-th] 4 Feb 2010 ; A note on gravity, entropy, and BF topological field theory) in which it seems Faraday's lines of force are about to re-emerge in a much more sophisticated guise.

    If only we could manipulate and experiment with gravity, as we so competently do with electromagnetism. Then there would be some hope of confirming theories of gravity with experiment and observation.
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