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Steve Hsu and Loop Gravity's black hole bounce

  1. Aug 29, 2006 #1


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    Steve Hsu has started a conversation that closely parallels discussion in Loop Quantum Cosmology regarding black hole bounce.

    In LQC the equation that replaces the classical Friedman or the Wheeler-DeWitt equations has gravity turn out to be repulsive at high densities and many solutions avoid singularity and exhibit a bounce.

    Ashtekar has pointed to evidence that in the LQC model gravitational collapse and re-expansion can connect two large classical regions of spacetime.
    http://arxiv.org/abs/gr-qc/0509075 (Class. Quant. Grav.)

    A black hole can (at least in some circumstances in the LQC model) produce a new region of the universe. The bounce phenomenon is being studied both analytically and by numerical simulations at Penn State.
    http://arxiv.org/abs/gr-qc/0604013 (Phys. Rev. D)

    It turns out, in other words, that Martin Bojowald, Abhay Ashtekar, and several collaborators have been making a detailed study of a mechanism by which what Steve Hsu is talking about might actually happen: a quantum gravity mechanism by which a new region of space might expand from the pit of a black hole. This is not something that quantum gravitists put into their model by hand. The bounce phenomenon appeared in a 2001 analysis by Bojowald ("Absence of Singularity in Loop Quantum Cosmology") and a number of LQG papers have been published since then, investigating the subject.

    Here is a list of 122 papers which cite Bojowald's original "Absence of Singularity in LQC" paper:

    These things seem to fit rather nicely together and it is noteworthy that Stephen Hsu seems to be coming to it from an entirely different direction. So I want in this thread to look at what his message is and at some of the material he gives links to.
    Last edited: Aug 29, 2006
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  3. Aug 29, 2006 #2


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    First off, here's the abstract of the Easson Brandenberger paper that Steve Hsu cited in his blog today:

    Universe Generation from Black Hole Interiors
    Damien A. Easson, Robert H. Brandenberger
    6 pages, 2 figures
    JHEP 0106 (2001) 024

    "We point out that scenarios in which the universe is born from the interior of a black hole may not posses many of the problems of the Standard Big-Bang (SBB) model. In particular we demonstrate that the horizon problem, flatness, and the structure formation problem might be solved naturally, not necessarily requiring a long period of cosmological inflation. The black hole information loss problem is also discussed. Our conclusions are completely independent of the details of general models."

    It is actually kind of strange to encounter this because it seems to echo so closely Lee Smolin's 1994 paper (and yet comes from a different part of the research "map")

    The fate of black hole singularities and the parameters of the standard models of particle physics and cosmology
    Lee Smolin
    27 pages

    "A cosmological scenario which explains the values of the parameters of the standard models of elementary particle physics and cosmology is discussed. In this scenario these parameters are set by a process analogous to natural selection which follows naturally from the assumption that the singularities in black holes are removed by quantum effects leading to the creation of new expanding regions of the universe..."

    Smolin's special contribution at that point was to recognize a TESTABLE HYPOTHESIS in this connection----something that makes unambiguous predictions about future observations allowing it to be checked empirically and possibly falsified.

    Stated as a kind of challenge, the hypothesis is that you cannot find a modification of the dimensionless parameters of standard model physics and cosmology which would initially increase black hole abundance.

    Smolin calculated an upper limit on the mass of neutron stars which, if a neutron star were observed to exceed it, would falsify his hypothesis (because an adjustment of top quark mass would then be possible which would cause more neutron stars to collapse forming holes, increasing hole abundance). His conjecture is that we are at a local optimum in parameter space. So far the challenge, or conjecture, still stands.

    So it looks like Easson Brandenberger are giving PLAUSIBILITY reasons why this model might be attractive-----this picture of a universe FORKING at black holes to make a kind of branched structure.

    And Smolin has, in a way complementing this, given a TESTABLE hypothesis related to that picture.

    And Ashtekar and Bojowald have been studying the bounce MECHANISM.

    So the whole thing seems prepared to come together in an interesting collection of issues to investigate.
  4. Aug 29, 2006 #3


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    I see that Easson Brandenberger cite three publications by Smolin as follows:

    [9] L. Smolin, Class. Quant. Grav. 9, 173 (1992); L. Smolin, Experimental Signatures of Quantum Gravity in the proceedings of the Fourth Drexel Conference on Quantum Nonintegrability, ed. D. H. Feng and B. L. Hu (International Press, Cambridge, 1997), gr-qc/9503027; L. Smolin, Using neutrons stars and primordial black holes to test theories of quantum gravity, astroph/9712189; L. Smolin, The Life of the Cosmos (Oxford University Press, Oxford, 1997).

    Yesterday I put the Steve Hsu paper on our bibliography thread (post #510)
    with the abstract, an exerpt, and some brief comment. For convenience here are the abstract and quote:

    Spacetime topology change and black hole information
    Stephen D.H. Hsu
    5 pages, 5 figures

    "Topology change -- the creation of a disconnected baby universe -- due to black hole collapse may resolve the information loss paradox. Evolution from an early time Cauchy surface to a final surface which includes a slice of the disconnected region can be unitary and consistent with conventional quantum mechanics. We discuss the issue of cluster decomposition, showing that any violations thereof are likely to be unobservably small. Topology change is similar to the black hole remnant scenario and only requires assumptions about the behavior of quantum gravity in planckian regimes. It does not require non-locality or any modification of low-energy physics."

    ===quote from conclusions section===
    "We have proposed a solution of the black hole information paradox which depends entirely on details of planckian physics — no modifications of low-energy physics, such as non-locality, are required. The main assumptions are that the endpoint of interior black hole evolution is topology change and that the quantum gravitational dynamics of pinching off are strongly coupled. Thus, small perturbations to the initial state of a black hole lead to different internal state vectors describing the resulting baby universe, even if the semiclassical properties are only slightly changed. Under this assumption, any violation of cluster decomposition will be practically unobservable.

    If our scenario is correct, there is no violation of causality or locality at the semiclassical black hole horizon, and no stable planck mass remnant of black hole evaporation. Instead, much as Hawking first proposed, information is lost: to a baby universe, from which it may or may not someday emerge via tunneling. If the information emerges again, evolution within the parent universe is unitary. If information remains in the baby universe, the parent universe appears to evolve from a pure to mixed state, but the evolution of parent and baby together is unitary. There are no dire consequences, such as energy non-conservation."
    Last edited: Aug 29, 2006
  5. Aug 30, 2006 #4
    lubos seems to think that if this is true, it would falsify the malcadena conjecture so hsu must be wrong.

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