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What do the gravity waves seen by LIGO imply for QG?

  1. Feb 8, 2016 #1


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    LIGO will apparently announce on February 11, the detection at five plus sigma of gravity waves (3 solar masses of energy worth in about 100 seconds) emitted as two medium sized black holes of about 65 solar masses combined spiral into each other and merge into a single Kerr (i.e. spinning) black hole.

    Apparently, the results are dead on to the GR prediction, which begs the question, what limits does this place on strong field modifications to GR predictions due to quantum gravity?

    It looks like any quantum gravity theory that predicts meaningful deviations from GR in this regime will now be ruled out experimentally. Is anyone familiar enough with QG phenomenology to make any meaningful statements about what theories will be casualties of this new data point?

    For example, are the energy scales involved in this event high enough that a running of the gravitational constant G with energy scale would be discernible relative to GR's predictions?

    Also, does it place any meaningful limitations on dark matter? It is hard to imagine that dark matter could manage to have absolutely no impact whatsoever on something like this, but on the other hand, this is taking place in an area of space smaller than Australia and the amount of dark matter in that region in the solar system would be miniscule because it is so diffuse.
    Last edited: Feb 9, 2016
  2. jcsd
  3. Feb 9, 2016 #2


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    Ask and you shall receive, I guess: http://arxiv.org/abs/1602.02413 and http://arxiv.org/abs/1602.02453

    Black Hole Based Tests of General Relativity (95 pages)
    Kent Yagi, Leo C. Stein
    (Submitted on 7 Feb 2016)
    General relativity has passed all solar system experiments and neutron star based tests, such as binary pulsar observations, with flying colors. A more exotic arena for testing general relativity is in systems that contain one or more black holes. Black holes are the most compact objects in the universe, providing probes of the strongest-possible gravitational fields. We are motivated to study strong-field gravity since many theories give large deviations from general relativity only at large field strengths, while recovering the weak-field behavior. In this article, we review how one can probe general relativity and various alternative theories of gravity by using electromagnetic waves from a black hole with an accretion disk, and gravitational waves from black hole binaries. We first review model-independent ways of testing gravity with electromagnetic/gravitational waves from a black hole system. We then focus on selected examples of theories that extend general relativity in rather simple ways. Some important characteristics of general relativity include (but are not limited to) (i) only tensor gravitational degrees of freedom, (ii) the graviton is massless, (iii) no quadratic or higher curvatures in the action, and (iv) the theory is 4 dimensional. Altering a characteristic leads to a different extension of general relativity: (i) scalar-tensor theories, (ii) massive gravity theories, (iii) quadratic gravity, and (iv) theories with large extra dimensions. Within each theory, we describe black hole solutions, their properties, and current and projected constraints on each theory using black hole-based tests of gravity. We close this review by listing some of the open problems in model-independent tests and within each specific theory.

    Testing general relativity using golden black-hole binaries (4 pages)
    Abhirup Ghosh, Archisman Ghosh, Nathan K. Johnson-McDaniel, Chandra Kant Mishra, Parameswaran Ajith, Walter Del Pozzo, David A. Nichols,Yanbei Chen, Alex B. Nielsen, Christopher P. L. Berry, Lionel London
    (Submitted on 8 Feb 2016)
    The coalescences of stellar-mass black-hole binaries through their inspiral, merger, and ringdown are among the most promising sources for ground-based gravitational-wave (GW) detectors. If a GW signal is observed with sufficient signal-to-noise ratio, the masses and spins of the black holes can be estimated from just the inspiral part of the signal. Using these estimates of the initial parameters of the binary, the mass and spin of the final black hole can be uniquely predicted making use of general-relativistic numerical simulations. In addition, the mass and spin of the final black hole can be independently estimated from the merger-ringdown part of the signal. If the binary black hole dynamics is correctly described by general relativity, these independent estimates have to be consistent with each other. We present a Bayesian implementation of such a test of general relativity, and outline the expected constraints from upcoming GW observations using the second-generation of ground-based GW detectors.
  4. Feb 9, 2016 #3
    how does this affect string theory vs lqg vs AsGr? its my understanding string theory is scalar-tensor theory of gravity, with higher order corrections, and additional dimensions, vs GR.

    if you can rule out scalar-tensor theories of gravity via black holes, would this falsify string theory as a theory of QG?
  5. Feb 9, 2016 #4


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    I don't think many theories can be completely ruled out. But, the parameter space could be made a lot narrower for a lot of theories. I think I recall reading in the longer article, that the bound on the mass of a massive graviton, for example, could be made tighter by a factor of 10^6.
  6. Feb 9, 2016 #5
    i understand parameter space based on susy-mssm.

    if what LIGO finds is in accordance with GR in the strong field regime, what QG gives GR in the strong field regime most parsimoniously?
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