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What do we want to calculate with quantum gravity?

  1. Apr 18, 2012 #1
    Einstein's General Realitivity acurately computed the precession of the perihelion of the Mercury, Schwinger's and Feynman's QED was able to calculate the Lamp shift, Bohr's quantum mechanics gave precise explanations of the hydrogen atom.

    Question: what experimental phenomenon is there that we can not acurately enough compute, but where we expect that quantum gravity calculations would give an exact or more precise solution?

    Is there any?

    The black hole information loss problem? What is dark energy? What happened at the Big Bang?

    What is the Lamp shift for quantum gravity?!
  2. jcsd
  3. Apr 18, 2012 #2


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    The power spectrum of the CMB
    See papers by Ashtekar + Nelson + Agullo
    by Zhao + Brown
    by Barrau + Grain

    Just look in arxiv with the "AND" of their names.

    Basically one wants a model of the geometry of the universe which is good back to very high density e.g. quantum corrections causing a bounce. The beginning of expansion will leave an imprint in the magnified fluctuations which one sees in the CMB ancient light sky.
    Last edited: Apr 18, 2012
  4. Apr 18, 2012 #3


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    Another thing, more of a longshot, is using QG folks would like to calculate the lifetime and endbehavior of primordial black holes. It is not clear that they evaporate finally according to Hawking model and they could have a longer lifetime and a different (observable) radiation signature.

    Search arxiv for Modesto+Hossenfelder
  5. Apr 18, 2012 #4


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    This is just a sample of the papers suggesting what we might want to calculate (in order to confront with observational data). I see there are a lot more.
    Perturbations in loop quantum cosmology
    Ivan Agullo, Abhay Ashtekar, William Nelson
    (Submitted on 5 Apr 2012)
    The era of precision cosmology has allowed us to accurately determine many important cosmological parameters, in particular via the CMB. Confronting Loop Quantum Cosmology with these observations provides us with a powerful test of the theory. For this to be possible we need a detailed understanding of the generation and evolution of inhomogeneous perturbations during the early, Quantum Gravity, phase of the universe. Here we describe how Loop Quantum Cosmology provides a completion of the inflationary paradigm, that is consistent with the observed power spectra of the CMB...

    Constraints on standard and non-standard early Universe models from CMB B-mode polarization
    Yin-Zhe Ma, Wen Zhao, Michael L. Brown
    (Submitted on 14 Jul 2010 (v1), last revised 21 Sep 2010 (this version, v2))
    We investigate the observational signatures of three models of the early Universe in the B-mode polarization of the Cosmic Microwave Background (CMB) radiation. In addition to the standard single field inflationary model, we also consider the constraints obtainable on the loop quantum cosmology model (from Loop Quantum Gravity) and ...

    Probing Loop Quantum Gravity with Evaporating Black Holes
    Aurelien Barrau, Xiangyu Cao, Jacobo Diaz-Polo, Julien Grain, Thomas Cailleteau

    Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters
    J. Grain, A. Barrau, T. Cailleteau, J. Mielczarek

    Inflation in loop quantum cosmology: Dynamics and spectrum of gravitational waves
    Jakub Mielczarek, Thomas Cailleteau, Julien Grain, Aurelien Barrau

    [The point is that ancient gravitational wave are FROZEN in the microwave sky, greatly magnified by subsequent expansion. One can make predictions about what will be seen as higher resolution maps of the CMB are made.]

    Tensor Tilt from Primordial B-modes
    Brian A. Powell
    (Submitted on 24 Jun 2011...)
    A primordial cosmic microwave background B-mode ... These models can be differentiated by the scale dependence of their tensor spectra: inflation predicts a red tilt (nT<0), string gases and loop quantum cosmology predict a blue tilt (nT>0), while a nonsingular matter bounce gives zero tilt (nT=0). We perform a Bayesian analysis to determine ... While a future mission like CMBPol will offer improvement, only an ideal satellite mission will be capable of providing sufficient Bayesian evidence to distinguish between each model considered.
    8 pages, 4 figures. ... Version to appear in Mon. Not. R. Astron. Soc

    Cosmological footprints of loop quantum gravity
    J. Grain, A. Barrau

    A model for non-singular black hole collapse and evaporation
    Sabine Hossenfelder, Leonardo Modesto, Isabeau Prémont-Schwarz
    (Submitted on 9 Dec 2009 (v1), last revised 24 Feb 2010 (this version, v3))
    We study the formation of a black hole and its subsequent evaporation in a model employing a minisuperspace approach to loop quantum gravity. In previous work the static solution was obtained and shown to be singularity-free. Here, we examine the more realistic dynamical case by generalizing the static case with help of the Vaidya metric. We track the formation and evolution of trapped surfaces during collapse and evaporation and examine the buildup of quantum gravitationally caused stress-energy preventing the formation of a singularity...

    Emission spectra of self-dual black holes
    Sabine Hossenfelder, Leonardo Modesto, Isabeau Prémont-Schwarz
    (Submitted on 2 Feb 2012 (v1), last revised 15 Feb 2012 (this version, v2))
    We calculate the particle spectra of evaporating self-dual black holes that are potential dark matter candidates. We first estimate the relevant mass and temperature range and find that the masses are below the Planck mass, and the temperature of the black holes is small compared to their mass. In this limit, we then derive the number-density of the primary emission particles,... We finally arrive at the expression for the spectrum of secondary particle emission from a dark matter halo constituted of self-dual black holes.

    [This is a longshot high risk research direction but its the kind of thing I am hoping to see more of. Small LQG black holes behave distinctly differently from small Hawking ones. I'd like to see more observable stuff calculated about this. It's barely possible that clouds of dark matter contain small black holes and have a distinctive radiation spectrum.]

    However the main LQG phenomenology is in the area of early universe cosmology.
    Agullo gave an invited talk about this at the April meeting of the American Physical Society (APS) in Atlanta.
    Last edited: Apr 19, 2012
  6. Apr 18, 2012 #5


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    It's about sheep!

    Strominger, Five Problems in Quantum Gravity: We present five open problems in quantum gravity which one might reasonably hope to solve in the next decade. Hints appearing in the literature are summarized for each one.
    Last edited: Apr 18, 2012
  7. Apr 19, 2012 #6
    Thanks for the replies!!
  8. Apr 19, 2012 #7


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    We all, like sheep, have gone astray!
    Bottom-line for me: Quantum-gravity should solve the mystery of mass. Mass bends space-time, so -in a sense- space-time should define the masses of the particles (is mass!). A classical, continuous space-time gives no clue whatsoever. So we should zoom in and find some triangled braided, bubbling or foaming spacetime, capable of predicting particle families and properties. It is a pity that we will not be able to built a real microscope for this and must relay on indirect proof..

    My best bet: Christoph Wetterich with his Ising model for fermions.

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