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See QG magnified by expansion

  1. Sep 22, 2013 #1

    marcus

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    Frank Wilczek and Larry Krauss point out (what is perhaps obvious but nevertheless important) that the universe acts as a giant magnifying glass for observing Quantum Geometry effects. Because of its expansion the cosmos, they point out, can serve as a classical detector of microscopic QG processes---precisely analogous to the classical detectors in experimental particle physics.http://arxiv.org/abs/1309.5343
    Using Cosmology to Establish the Quantization of Gravity
    Lawrence M. Krauss (1,2), Frank Wilczek (3) ((1) Arizona State University, (2) Australian National Univeresity, (3) MIT)
    (Submitted on 20 Sep 2013)
    While many aspects of general relativity have been tested, and general principles of quantum dynamics demand its quantization, there is no direct evidence for that. It has been argued that development of detectors sensitive to individual gravitons is unlikely, and perhaps impossible. We argue here, however, that measurement of polarization of the Cosmic Microwave Background due to a long wavelength stochastic background of gravitational waves from Inflation in the Early Universe would firmly establish the quantization of gravity.
    3 pages
     
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  3. Sep 22, 2013 #2

    MTd2

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    And what if inflation is due an aeon transition? How to distinguish this waves from the previous aeon's black hole evaporation or collision?
     
  4. Sep 22, 2013 #3

    marcus

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    Before we speculate too much, we could wait until the Planck mission results are released and see what they look like.
    They argue that "measurement of polarization of the Cosmic Microwave Background due to a long wavelength stochastic background of gravitational waves from Inflation" might tell us something. The polarization spectrum would have to have some distinctive character making it plausible that it WAS due to inflationary uniform expansion of a stochastic background by many orders of magnitude.

    Without actual seeing polarization maps and/or spectral info extracted from them, it seems premature to start imagining and interpreting hypothetical feature (at least to any great extent.)
    Krauss Wilczek DO however take some steps in the direction you indicate, on page 3:
    ==quote page 3==
    Thus, a gravitational wave background due to Inflation at the Grand Unified Scale, which corresponds to H ≈ 2.5×1013 GeV, could be observed in the near future. While the current observations of CMB temperature fluctuations, and the observed flatness of the universe are strongly suggestive of an inflationary origin, the mere observation of polarization in the CMB compatible with a gravitational wave background, as exciting as that may be, will not alone prove that it originates in quantum phenomena associated with gravitation (e.g. [10, 11]). Fortunately, there are a wide variety of consistency tests that can be performed to check for an inflationary origin (e.g. see [12]). These include a simple relationship between this ratio and the slope of the CMB temperature fluctuation power spectrum as a function of frequency. In addition, inflation predicts super-horizon size correlations in the gravitational wave spectrum that might be discernible (e.g. see [13]).
    If these consistency tests were quantitatively satisfied, we would therefore have reasonably unambiguous evidence that Inflation did indeed occur, and that that linearized fluctuations in the gravitational field are quantized, with the power spectrum produced as a result of quantum zero-point fluctuations in the gravitational field.
    We should contrast the joint appearance of G and [STRIKE]h[/STRIKE] in Eqs. (7) and (8), which really does implicate quantization of the gravitational field, with other cases, including specifically neutron interferometry, in which both appear. These have sometimes been put forward as “quantum gravitational” phenomena, but more properly they are manifestations of the ordinary quantum mechanics of particles (e.g., neutrons) in classical gravitational fields. Indeed it is more natural to express the effect in terms of the quantity g, the gravitational acceleration near Earth’s surface, which is the relevant aspect of the experimental environment, and then G, which indicates intrinsically gravitational dynamics, does not appear at all.
    It is also possible, of course, that a fully realized theory of quantum gravity would have other indirect consequences that could be observed,..
    ==endquote==
     
  5. Sep 24, 2013 #4

    bapowell

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    We should be careful to distinguish between "observe quantum gravity" and "ascertain quantum gravitational effects". My impression of the Krauss & Wilczek paper is that they claim by observing primordial gravitational waves (or more appropriately the primordial B-mode polarization of the CMB), we will have verified that gravity is, in fact, quantized. Sort of a proof of principle. This is quite different from saying that we can study quantum gravitational effects simply by discovering B-modes. Studying the characteristics of quantum gravity via the CMB would require that we observe so-called "trans-Planckian" effects, effects that get imprinted on fluctuation modes during inflation while they are evolving out of the vacuum with sub-Planckian wavelengths. That we can use the CMB to study quantum gravity in this way has been known for a long time, and so their point was this broader one -- that the presence of the primordial tensor spectrum alone asserts that gravity is quantized.
     
  6. Sep 26, 2013 #5

    marcus

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    Hi Brian, if I understand correctly you may be rephrasing one of the points Krauss and Wilczek made in their abstract:
    http://arxiv.org/abs/1309.5343
    Using Cosmology to Establish the Quantization of Gravity
    Lawrence M. Krauss, Frank Wilczek

    ".. that measurement of polarization of the Cosmic Microwave Background due to a long wavelength stochastic background of gravitational waves from Inflation in the Early Universe would firmly establish the quantization of gravity."

    If this is right, it's kind of exciting. We may soon be seeing evidence that the geometry of spacetime itself is quantized!

    This article relates to that, and was just posted today on arxiv. It is a review for Classical and Quantum Gravity by four of the best-known people in observational LQC.

    ]http://arxiv.org/abs/1309.6896
    Observational issues in loop quantum cosmology
    A. Barrau, T. Cailleteau, J. Grain, J. Mielczarek
    (Submitted on 26 Sep 2013)
    Quantum gravity is sometimes considered as a kind of metaphysical speculation. In this review, we show that, although still extremely difficult to reach, observational signatures can in fact be expected. The early universe is an invaluable laboratory to probe "Planck scale physics". Focusing on Loop Quantum Gravity as one of the best candidates for a non-perturbative and background-independant quantization of gravity, we detail some expected features.
    75 pages, invited topical review for Classical and Quantum Gravity
     
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