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2 results

  • 3 - Emergence of Time in Loop Quantum Gravity

  • from Part I - Spacetime Emergence
  • Edited by Nick Huggett, University of Illinois, Chicago, Keizo Matsubara, Uppsala Universitet, Sweden, Christian Wüthrich, Université de Genève
  • Book:
    Beyond Spacetime
    Published online:
    24 April 2020
    Print publication:
    14 May 2020, pp 53-78

  • Summary

    Loop quantum gravity has formalized a robust scheme in resolving classical singularities in a variety of symmetry-reduced models of gravity. In this essay, we demonstrate that the same quantum correction that is crucial for singularity resolution is also responsible for the phenomenon of signature change in these models, whereby one effectively transitions from a `fuzzy' Euclidean space to a Lorentzian space-time in deep quantum regimes. As long as one uses a quantization scheme that respects covariance, holonomy corrections from loop quantum gravity generically leads to nonsingular signature change, thereby giving an emergent notion of time in the theory. Robustness of this mechanism is established by comparison across a large class of midisuperspace models and allowing for diverse quantization ambiguities. Conceptual and mathematical consequences of such an underlying quantum-deformed spacetime are briefly discussed.

  • X-ray fluctuation timescale and Black Hole mass relation in AGN

  • Amri Wandel, Mathew Malkan
  • Journal:
    Proceedings of the International Astronomical Union / Volume 12 / Issue S324 / September 2016
    Published online by Cambridge University Press:
    23 June 2017, pp. 172-175
    Print publication:
    September 2016
    • Article
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  • We analyze the fluctuations in the X-ray flux of 20 AGN (mainly Seyfert 1 galaxies) monitored by RXTE and XMM-Newton with a sampling frequency ranging from hours to years, using structure function (SF) analysis. We derive SFs over four orders of magnitude in the time domain (0.03-300 days). Most objects show a characteristic time scale, where the SF flattens or changes slope. For 10 objects with published power-spectral density (PSD) the break time scales in the SF and PSD are similar and show a good correlation. We also find a significant correlation between the SF timescale and the mass of the central black hole, determined for most objects by reverberation mapping.