18 research outputs found
The Semiclassical Limit of Causal Dynamical Triangulations
Previous work has shown that the macroscopic structure of the theory of
quantum gravity defined by causal dynamical triangulations (CDT) is compatible
with that of a de Sitter universe. After emphasizing the strictly
nonperturbative nature of this semiclassical limit we present a detailed study
of the three-volume data, which allows us to re-confirm the de Sitter
structure, exhibit short-distance discretization effects, and make a first
detailed investigation of the presence of higher-order curvature terms in the
effective action for the scale factor. Technically, we make use of a novel way
of fixing the total four-volume in the simulations.Comment: 30 pages, 10 figure
The flare model for X-ray variability of NGC 4258
We study the variability mechanism of active galactic nuclei (AGN) within the
framework of the flare model. To this end we examine the case of Seyfert/LINER
galaxy NGC 4258, which is observed at high inclination angle and exhibits rapid
fluctuations of the X-ray light curve. We construct a model light curve based
on the assumption of magnetic flares localized in the equatorial plane and
orbiting with Keplerian speed at each given radius. We calculate the level of
variability as a function of the inclination of an observer, taking into
account all effects of general relativity near a rotating supermassive black
hole. The variability level is a monotonic function of the source inclination.
It rises more rapidly for larger values of the black hole spin (Kerr parameter)
and for steeper emissivity (index beta of the radial profile). We compare the
expected level of variability for the viewing angle 81.6 deg, as inferred for
NGC 4258, with the case of moderate viewing angles about 30 deg, typical for
Seyfert type-1 galaxies. Highly inclined sources such as this one are
particularly suitable to test the flare model because the effects of orbital
motion, Doppler boosting and light bending are all expected to have maximum
when the accretion disk is seen almost edge-on. The model is consistent with
the NGC 4258 variability, where the obscuring material is thought to be
localized mainly towards the equatorial plane rather than forming a
geometrically thick torus. Once the intrinsic time-scales of the flare duration
are determined to better precision, this kind of highly inclined objects with a
precisely known mass of the black hole can be used to set independent
constraints on the spin parameter.Comment: 7 pages, 3 figures; Astronomy & Astrophysics (this version includes
minor language corrections
Quantum gravity phenomenology at the dawn of the multi-messenger era -- A review
The exploration of the universe has recently entered a new era thanks to the
multi-messenger paradigm, characterized by a continuous increase in the
quantity and quality of experimental data that is obtained by the detection of
the various cosmic messengers (photons, neutrinos, cosmic rays and
gravitational waves) from numerous origins. They give us information about
their sources in the universe and the properties of the intergalactic medium.
Moreover, multi-messenger astronomy opens up the possibility to search for
phenomenological signatures of quantum gravity. On the one hand, the most
energetic events allow us to test our physical theories at energy regimes which
are not directly accessible in accelerators; on the other hand, tiny effects in
the propagation of very high energy particles could be amplified by
cosmological distances. After decades of merely theoretical investigations, the
possibility of obtaining phenomenological indications of Planck-scale effects
is a revolutionary step in the quest for a quantum theory of gravity, but it
requires cooperation between different communities of physicists (both
theoretical and experimental). This review is aimed at promoting this
cooperation by giving a state-of-the art account of the interdisciplinary
expertise that is needed in the effective search of quantum gravity footprints
in the production, propagation and detection of cosmic messengers