arXiv subjects

Aurélien Barrau

Publications and source records attributed to Aurélien Barrau.

53 records · Page 3Linked to original sources

arXiv2018

Primordial power spectra from an emergent universe: basic results and clarifications

Emergent cosmological models, together with the Big Bang and bouncing scenarios, are among the possible descriptions of the early Universe. This work aims at clarifying some general features of the primordial tensor power spectrum in this specific framework. In particular, some naive beliefs are corrected. Using a toy model, we investigate the conditions required to produce a scale invariant spectrum and show to which extent this spectrum can exhibit local features sensitive to the details of the scale factor evolution near the transition time.

gr-qc↗

arXiv2018

Phenomenology of Quantum Reduced Loop Gravity in the isotropic cosmological sector

Quantum reduced loop gravity is designed to consistently study symmetry reduced systems within the loop quantum gravity framework. In particular, it bridges the gap between the effective cosmological models of loop quantum cosmology and the full theory, addressing the dynamics before the minisuperspace reduction. This mostly preserves the graph structure and SU(2) quantum numbers. In this article, we study the phenomenological consequences of the isotropic sector of the theory, the so-called emergent bouncing universe model. In particular, the parameter space is scanned and we show that the number of inflationary e-folds is almost always higher than the observational lower-bound. We also compute the primordial tensor power spectrum and study its sensitivity upon the fundamental parameters used in the model.

gr-qc↗

arXiv2018

Scalar spectra of primordial perturbations in loop quantum cosmology

This article is devoted to the study of scalar perturbations in loop quantum cosmology. It aims at clarifying the situation with respect to the way initial conditions are set and to the specific choice of an inflaton potential. Several monomial potentials are studied. Both the dressed metric and deformed algebra approaches are considered. We show that the calculation of the ultraviolet part of the spectrum, which is the physically relevant region for most background trajectories, is reliable, whereas the infrared and intermediate parts do depend on some specific choices that are made explicit.

gr-qc↗

arXiv2018

A status report on the phenomenology of black holes in loop quantum gravity: Evaporation, tunneling to white holes, dark matter and gravitational waves

The understanding of black holes in loop quantum gravity is becoming increasingly accurate. This review focuses on the possible experimental or observational consequences of the underlying spinfoam structure of space-time. It adresses both the aspects associated with the Hawking evaporation and the ones due to the possible existence of a bounce. Finally, consequences for dark matter and gravitational waves are considered.

gr-qc↗

arXiv2018

Fast radio bursts and the stochastic lifetime of black holes in quantum gravity

Non-perturbative quantum gravity effects might allow a black-to-white hole transition. We revisit this increasingly popular hypothesis by taking into account the fundamentally random nature of the bouncing time. We show that if the primordial mass spectrum of black holes is highly peaked, the expected signal can in fact match the wavelength of the observed fast radio bursts. On the other hand, if the primordial mass spectrum is wide and smooth, clear predictions are suggested and the sensitivity to the shape of the spectrum is studied.

gr-qc↗

arXiv2018

A first step towards the inflationary trans-planckian problem treatment in Loop Quantum Cosmology

For most initial conditions, cosmologically relevant physical modes were trans-planckian at the bounce time, often by many magnitude orders. We improve the usual loop quantum cosmology calculation of the primordial power spectra -- in the inflationary framework -- by accounting for those trans-planckian effects through modified dispersion relations. This can induce drastic changes in the spectrum, making it either compatible or incompatible with observational data, depending on the details of the choices operated.

gr-qc↗

arXiv2017

Seeing through the cosmological bounce: Footprints of the contracting phase and luminosity distance in bouncing models

The evolution of the luminosity distance in a contracting universe is studied. It is shown that for quite a lot of natural dynamical evolutions, its behavior is far from trivial and its value can even decrease with an increasing time interval between events. The consequences are investigated and it is underlined that this could both put stringent consistency conditions on bouncing models and open a new observational window on "pre Big Bang" physics using standard gravitational waves.

gr-qc↗

arXiv2017

Bouncing black holes in quantum gravity and the Fermi gamma-ray excess

Non-perturbative quantum-gravity effects can change the fate of black holes and make them bounce in a time scale shorter than the Hawking evaporation time. In this article, we show that this hypothesis can account for the GeV excess observed from the galactic center by the Fermi satellite. By carefully taking into account the secondary component due to the decay of unstable hadrons, we show that the model is fully self-consistent. This phenomenon presents a specific redshift-dependance that could allow to distinguish it from other astrophysical phenomena possibly contributing to the GeV excess.

gr-qc↗

arXiv2017

Testing different approaches to quantum gravity with cosmology: An overview

Among the available quantum gravity proposals, string theory, loop quantum gravity, non-commutative geometry, group field theory, causal sets, asymptotic safety, causal dynamical triangulation, emergent gravity are among the best motivated models. As an introductory summary to this special issue of Comptes Rendus Physique, I explain how those different theories can be tested or constrained by cosmological observations.

gr-qc↗

arXiv2017

Detailed investigation of the duration of inflation in loop quantum cosmology for a Bianchi-I universe with different inflaton potentials and initial conditions

There is a wide consensus on the correct dynamics of the background in loop quantum cosmology. In this article we make a systematic investigation of the duration of inflation by varying what we think to be the most important "unknowns" of the model: the way to set initial conditions, the amount of shear at the bounce and the shape of the inflaton potential.

gr-qc↗

arXiv2017

Some Clarifications on the Duration of Inflation in Loop Quantum Cosmology

The prediction of a phase of inflation whose number of e-folds is constrained is an important feature of loop quantum cosmology. This work aims at giving some elementary clarifications on the role of the different hypotheses leading to this conclusion. We show that the duration of inflation does not depend significantly on the modified background dynamics in the quantum regime.

gr-qc↗

arXiv2016

Evaporation Spectrum of Black Holes from a Local Quantum Gravity Perspective

We revisit the hypothesis of a possible line structure in the Hawking evaporation spectrum of black holes. Because of nonperturbative quantum gravity effects, this would take place arbitrarily far away from the Planck mass. We show, based on a speculative but consistent hypothesis, that this naive prediction might in fact hold in the specific context of loop quantum gravity. A small departure from the ideal case is expected for some low-spin transitions and could allow us to distinguish several quantum gravity models. We also show that the effect is not washed out by the dynamics of the process, by existence of a mass spectrum up to a given width, or by the secondary component induced by the decay of neutral pions emitted during the time-integrated evaporation.

gr-qc↗

arXiv2016

Cosmology without time: What to do with a possible signature change from quantum gravitational origin?

Within some approaches to loop quantum cosmology, the existence of an Euclidean phase at high density has been suggested. In this article, we try to explain clearly what are the observable consequences of this possible disappearance of time. Depending on whether it is a real fundamental effect or just an instability in the equation of motion, we show that very different conclusions should be drawn. We finally mention some possible consequences of this phenomenon in the black hole sector.

gr-qc↗

arXiv2016

Some conceptual issues in loop quantum cosmology

Loop quantum gravity is a mature theory. To proceed to explicit calculations in cosmology, it is necessary to make assumptions and simplifications based on the symmetries of the cosmological setting. Symmetry reduction is especially critical when dealing with cosmological perturbations. The present article reviews several approaches to the problem of building a consistent formalism that describes the dynamics of perturbations on a quantum spacetime and tries to address their respective strengths and weaknesses. We also review the main open issues in loop quantum cosmology.

gr-qc↗

arXiv2016

Primordial scalar power spectrum from the Euclidean Big Bounce

In effective models of loop quantum cosmology, the holonomy corrections are associated with deformations of space-time symmetries. The most evident manifestation of the deformations is the emergence of an Euclidean phase accompanying the non-singular bouncing dynamics of the scale factor. In this article, we compute the power spectrum of scalar perturbations generated in this model, with a massive scalar field as the matter content. Instantaneous and adiabatic vacuum-type initial conditions for scalar perturbations are imposed in the contracting phase. The evolution through the Euclidean region is calculated based on the extrapolation of the time direction pointed by the vectors normal to the Cauchy hypersurface in the Lorentzian domains. The obtained power spectrum is characterized by a suppression in the IR regime and oscillations in the intermediate energy range. Furthermore, the speculative extension of the analysis in the UV reveals a specific rise of the power.

gr-qc↗

arXiv2014

Fast Radio Bursts and White Hole Signals

We estimate the size of a primordial black hole exploding today via a white hole transition, and the power in the resulting explosion, using a simple model. We point out that Fast Radio Bursts, strong signals with millisecond duration, probably extragalactic and having unknown source, have wavelength not far from the expected size of the exploding hole. We also discuss the possible higher energy components of the signal.

gr-qc↗

arXiv2013

A new method to improve photometric redshift reconstruction. Applications to the Large Synoptic Survey Telescope

In the next decade, the LSST will become a major facility for the astronomical community. However accurately determining the redshifts of the observed galaxies without using spectroscopy is a major challenge. Reconstruction of the redshifts with high resolution and well-understood uncertainties is mandatory for many science goals, including the study of baryonic acoustic oscillations. We investigate different approaches to establish the accuracy that can be reached by the LSST six-band photometry. We construct a realistic mock galaxy catalog, based on the GOODS survey luminosity function, by simulating the expected apparent magnitude distribution for the LSST. To reconstruct the photometric redshifts (photo-z's), we consider a template-fitting method and a neural network method. The photo-z reconstruction from both of these techniques is tested on real CFHTLS data and also on simulated catalogs. We describe a new method to improve photo-z reconstruction that efficiently removes catastrophic outliers via a likelihood ratio statistical test. This test uses the posterior probability functions of the fit parameters and the colors. We show that the photometric redshift accuracy will meet the stringent LSST requirements up to redshift $\sim2.5$ after a selection that is based on the likelihood ratio test or on the apparent magnitude for galaxies with $S/N>5$ in at least 5 bands. The former selection has the advantage of retaining roughly 35% more galaxies for a similar photo-z performance compared to the latter. Photo-z reconstruction using a neural network algorithm is also described. In addition, we utilize the CFHTLS spectro-photometric catalog to outline the possibility of combining the neural network and template-fitting methods. We conclude that the photo-z's will be accurately estimated with the LSST if a Bayesian prior probability and a calibration sample are used.

astro-ph.CO↗