101 research outputs found
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.Comment: Invited introductory article for a special issue of Comptes Rendus
Physiqu
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.Comment: Invited article for an IJMP volume dedicated to loop quantum gravit
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.Comment: Invited contribution for CQ
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.Comment: 5 page
Catastrophe : signe ou concept pour la physique contemporaine ?
Considérées dans le cadre de la pensée scientifique, les catastrophes ne sont pas seulement des accidents. Elles constituent – à commencer par le Big-Bang, la catastrophe originelle – également de précieux jalons qui révèlent les limites d’un modèle, d’un monde ou d’un discours. Limites intrinsèques et extrinsèques qui plaident pour un constructivisme épistémique assumé et libéré de tout ontocentrisme physique.Within the framework of science, catastrophes are not only accidents. They are also – beginning with the Big-Bang seen as the primitive catastrophe – precious flags revealing the limits of models, worlds or discourses. Those limits, both intrinsic and extrinsic, favour an epistemic relativism free from any kind of physical ontocentrism.Betrachtet man die Katastrophen im Rahmen des Denkens über die Wissenschaften, so handelt es sich nicht nur um zufällige Unfälle. Sie bilden auch – mit der Urkatastrophe angefangen, dem Urschall- wertvolle Richtlinien, die die Grenzen eines Modells, einer Welt oder eines Diskurses erkennen lassen
Detailed analysis of the curvature bounce: background dynamics and imprints in the CMB
If the spatial sections of the Universe are positively curved, extrapolating
the inflationary stage backward in time inevitably leads to a classical bounce.
This simple scenario, non-singular and free of exotic physics, deserves to be
investigated in details. The background dynamics exhibits interesting features
and is shown to be mostly insensitive to initial conditions as long as
observational consequences are considered. The primordial scalar power spectrum
is explicitly computed, for different inflaton potentials, and the subsequent
CMB temperature anisotropies are calculated. The results are compatible with
current measurements. Some deviations with respect to the standard paradigm can
however appear at large scales and we carefully disentangle what is associated
with the vacuum choice with what is more fundamentally due to the bounce
itself
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.Comment: 13 pages, 4 figure
Etude des perturbations cosmologiques et dérivation des observables en Gravité Quantique à Boucles
La relativité générale est la théorie rendant compte de la gravitation via une déformation de l'espace-temps. Son application à l'Univers permet, dans le modèle Lambda-CDM, de bien rentre compte des observations cosmologiques. Cependant, à l'échelle de Planck, la théorie ne fonctionne plus et s'avère incohérente. Pour résoudre ce problème, il est sans doute essentiel de tenir compte des effets quantiques. Depuis près d'un siècle, concilier relativité générale et mécanique quantique est considéré comme une priorité de la physique théorique. La tâche s'avère néanmoins extraordinairement difficile et cette thèse est consacrée à l'une des pistes les plus sérieuses : la gravitation quantique à boucles. Pour aller de l'avant dans cette démarche nécessaire mais complexe, des confrontation avec des données expérimentales seraient essentielles. Nous nous sommes ainsi intéressés aux perturbations cosmologiques générées dans ce cadre. Nous avons étudié en détails les conséquences phénoménologiques des corrections de cosmologie quantique à boucles aux modes tensoriels dans un modèle d'univers en rebond. Une analyse de Fisher a été développée pour comparer ces prédictions aux éventuelles futures observations. Pour les autres modes, nous nous sommes placés dans un formalisme spécifique incluant le calcul de contre-termes permettant de prévenir l'apparition d'anomalies dans la structure de l'algèbre des contraintes. Ce formalisme a été appliqué aux cas des perturbations vectorielles puis scalaires. Les équations du mouvement invariantes de jauges permettant de calculer les spectres ont alors été dérivées.General relativity describes gravity as a deformation of space-time. Applied to the Universe as a whole, it explains well cosmological observations in the lambda-CDM paradigm. However, at the Planck scale, the theory is not anymore self-consistent. It is most probably necessary to include quantum effects. For a century, this has been considered as one of the main challenges for theoretical physics. This is however an extremely difficult aim to reach and this thesis is devoted to one of the main proposal: Loop Quantum Gravity. To go ahead in the construction of any quantum theory of gravity, it would be most useful to compare predictions with observations. To this aim, we have studied cosmological perturbations in this framework. We have investigated into the details the phenomenological consequences of loop quantum cosmology corrections in a bouncing universe. A Fisher analysis was carried out to compare the predictions with future data. For the other modes, we have used a specific formalism to include counterterms that prevent anomalies from appearing in the algebra of constraints. This formalism was applied to vector and scalar perturbations. The gauge-invariant equations of motion leading to the calculation of measurable spectra were derived.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF
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 after a selection
that is based on the likelihood ratio test or on the apparent magnitude for
galaxies with 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.Comment: 19 pages, 25 figures, accepted for publication in Astronomy and
Astrophysics Astronomy and Astrophysics, 201
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