arXiv subjects

Jean-Philippe Uzan

Publications and source records attributed to Jean-Philippe Uzan.

156 records · Page 9Linked to original sources

arXiv2000

Photon mixing in universes with large extra-dimensions

In presence of a magnetic field, photons can mix with any particle having a two-photon vertex. In theories with large compact extra-dimensions, there exists a hierachy of massive Kaluza-Klein gravitons that couple to any photon entering a magnetic field. We study this mixing and show that, in comparison with the four dimensional situation where the photon couples only to the massless graviton, the oscillation effect may be enhanced due to the existence of a large number of Kaluza-Klein modes. We give the conditions for such an enhancement and then investigate the cosmological and astrophysical consequences of this phenomenon; we also discuss some laboratory experiments. Axions also couple to photons in the same way; we discuss the effect of the existence of bulk axions in universes with large extra-dimensions. The results can also be applied to neutrino physics with extra-dimensions.

hep-ph↗

arXiv1999

Cosmic microwave background anisotropies seeded by coherent topological defects : a semi-analytic approach

We consider a perfectly homogeneous, isotropic and spatially flat universe which undergoes a sudden phase transition producing topological defects. We assume that these defects form a coherent network which scales like the background density during the radiation and matter dominated eras, and describe them in terms of a few free functions. We carefully model the loss of scaling invariance during the transition from radiation to matter dominated era. We choose a simple set for the free functions, compute the microwave background temperature anisotropies generated by such a network, and compare our results to previous calculations.

astro-ph↗

arXiv1999

A New Crystallographic Method for Detecting Space Topology

Multi-connected universe models with space identification scales smaller than the size of the observable universe produce topological images of cosmic sources. We generalise to locally hyperbolic spaces the crystallographic method, aimed to detect the topology from three-dimensional catalogs of cosmological objects. Our new method is based on the construction of a {\it collecting-correlated-pair} technique which enhances the topological signature and can make it detectable. The main idea is that in multi-connected universes, equal distances occur more often than by chance. We present an idealised version of this method as well as numerical simulations, we discuss the statistical relevance of the expected signature and we show how the extraction of a topological signal may also lead to a precise determination of the cosmological parameters. A more realistic version of the method which takes account of uncertainties in the position and velocity data is then discussed. We apply our technique to presently available data, namely a quasar catalog, and discuss the significance of the result. We show how the improved crystallographic method both competes with the two--dimensional methods based on cosmic microwave background analyses, and suffers from the same drawback: the shortage of present observational data.

astro-ph↗

arXiv1999

Cosmological scaling solutions of non-minimally coupled scalar fields

We study the existence and stability of cosmological scaling solutions of a non-minimally coupled scalar field evolving in either an exponential or inverse power law potential. We show that for inverse power law potentials there exist scaling solutions the stability of which does not depend on the coupling constant $ξ$. We then study the more involved case of exponential potentials and show that the scalar field will assymptotically behaves as a barotropic fluid when $ξ\ll 1$. The general case $ξ\not\ll 1$ is then discussed an we illustrate these results by some numerical examples.

gr-qc↗

arXiv1999

Topological lens effects in universes with non-Euclidean compact spatial sections

Universe models with compact spatial sections smaller than the observable universe produce a topological lens effect. Given a catalog of cosmic sources, we estimate the number of topological images in locally hyperbolic and locally elliptic spaces, as a function of the cosmological parameters, of the volume of the spatial sections and of the catalog depth. Next we apply the crystallographic method, aimed to detect a topological signal in the 3D distance histogram between images, to compact hyperbolic models. Numerical calculations in the Weeks manifold allows us to check the absence of crystallographic signature of topology, due to the fact that the number of copies of the fundamental domain in the observable covering space is low and that the points are not moved the same distance by the holonomies of space.

astro-ph↗

arXiv1998

Cosmological Perturbations seeded by Topological Defects: Setting the Initial Conditions

We consider a perfectly homogeneous and isotropic universe which undergoes a sudden phase transition. If the transition produces topological defects, which we assume, perturbations in the geometry and the cosmic fluid also suddenly appear. We apply the standard general relativistic junction conditions to match the pre- and post- transition eras and thus set the initial conditions for the perturbations. We solve their evolution equations analytically in the case when the defects act as a coherent source and their density scales like the background density. We show that isocurvature as well as adiabatic perturbations are created, in a ratio which is independent of the detailed properties of the defects. We compare our result to the initial conditions currently used in the literature and show how the cosmic fluid naturally "compensates" for the presence of the defects.

gr-qc↗

arXiv1998

The No-defect Conjecture: Cosmological Implications

When the topology of the universe is non trivial, it has been shown that there are constraints on the network of domain walls, cosmic strings and monopoles. I generalize these results to textures and study the cosmological implications of such constraints. I conclude that a large class of multi-connected universes with topological defects accounting for structure formation are ruled out by observation of the cosmic microwave background.

gr-qc↗

arXiv1998

Conservation Laws and Cosmological Perturbations in Curved Universes

When working in synchronous gauges, pseudo-tensor conservation laws are often used to set the initial conditions for cosmological scalar perturbations, when those are generated by topological defects which suddenly appear in an up to then perfectly homogeneous and isotropic universe. However those conservation laws are restricted to spatially flat (K=0) Friedmann-Lema\^ıtre spacetimes. In this paper, we first show that in fact they implement a matching condition between the pre- and post- transition eras and, in doing so, we are able to generalize them and set the initial conditions for all $K$. Finally, in the long wavelength limit, we encode them into a vector conservation law having a well-defined geometrical meaning.

gr-qc↗

arXiv1998

Conservation Laws for Cosmological Perurbations

We briefly recall the problem of defining conserved quantities such as energy in general relativity, and the solution by introducing a symmetric background. We apply the general formalism to perturbed Robertson-Walker spacetimes with de Sitter geometry as background. We relate the obtained conserved quantities to Traschen's integral constraints and mention a few applications in cosmology.

gr-qc↗

arXiv1998

Dynamics of Relativistic Interacting Gases : from a Kinetic to a Fluid Description

Starting from a microscopic approach, we develop a covariant formalism to describe a set of interacting gases. For that purpose, we model the collision term entering the Boltzmann equation for a class of interactions and then integrate this equation to obtain an effective macroscopic description. This formalism will be useful to study the cosmic microwave background non-perturbatively in inhomogeneous cosmologies. It should also be useful for the study of the dynamics of the early universe and can be applied, if one considers fluids of galaxies, to the study of structure formation.

gr-qc↗

arXiv1996

Integral Constraints On cosmological Perturbations and their Energy

We show the relation between Traschen's integral equations and the energy, and ``position of the centre of mass'', of the matter perturbations in a Robertson-Walker spacetime. When the perturbations are ``localised'' we get a set of integral constraints that includes hers. We illustrate them on a simple example.

gr-qc↗

arXiv1996

The no-defect conjecture in cosmic crystallography

The topology of space is usually assumed simply connected, but could be multi-connected. We review in the latter case the possibility that topological defects arising at high energy phase transitions might still be present and find that either they are very unlikely to form at all, or space is effectively simply connected on scales up to the horizon size.

gr-qc↗