arXiv subjects

Jean-Philippe Uzan

Publications and source records attributed to Jean-Philippe Uzan.

At least 145 records · Page 8Linked to original sources

arXiv2002

Eigenmodes of 3-dimensional spherical spaces and their application to cosmology

This article investigates the computation of the eigenmodes of the Laplacian operator in multi-connected three-dimensional spherical spaces. General mathematical results and analytical solutions for lens and prism spaces are presented. Three complementary numerical methods are developed and compared with our analytic results and previous investigations. The cosmological applications of these results are discussed, focusing on the cosmic microwave background (CMB) anisotropies. In particular, whereas in the Euclidean case too small universes are excluded by present CMB data, in the spherical case there will always exist candidate topologies even if the total energy density parameter of the universe is very close to unity.

gr-qc↗

arXiv2002

Gradient formula for linearly self-interacting branes

The computation of long range linear self-interaction forces in string and higher dimensional brane models requires the evaluation of the gradients of regularised values of divergent self-interaction potentials. It is shown that the appropriately regularised gradient in directions orthogonal to the brane surface will always be obtainable simply by multiplying the regularised potential components by just half the trace of the second fundamental tensor, except in the hypermembrane case for which the method fails. Whatever the dimension of the background this result is valid provided the codimension is two (the hyperstring case) or more, so it can be used for investigating brane-world scenarios with more than one extra space dimension.

hep-th↗

arXiv2001

Dimming of Supernovae by Photon-Pseudoscalar Conversion and the Intergalactic Plasma

Recently Csaki, Kaloper and Terning (hep-ph/0111311) suggested that the observed dimming of distant type Ia supernovae may be a consequence of mixing of the photons with very light axions. We point out that the effect of the plasma, in which the photons are propagating, must be taken into account. This effect changes the oscillation probability and renders the dimming frequency-dependent, contrary to observations. One may hope to accommodate the data by averaging the oscillations over many different coherence domains. We estimate the effect of coherence loss, either due to the inhomogeneities of the magnetic field or of the intergalactic plasma. These estimates indicate that the achromaticity problem can be resolved only with very specific, and probably unrealistic, properties of the intergalactic medium.

hep-ph↗

arXiv2001

The stress-energy tensor for trans-Planckian cosmology

This article presents the derivation of the stress-energy tensor of a free scalar field with a general non-linear dispersion relation in curved spacetime. This dispersion relation is used as a phenomelogical description of the short distance structure of spacetime following the conventional approach of trans-Planckian modes in black hole physics and in cosmology. This stress-energy tensor is then used to discuss both the equation of state of trans-Planckian modes in cosmology and the magnitude of their backreaction during inflation. It is shown that gravitational waves of trans-Planckian momenta but subhorizon frequencies cannot account for the form of cosmic vacuum energy density observed at present, contrary to a recent claim. The backreaction effects during inflation are confirmed to be important and generic for those dispersion relations that are liable to induce changes in the power spectrum of metric fluctuations. Finally, it is shown that in pure de Sitter inflation there is no modification of the power spectrum except for a possible magnification of its overall amplitude independently of the dispersion relation.

hep-th↗

arXiv2001

A Dynamical Study of the Friedmann Equations

Cosmology is an attracting subject for students but usually difficult to deal with if general relativity is not known. In this article, we first recall the Newtonian derivation of the Friedmann equations which govern the dynamics of our universe and discuss the validity of such a derivation. We then study the equations of evolution of the universe in terms of a dynamical system. This sums up the different behaviors of our universe and enables to address some cosmological problems.

physics.ed-ph↗

arXiv2001

Simulated gravity without true gravity in asymmetric brane-world scenarios

This article investigates asymmetric brane-world scenarios in the limit when the bulk gravity is negligible. We show that, even when true self gravity is negligible, local mass concentrations will be subject to a mutual attraction force which simulates the effect of Newtonian gravity in the non-relativistic limit. Cosmological and also post-Newtonian constraints are examined.

gr-qc↗

arXiv2001

Topological Lensing in Spherical Spaces

This article gives the construction and complete classification of all three-dimensional spherical manifolds, and orders them by decreasing volume, in the context of multiconnected universe models with positive spatial curvature. It discusses which spherical topologies are likely to be detectable by crystallographic methods using three-dimensional catalogs of cosmic objects. The expected form of the pair separation histogram is predicted (including the location and height of the spikes) and is compared to computer simulations, showing that this method is stable with respect to observational uncertainties and is well suited for detecting spherical topologies.

gr-qc↗

arXiv2001

Einstein equations for an asymmetric brane-world

We consider a brane-world of co-dimension one without the reflection symmetry that is commonly imposed between the two sides of the brane. Using the coordinate-free formalism of the Gauss-Codacci equations, we derive the effective Einstein equations by relating the local curvature to the matter on the brane in the case when its bare tension is much larger than the localized matter, and hence show that Einstein gravity is a natural consequence of such models in the weak field limit. We find agreement with the recently derived cosmological case, which can be solved exactly, and point out that such models can be realized naturally in the case where there is a minimally coupled form field in the bulk.

hep-th↗

arXiv2001

Reflection symmetry breaking scenarios with minimal gauge form coupling in brane world cosmology

This article synthesises and extends recent work on the cosmological consequences of dropping the usual Z_2 reflection symmetry postulate in brane world scenarios. It is observed that for a cosmological model of homogeneous isotropic type, the relevant generalised Birkhoff theorem establishing staticity of the external vacuum in the maximally symmetric ``bulk'' outside a freely moving world brane will remain valid for the case of motion that is forced by minimal (generalised Wess Zumino type) coupling to an external antisymmetric gauge field provided its kinetic action contribution has the usual homogeneous quadratic form. This means that the geometry on each side of the brane worldsheet will still be of the generalised Schwarzschild anti de Sitter type. The usual first integrated Friedmann equation for the Hubble expansion rate can thereby be straightforwardly generalised by inclusion of new terms involving 2 extra parameters respectively measuring the strength of the gauge coupling and the degree of deviation from reflection symmetry. Some conceivable phenomenological implications are briefly outlined, and corresponding limitations are derived for possible values of relevant parameters.

gr-qc↗

arXiv2000

Lensing at cosmological scales: a test of higher dimensional gravity

Recent developments in gravitational lensing astronomy have paved the way to genuine mappings of the gravitational potential at cosmological scales. We stress that comparing these data with traditional large scale structure surveys will provide us with a test of gravity at such scales. These constraints could be of great importance in the framework of higher dimensional cosmological models.

hep-ph↗

arXiv2000

Cosmic Strings Lens Phenomenology Revisited

We present investigations of lens phenomenological properties of cosmic strings for deep galaxy surveys. General results that have obtained for lineic energy distribution are presented first. We stress that generically the local convergence always vanishes in presence of strings although there might be some significant distortions. We then propose a simplified model of strings, we call ``Poisson strings'', for which exhaustive investigations can be done either numerically or analytically.

astro-ph↗

arXiv2000

Cosmic Strings Lens Phenomenology: General Properties of Distortion Fields

We reconsider the general properties of gravitational lensing effects induced by cosmic string systems, taking into account their equation of state and motion equations. We explicitly show that the deflection patterns induced by a string is equivalent to the one of a lineic and static distribution of matter. We then rigorously show that the convergence part of the deformation field is always zero, except on the intersection of the string worldsheet and the observer past light cone, extending previous results obtained in peculiar cases. Phenomenological consequences of this result on multiple image systems are investigated.

astro-ph↗

arXiv2000

Cosmic Strings Lens Phenomenology: Model of Poisson Energy Distribution

We present a novel approach for investigating lens phenomenology of cosmic strings in order to elaborate detection strategies in galaxy deep field images. To account for the complexity of the projected energy distribution of string networks we assume their lens effects to be similar to those of a straight string carrying a {\em random} lineic energy distribution. In such a model we show that, unlike the case of uniform strings, critical phenomena naturally appear. We explore the properties of the critical lines and caustics. In particular, assuming that the energy coherence length along the string is much smaller than the observation scale, we succeeded in computing the total length of critical lines per unit string length and found it to be $4/\sqrt{3} {\bf E}(3/4)$. The length of the associated caustic lines can also be computed to be $16/(π\sqrt{3}) {\bf E}(3/4)$. The picture we obtain here for the phenomenology of cosmic string detection is clearly at variance with common lore.

astro-ph↗

arXiv2000

Twin paradox and space topology

If space is compact, then a traveller twin can leave Earth, travel back home without changing direction and find her sedentary twin older than herself. We show that the asymmetry between their spacetime trajectories lies in a topological invariant of their spatial geodesics, namely the homotopy class. This illustrates how the spacetime symmetry invariance group, although valid {\it locally}, is broken down {\it globally} as soon as some points of space are identified. As a consequence, any non--trivial space topology defines preferred inertial frames along which the proper time is longer than along any other one.

physics.class-ph↗

arXiv2000

Quintessence and Gravitational Waves

We investigate some aspects of quintessence models with a non-minimally coupled scalar field and in particular we show that it can behave as a component of matter with $-3 \lesssim P/ρ\lesssim 0$. We study the properties of gravitational waves in this class of models and discuss their energy spectrum and the cosmic microwave background anisotropies they induce. We also show that gravitational waves are damped by the anisotropic stress of the radiation and that their energy spectrum may help to distinguish between inverse power law potential and supergravity motivated potential. We finish by a discussion on the constraints arising from their density parameter $Ω_\GW$.

astro-ph↗

arXiv2000

New Developments in the Search for the Topology of the Universe

Multi-connected Universe models with space idenfication scales smaller than the size of the observable universe produce topological images in the catalogs of cosmic sources. In this review, we present the recent developments for the search of the topology of the universe focusing on three dimensional methods. We present the crystallographic method, we give a new lower bound on the size of locally Euclidean multi-connected universe model of $3000 h^{-1} \hbox{Mpc}$ based on this method and a quasar catalog, we discuss its successes and failures, and the attemps to generalise it. We finally introduce a new statistical method based on a collecting correlated pair (CCP) technique.

gr-qc↗

arXiv2000

Limits of crystallographic methods for detecting space topology

We investigate to what extent the cosmic crystallographic methods aimed to detect the topology of the universe using catalogues of cosmic objects would be damaged by various observational uncertainties. We find that the topological signature is robust in the case of Euclidean spaces, but is very fragile in the case of compact hyperbolic spaces. Comparing our results to the presently accepted range of values for the curvature parameters, the best hopes for detecting space topology rest on elliptical space models.

astro-ph↗

arXiv2000

Skewness as a probe of non-Gaussian initial conditions

We compute the skewness of the matter distribution arising from non-linear evolution and from non-Gaussian initial perturbations. We apply our result to a very generic class of models with non-Gaussian initial conditions and we estimate analytically the ratio between the skewness due to non-linear clustering and the part due to the intrinsic non-Gaussianity of the models. We finally extend our estimates to higher moments.

astro-ph↗