arXiv subjects

David Langlois

Publications and source records attributed to David Langlois.

At least 91 records · Page 5Linked to original sources

arXiv2008

Nonlinear perturbations of cosmological scalar fields

We present a covariant formalism for studying nonlinear perturbations of scalar fields. In particular, we consider the case of two scalar fields and introduce the notion of adiabatic and isocurvature covectors. We obtain differential equations governing the evolution of these two covectors, as well as the evolution equation for the covector associated with the curvature perturbation. The form of these equations is very close to the analogous equations obtained in the linear theory, but our equations are fully nonlinear and exact. As an application of our formalism, we expand these equations at second order in the perturbations. On large scales, we obtain a closed system of coupled scalar equations giving the evolution of the second-order adiabatic and entropy perturbations in terms of the first-order perturbations. These equations in general contain a nonlocal term which, however, rapidly decays in an expanding universe.

astro-ph↗

arXiv2008

Cosmological perturbations from multi-field inflation

We briefly review the standard derivation of the spectra of cosmological perturbations in the simplest models of inflation. We then consider models with several scalar fields, described by Lagrangians with an arbitrary dependence on the kinetic terms. We illustrate our general formalism with the case of multi-field DBI inflation.

astro-ph↗

arXiv2008

Primordial perturbations and non-Gaussianities in DBI and general multi-field inflation

We study cosmological perturbations in general inflation models with multiple scalar fields and arbitrary kinetic terms, with special emphasis on the multi-field extension of Dirac-Born-Infeld (DBI) inflation. We compute the second-order action governing the dynamics of linear perturbations in the most general case. Specializing to DBI, we show that the adiabatic and entropy modes propagate with a {\it common} effective sound speed and are thus amplified at sound horizon crossing. In the small sound speed limit, we find that the amplitude of the entropy modes is much higher than that of the adiabatic modes. We also derive, in the general case, the third-order action which is useful for studying primordial non-Gaussianities generated during inflation. In the DBI case, we compute the dominant contributions to non-Gaussianities, which depend on both the adiabatic and entropy modes.

hep-th↗

arXiv2008

Perturbations in generalized multi-field inflation

We study the linear perturbations of multi-field inflationary models governed by a Lagrangian which is a general function of the scalar fields and of a global kinetic term combining their spacetime gradients with an arbitrary field space metric. Our analysis includes k-inflation, DBI inflation and its multi-field extensions which have been recently studied. For this general class of models, we calculate the action to second order in the linear perturbations. We decompose the perturbations into an (instantaneous) adiabatic mode, parallel to the background trajectory, and entropy modes. We show that all the entropy modes propagate with the speed of light whereas the adiabatic mode propagates with an effective speed of sound. We also identify the specific combination of entropy modes which sources the curvature perturbation on large scales. We then study in some detail the case of two scalar fields: we write explicitly the equations of motion for the adiabatic and entropy modes in a compact form and discuss their quantum fluctuations and primordial power spectra.

hep-th↗

arXiv2008

Primordial fluctuations and non-Gaussianities in multi-field DBI inflation

We study Dirac-Born-Infeld (DBI) inflation models with multiple scalar fields. We show that the adiabatic and entropy modes propagate with a common effective sound speed and are thus amplified at the sound horizon crossing. In the small sound speed limit, we find that the amplitude of the entropy modes is much higher than that of the adiabatic modes. We show that this could strongly affect the observable curvature power spectrum as well as the amplitude of non-Gaussianities, although their shape remains as in the single-field DBI case.

hep-th↗

arXiv2007

Cosmological evolution of regularized branes in 6D warped flux compactifications

We study the cosmological evolution of extended branes in 6D warped flux compactification models. The branes are endowed with the three ordinary spatial dimensions, which are assumed to be homogeneous and isotropic, as well as an internal extra dimension compactified on a circle. We embed these codimension 1 branes in a static bulk 6D spacetime, whose geometry is a solution of 6D Einstein-Maxwell or Einstein-Maxwell-dilaton theories, corresponding to a warped flux compactification. The brane matter consists of a complex scalar field which is coupled to the bulk U(1) gauge field. In both models, we show that there is critical point which the brane cannot cross as it moves in the bulk. We study the cosmological behaviour, especially when the brane approaches this critical point or one of the two conical singularities. In the present setup where the bulk geometry is fixed, we find that the brane cosmology does not coincide with the standard one in the low energy limit.

hep-th↗

arXiv2007

Kaluza-Klein braneworld cosmology with static internal dimensions

We investigate the Kaluza-Klein braneworld cosmology from the point of view of observers on the brane. We first generalize the Shiromizu-Maeda-Sasaki (SMS) equations to higher dimensions. As an application, we study a (4+n)-dimensional brane with n dimensions compactified on the brane, in a (5+n)-dimensional bulk. By assuming that the size of the internal space is static, that the bulk energy-momentum tensor can be neglected, we determine the effect of the bulk geometry on the Kaluza-Klein braneworld. Then we derive the effective Friedmann equation on the brane. It turns out that the Friedmann equation explicitly depends on the equation of state, in contrast to the braneworld in a 5-dimensional bulk spacetime. In particular, in a radiation-dominated era, the effective Newton constant depends on the scale factor logarithmically. If we include a pressureless matter on the brane, this dependence disappears after the radiation-matter equality. This may be interpreted as stabilization of the Newton constant by the matter on the brane. Our findings imply that the Kaluza-Klein braneworld cosmology is quite different from the conventional Kaluza-Klein cosmology even at low energy.

hep-th↗

arXiv2007

Rotating spacetimes with a cosmological constant

We develop solution-generating techniques for stationary metrics with one angular momentum and axial symmetry, in the presence of a cosmological constant and in arbitrary spacetime dimension. In parallel we study the related lower dimensional Einstein-Maxwell-dilaton static spacetimes with a Liouville potential. For vanishing cosmological constant, we show that the field equations in more than four dimensions decouple into a four dimensional Papapetrou system and a Weyl system. We also show that given any four dimensional 'seed' solution, one can construct an infinity of higher dimensional solutions parametrised by the Weyl potentials, associated to the extra dimensions. When the cosmological constant is non-zero, we discuss the symmetries of the field equations, and then extend the well known works of Papapetrou and Ernst (concerning the complex Ernst equation) in four-dimensional general relativity, to arbitrary dimensions. In particular, we demonstrate that the Papapetrou hypothesis generically reduces a stationary system to a static one even in the presence of a cosmological constant. We also give a particular class of solutions which are deformations of the (planar) adS soliton and the (planar) adS black hole. We give example solutions of these techniques and determine the four-dimensional seed solutions of the 5 dimensional black ring and the Myers-Perry black hole.

gr-qc↗

arXiv2006

Nonlinear perturbations for dissipative and interacting relativistic fluids

We develop a covariant formalism to study nonlinear perturbations of dissipative and interacting relativistic fluids. We derive nonlinear evolution equations for various covectors defined as linear combinations of the spatial gradients of the local number of e-folds and of some scalar quantities characterizing the fluid, such as the energy density or the particle number density. For interacting fluids we decompose perturbations into adiabatic and entropy components and derive their coupled evolution equations, recovering and extending the results obtained in the context of the linear theory. For non-dissipative and noninteracting fluids, these evolution equations reduce to the conservation equations that we have obtained in recent works. We also illustrate geometrically the meaning of the covectors that we have introduced.

astro-ph↗

arXiv2006

Conserved non-linear quantities in cosmology

We give a detailed and improved presentation of our recently proposed formalism for non-linear perturbations in cosmology, based on a covariant and fully non-perturbative approach. We work, in particular, with a covector combining the gradients of the energy density and of the local number of e-folds to obtain a non-linear generalization of the familiar linear uniform density perturbation. We show that this covector obeys a remarkably simple conservation equation which is exact, fully non-linear and valid at all scales. We relate explicitly our approach to the coordinate-based formalisms for linear perturbations and for second-order perturbations. We also consider other quantities, which are conserved on sufficiently large scales for adiabatic perturbations, and discuss the issue of gauge invariance.

astro-ph↗

arXiv2005

Is our Universe brany ?

In brane-worlds, our universe is assumed to be a submanifold, or brane, embedded in a higher-dimensional bulk spacetime. Focusing on scenarios with a curved five-dimensional bulk spacetime, I discuss their gravitational and cosmological properties.

hep-th↗

arXiv2005

Evolution of non-linear cosmological perturbations

We define fully non-perturbative generalizations of the uniform density and comoving curvature perturbations, which are known, in the linear theory, to be conserved on sufficiently large scales for adiabatic perturbations. Our non-linear generalizations are defined geometrically, independently of any coordinate system. We give the equations governing their evolution on all scales. Also, in order to make contact with previous works on first and second order perturbations, we introduce a coordinate system and show that previous results can be recovered, on large scales, in a remarkably simple way, after restricting our definitions to first and second orders in a perturbative expansion.

astro-ph↗

arXiv2005

From heaviness to lightness during inflation

We study the quantum fluctuations of scalar fields with a variable effective mass during an inflationary phase. We consider the situation where the effective mass depends on a background scalar field, which evolves during inflation from being frozen into a damped oscillatory phase when the Hubble parameter decreases below its mass. We find power spectra with suppressed amplitude on large scales, similar to the standard massless spectrum on small scales, and affected by modulations on intermediate scales. We stress the analogies and differences with the parametric resonance in the preheating scenario. We also discuss some potentially observable consequences when the scalar field behaves like a curvaton.

astro-ph↗

arXiv2005

Kaluza-Klein gravitons are negative energy dust in brane cosmology

We discuss the effect of Kaluza-Klein (KK) modes of bulk metric perturbations on the second Randall-Sundrum (RS II) type brane cosmology, taking the possible backreaction in the bulk and on the brane into account. KK gravitons may be produced via quantum fluctuations during a de Sitter (dS) inflating phase of our brane universe. In an effective 4-dimensional theory in which one integrates out the extra-dimensional dependence in the action, KK gravitons are equivalent to massive gravitons on the brane with masses $m>3H/2$, where $H$ represents the expansion rate of a dS brane. Thus production of even a tiny amount of KK gravitons may eventually have a significant impact on the late-time brane cosmology. As a first step to quantify the effect of KK gravitons on the brane, we calculate the effective energy density and pressure for a single KK mode. Surprisingly, we find that a KK mode behaves as cosmic dust with a negative energy density on the brane. We note that the bulk energy density of a KK mode is positive definite and there occurs no singular phenomenon in the bulk.

gr-qc↗

arXiv2004

Gravitation and cosmology in brane-worlds

gr-qc↗

arXiv2004

Mixed inflaton and curvaton perturbations

A recent variant of the inflationary paradigm is that the ``primordial'' curvature perturbations come from quantum fluctuations of a scalar field, subdominant and effectively massless during inflation, called the ``curvaton'', instead of the fluctuations of the inflaton field. We consider the situation where the primordial curvature perturbations generated by the quantum fluctuations of an inflaton and of a curvaton field are of the same order of magnitude. We compute the curvature perturbation and its spectrum in this case and we discuss the observational consequences.

astro-ph↗

arXiv2004

Inflation, quantum fluctuations and cosmological perturbations

These lectures are intended to give a pedagogical introduction to the main current picture of the very early universe. After elementary reviews of general relativity and of the standard Big Bang model, the following subjects are discussed: inflation, the classical relativistic theory of cosmological perturbations and the generation of perturbations from scalar field quantum fluctuations during inflation.

hep-th↗

arXiv2004

Cosmology of Brane-worlds

This talk presents an overview of the brane cosmology scenario, based on the idea that our Universe is a 3-brane embedded in a five-dimensional anti-de Sitter bulk space-time. Special emphasis is put on the novel features of this scenario: an unconventional cosmological evolution at high energy densities, i.e. in the early universe, and dark radiation, that embodies the gravitational effects of the bulk onto the brane, and which is shown to be generated during the high energy era by the production of bulk gravitons.

astro-ph↗