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B. Boisseau

Publications and source records attributed to B. Boisseau.

At least 19 recordsLinked to original sources
arXiv2019

Inflationary models that generalize the constant roll constraint

Recently a class of inflationary models satisfying the constant rate of roll constraint, $\ddotϕ+(3+α)H\dotϕ=0$, has been studied and compared with the latest cosmological observational data. We consider the broader class of constraints $ \ddotϕ+\frac{α_1}{2}\dotϕ+α_2 H\dotϕ=0$ and find an exact particular solution, without initial singularity, which is an attractor of the dynamics governed by a positive periodic potential $V(ϕ)=E+ F \cos (Ωϕ)+G \cos (2 Ωϕ)$. The spectral index $n_s$ and the tensor to scalar ratio $r$ are consistent with the recent observational data.

gr-qc↗
arXiv2016

Bouncing Universes in Scalar-Tensor Gravity Around Conformal Invariance

We consider the possibility to produce a bouncing universe in the framework of scalar-tensor gravity when the scalar field has a nonconformal coupling to the Ricci scalar. We prove that bouncing universes regular in the future with essentially the same dynamics as for the conformal coupling case do exist when the coupling deviates slightly from it. This is found numerically for more substantial deviations as well. In some cases however new features are found like the ability of the system to leave the effective phantom regime.

gr-qc↗
arXiv2015

Scalar Field Cosmologies With Inverted Potentials

Regular bouncing solutions in the framework of a scalar-tensor gravity model were found in a recent work. We reconsider the problem in the Einstein frame (EF) in the present work. Singularities arising at the limit of physical viability of the model in the Jordan frame (JF) are either of the Big Bang or of the Big Crunch type in the EF. As a result we obtain integrable scalar field cosmological models in general relativity (GR) with inverted double-well potentials unbounded from below which possess solutions regular in the future, tending to a de Sitter space, and starting with a Big Bang. The existence of the two fixed points for the field dynamics at late times found earlier in the JF becomes transparent in the EF.

gr-qc↗
arXiv2015

Bouncing Universes in Scalar-Tensor Gravity Models admitting Negative Potentials

We consider the possibility to produce a bouncing universe in the framework of scalar-tensor gravity models in which the scalar field potential may be negative, and even unbounded from below. We find a set of viable solutions with nonzero measure in the space of initial conditions passing a bounce, even in the presence of a radiation component, and approaching a constant gravitational coupling afterwards. Hence we have a model with a minimal modification of gravity in order to produce a bounce in the early universe with gravity tending dynamically to general relativity (GR) after the bounce.

gr-qc↗
arXiv2011

Exact cosmological solution of a Scalar-Tensor Gravity theory compatible with the $ΛCDM$ model

We consider the massive scalar-tensor theory in the Jordan frame $F(Φ) =K^{2}Φ^2$ and $U(Φ) =(1/2)m^{2}Φ^2$, where $F(Φ)$ corresponds to a constant Brans-Dicke parameter $ω_{BD}=1/4K^2$. The constraint of the Solar System experiments is $K^2<(1/400)^2$. For dustlike matter in a spatially flat homogeneous isotropic universe, we reduce the equations of motion to a system of two differential equations of first order which can be exactly solved. We obtain simple and explicit expressions for $\frac{Φ(z)}{Φ(0)}$ and $\frac{H(z)}{H_{0}}$ that depend only on two parameters, $K^2$ and $Ω_{m,0}$. For $K\leq1/400$ the expansion rate $H(z)$ can be practically superposed on the $Λ$CDM solution $H_Λ(z)$, up to high redshift $z$, but the equation of state $w_{DE}(z)$ of the dark energy is not constant: it presents a very slight crossing of the phantom divide line $w=-1$ in the neighborhood of $z=0$ and becomes very slightly positive at high redshifts.

astro-ph.CO↗
arXiv2008

Analytical approximation schemes for solving exact renormalization group equations. II Conformal mappings

We present a new efficient analytical approximation scheme to two-point boundary value problems of ordinary differential equations (ODEs) adapted to the study of the derivative expansion of the exact renormalization group equations. It is based on a compactification of the complex plane of the independent variable using a mapping of an angular sector onto a unit disc. We explicitly treat, for the scalar field, the local potential approximations of the Wegner-Houghton equation in the dimension $d=3$ and of the Wilson-Polchinski equation for some values of $d\in ] 2,3] $. We then consider, for $d=3$, the coupled ODEs obtained by Morris at the second order of the derivative expansion. In both cases the fixed points and the eigenvalues attached to them are estimated. Comparisons of the results obtained are made with the shooting method and with the other analytical methods available. The best accuracy is reached with our new method which presents also the advantage of being very fast. Thus, it is well adapted to the study of more complicated systems of equations.

hep-th↗
arXiv2007

Analytical approximation schemes for solving exact renormalization group equations in the local potential approximation

The relation between the Wilson-Polchinski and the Litim optimized ERGEs in the local potential approximation is studied with high accuracy using two different analytical approaches based on a field expansion: a recently proposed genuine analytical approximation scheme to two-point boundary value problems of ordinary differential equations, and a new one based on approximating the solution by generalized hypergeometric functions. A comparison with the numerical results obtained with the shooting method is made. A similar accuracy is reached in each case. Both two methods appear to be more efficient than the usual field expansions frequently used in the current studies of ERGEs (in particular for the Wilson-Polchinski case in the study of which they fail).

hep-th↗
arXiv2004

Vortex in a weakly relativistic Bose gas at zero temperature and relativistic fluid approximation

The Bogoliubov procedure in quantum field theory is used to describe a relativistic almost ideal Bose gas at zero temperature. Special attention is given to the study of a vortex. The radius of the vortex in the field description is compared to that obtained in the relativistic fluid approximation. The Kelvin waves are studied and, for long wavelengths, the dispersion relation is obtained by an asymptotic matching method and compared with the non relativistic result.

gr-qc↗
arXiv2000

Reconstruction of a scalar-tensor theory of gravity in an accelerating universe

The present acceleration of the Universe strongly indicated by recent observational data can be modeled in the scope of a scalar-tensor theory of gravity. We show that it is possible to determine the structure of this theory (the scalar field potential and the functional form of the scalar-gravity coupling) along with the present density of dustlike matter from the following two observable cosmological functions: the luminosity distance and the linear density perturbation in the dustlike matter component as functions of redshift. Explicit results are presented in the first order in the small inverse Brans-Dicke parameter 1/omega.

gr-qc↗
arXiv1999

Dynamics of a relativistic Rankine vortex for a two-constituent superfluid in a weak perturbation of cylindrical symmetry

From a recent study of a stationary cylindrical solution for a relativistic two-constituent superfluid at low temperature limit, we propose to specify this solution under the form of a relativistic generalisation of a Rankine vortex (Potential vortex whose the core has a solid body rotation).Then we establish the dynamics of the central line of this vortex by supposing that the deviation from the cylindrical configuration is weak in the neighbourhood of the core of the vortex. In "stiff" material the Nambu-Goto equations are obtained.

gr-qc↗
arXiv1998

Dynamics of a global string with large Higgs boson mass

We consider a self-gravitating string generated by a global vortex solution in general relativity. We investigate the Einstein and field equations of a global vortex in the region of its central line and at a distance from the centre of the order of the inverse of its Higgs boson mass. By combining the two we establish by a limiting process of large Higgs mass the dynamics of a self-gravitating global string. Under our assumptions the presence of gravitation restricts the world sheet of the global string to be totally geodesic.

gr-qc↗
arXiv1997

Can a self-gravitating thin cosmic string obey the Nambu-Goto dynamics ?

We assume that a self-gravitating string is locally described by a thin tube of matter represented by a ``smoothed conical metric''. If we impose a specific constraint on the model of string then its central line obeys the Nambu-Goto dynamics in the limit where the radius of the tube tends to zero. If no constraint is added then the world sheet of the central line is totally geodesic.

gr-qc↗
arXiv1997

Dynamics of a self-gravitating thin string in scalar-tensor theories of gravitation

We examine the dynamics of a self-gravitating string in the scalar-tensor theories of gravitation by considering a thin tube of matter to describe it. For a class of solutions, we obtain in the generic case that the extrinsic curvature of the world sheet of the central line is null in the limit where the radius of the string tends to zero. However, if we impose a specific constraint on the behaviour of the solution then we find that only the mean curvature of the world sheet of the central line vanishes which is just the Nambu-Goto dynamics. This analysis can include the massless dilatonic theories of gravity.

gr-qc↗
arXiv1997

Dynamics of a self-gravitating thin cosmic string

We assume that a self-gravitating thin string can be locally described by what we shall call a smoothed cone. If we impose a specific constraint on the model of the string, then its central line obeys the Nambu-Goto equations. If no constraint is added, then the worldsheet of the central line is a totally geodesic surface.

gr-qc↗
arXiv1995

Orbits, forces and accretion dynamics near spinning black holes

We analyze the relativistic dynamical properties of Keplerian and non-Keplerian circular orbits in a general axisymmetric and stationary gravitational field, and discuss the implications for the stability of co- and counter-rotating accretion disks and tori surrounding a spinning black hole. Close to the horizon there are orbital peculiarities which can seem counterintuitive, but are elucidated by formulating the dynamics in terms of the orbital velocity actually measured by a local, zero-angular-momentum observer.

gr-qc↗