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Stephane Fay

Publications and source records attributed to Stephane Fay.

At least 19 recordsLinked to original sources
arXiv2020

$\Lambda CDM$ periodic cosmology

We examine the possibility that Universe expansion be made of some $\Lambda CDM$ expansions repeating periodically, separated by some inflation and radiation dominated phases. This so-called $\Lambda CDM$ periodic cosmology is motivated by the possibility that inflation and the present phase of accelerated expansion be due to the same dark energy. Then, in a phase space showing the variation of matter density parameter $\Omega_m$ with respect to this of the radiation $\Omega_r$, the curve $\Omega_m(\Omega_r)$ looks like a closed trajectory that Universe could run through forever. In this case, the end of the expansion acceleration of the $\Lambda CDM$ phase is the beginning of a new inflation phase. We show that such a scenario implies the coupling of matter and/or radiation to dark energy. We consider the simplest of these $\Lambda CDM$ periodic models i.e. a vacuum energy coupled to radiation. From matter domination phase to today, it behaves like a $\Lambda CDM$ model, then followed by an inflation phase. But a sudden and fast decay of the dark energy into radiation periodically ends the expansion acceleration. This leads to a radiation dominated Universe preceding a new $\Lambda CDM$ type expansion. The model is constrained with Markov Chain Monte Carlo simulations using supernovae, Hubble expansion, BAO and CMB data and fits the data as well as the $\Lambda CDM$ one.

gr-qc↗
arXiv2019

Phase space of static wormholes sustained by an isotropic perfect fluid

A phase space is built that allows to study, classify and compare easily large classes of static spherically symmetric wormholes solutions, sustained by an isotropic perfect fluid in General Relativity. We determine the possible locations of equilibrium points, throats and curvature singularities in this phase space. Throats locations show that the spatial variation of the gravitational redshift at the throat of a static spherically symmetric wormhole sustained by an isotropic perfect fluid is always diverging, generalising the result that there is no such wormhole with zero-tidal force. Several specific static spherically symmetric wormholes models are studied. A vanishing density model leads to an exact solution of the field equation allowing to test our dynamical system formalism. It also shows how to extend it to the description of static black holes. Hence, the trajectory of the Schwarzschild black hole is determined. The static spherically symmetric wormhole solutions of several usual isotropic dark energy (generalised Chaplygin gas, constant, linear and Chevallier-Polarski-Linder equations of state) and dark matter (Navarro-Frenk-White profile) models are considered. They show various behaviours far from the throat: singularities, spatial flatness, cyclic behaviours, etc. None of them is asymptotically Minkowski flat. This discards the natural formation of static spherically symmetric and isotropic wormholes from these dark fluids. Last we consider a toy model of an asymptotically Minkowski flat wormhole that is a counterexample to a recent theorem claiming that a static wormhole sustained by an isotropic fluid cannot be asymptotically flat on both sides of its throat.

gr-qc↗
arXiv2016

Constraints from growth-rate data on some coupled dark energy models mimicking a $\Lambda CDM$ expansion

The $\Lambda CDM$ expansion could be mimicked by a dark energy coupled to matter. Then, the equation of state $\bar w$ and coupling $\bar Q$ of this coupled dark energy could not be constrained by observations of the Hubble function alone. Also, in this paper, we determine the constraints on two such coupled dark energy models considering some current and forecast Euclid-like growth-rate data and assuming the prior on the $\Lambda CDM$ dark matter density parameter today $\Omega_{m0}=0.295\pm 0.04$. The first model is defined by a constant equation of state. We find that at $2\sigma$, $\bar w=-1.02_{-0.22}^{+0.06}$ and the coupling function $\bar Q_0$ today is $\bar Q_0H_0^{-3}=0.057_{-0.148}^{+0.353}$ with $H_0$ the Hubble constant. The second model is defined by a varying equation of state $\bar w=\bar w_a-\bar w_b\ln(1+z)$, with $z$ the redshift and $(\bar w_a,\bar w_b)$, two constants. We find that at $2\sigma$, $\bar w_a=-0.99_{-0.90}^{+0.17}$, $\bar w_b=-0.04_{-1.17}^{+0.31}$ and $\bar Q_0H_0^{-3}=0.0002_{-0.18}^{+1.35}$. These constraints on coupled dark energy agreed with a $\Lambda CDM$ model but are too poor to discard confidently a coupled dark energy different from vacuum but mimicking a $\Lambda CDM$ expansion.

astro-ph.CO↗
arXiv2015

An easy journey from Galilean to General Relativity

We explain in a very concise way the basic principles that lead from Galilean to General Relativity to make them understandable to students or general audience, even with little knowledge in physics and mathematics.

physics.pop-ph↗
arXiv2015

A coupled vacuum energy model producing endless alternated phases of accelerated and decelerated expansion

We consider a flat Universe filled with a vacuum energy coupled to matter and radiation by respectively positive coupling functions $Q_m$ and $Q_r$. We require that these functions be such as Universe exits from inflation to go to a radiation dominated epoch and allow to reproduce the observed $\Lambda CDM$ expansion after this last epoch. These requirements lead to some necessary constraints on the coupling functions. We then look at one of the simplest forms of $Q_m$ and $Q_r$ able to satisfy them. The cosmological model thus defined describes a Universe with an initial singularity and endless alternated periods of accelerated and decelerated expansion (one of them being our $\Lambda CDM$ Universe) unifying inflation and present time expansion acceleration.

gr-qc↗
arXiv2015

A scientometric study of General Relativity and Quantum Cosmology from 2000 to 2012

2015 is the centennial of Einstein General Relativity. On this occasion, we examine the General Relativity and Quantum Cosmology (GRQC) field of research by analysing 38291 papers uploaded on the electronic archives arXiv.org from 2000 to 2012. We establish a map of the countries contributing to GRQC in 2012. We determine the main journals publishing GRQC papers and which countries publish in which journals. We find that more and more papers are written by groups (instead of single) of authors with more and more international collaborations. There are huge differences between countries. Hence Russia is the country where most of papers are written by single authors whereas Canada is one of the countries where the most of papers imply international collaborations. We also study authors mobility, determining how some groups of authors spread worldwide with time in different countries. The largest mobilities are between USA-UK and USA-Germany. Countries attracting the most of GRQC authors are Netherlands and Canada whereas those undergoing a brain drain are Italy and India. There are few mobility between Europe and Asia contrarily to mobility between USA and Asia.

physics.hist-ph↗
arXiv2015

Ten scenarios from early radiation to late time acceleration with a minimally coupled dark energy

We consider General Relativity with matter, radiation and a minimally coupled dark energy defined by an equation of state w. Using dynamical system method, we find the equilibrium points of such a theory assuming an expanding Universe and a positive dark energy density. Two of these points correspond to classical radiation and matter dominated epochs for the Universe. For the other points, dark energy mimics matter, radiation or accelerates Universe expansion. We then look for possible sequences of epochs describing a Universe starting with some radiation dominated epoch(s) (mimicked or not by dark energy), then matter dominated epoch(s) (mimicked or not by dark energy) and ending with an accelerated expansion. We find ten sequences able to follow this Universe history without singular behaviour of w at some saddle points. Most of them are new in dark energy literature. To get more than these ten sequences, w has to be singular at some specific saddle equilibrium points. This is an unusual mathematical property of the equation of state in dark energy literature, whose physical consequences tend to be discarded by observations. This thus distinguishes the ten above sequences from an infinity of ways to describe Universe expansion.

gr-qc↗
arXiv2007

f(R) gravity theories in Palatini formalism: cosmological dynamics and observational constraints

We make a systematic study of the cosmological dynamics for a number of f(R) gravity theories in Palatini formalism. We find a number of interesting results: (i) models based on theories of the type (a) f(R)=R-beta R^n and (b) f(R)=R+alpha ln R -beta, unlike the metric formalism, are capable of producing the sequence of radiation-dominated, matter-dominated and de-Sitter periods, and (ii) models based on theories of the type (c) f(R)=R+alpha R^m -beta R^n can produce early as well as late accelerating phases. However for the classes of models considered here, we have been unable to find the sequence of all four dynamical epochs required to account for the complete cosmological dynamics, even though three out of four phases are possible. We also place observational constraints on these models using the recently released supernovae data (SNLS) as well as the baryon acoustic oscillation peak and the CMB shift parameter. The best-fit values are found to be n=0.027, alpha=4.63 for the models based on (a) and alpha=0.11, beta=4.62 for the models based on (b), neither of which are significantly preferred over the LCDM model. The models based on (c) are also consistent with the data with suitable choices of their parameters.

astro-ph↗
arXiv2006

A model-independent dark energy reconstruction scheme using the geometrical form of the luminosity-distance relation

We put forward a new model-independent reconstruction scheme for dark energy which utilises the expected geometrical features of the luminosity-distance relation. The important advantage of this scheme is that it does not assume explicit ansatzes for cosmological parameters but only some very general cosmological properties via the geometrical features of the reconstructed luminosity-distance relation. Using the recently released supernovae data by the Supernova Legacy Survey together with a phase space representation, we show that the reconstructed luminosity-distance curves best fitting the data correspond to a slightly varying dark energy density with the Universe expanding slightly slower than the Lambda CDM model. However, the Lambda CDM model fits the data at 1 sigma significance level and the fact that our best fitting luminosity-distance curve is lower than that of the corresponding Lambda CDM model could be due to systematics. The transition from an accelerating to a decelerating expansion occurs at a redshift larger than z=0.35. Interpreting the dark energy as a minimally coupled scalar field we also reconstruct the scalar field and its potential. We constrain $Ω_{m_0}$ using the baryon acoustic oscillation peak in the SDSS luminous red galaxy sample and find that the best fit is obtained with $Ω_{m_0}=0.27$, in agreement with the CMB data.

astro-ph↗
arXiv2006

Large-scale structure and the Cardassian fluid

In this paper, we confront the predictions of the power law cardassian model for the baryon power spectrum with the observations of the SDSS galaxy survey. We show that they fit only for very unusual values of the cold dark matter or baryon density parameters, the Hubble parameter or the spectral index of the initial power spectrum. Moreover, the best-fit Cardassian models turn out to be phantom models. If one wants to recover the usual values for these constants, as quoted by the WMAP team, the power law Cardassian model turns out to be indistinguishable from a LCDM model.

gr-qc↗
arXiv2006

Branes: cosmological surprise and observational deception

Using some supernovae and CMB data, we constrain the Cardassian, Randall-Sundrum, and Dvali-Gabadadze-Porrati brane-inspired cosmological models. We show that a transient acceleration and an early loitering period are usually excluded by the data. Moreover, the three models are equivalent to some usual quintessence/ghost dark energy models defined by a barotropic index $γ_ϕ$ depending on the redshift. We calculate this index for each model and show that they mimic a universe close to a $ΛCDM$ model today.

gr-qc↗
arXiv2005

Properties of homogeneous cosmologies in scalar tensor theories

We study the isotropisation of the homogeneous but anisotropic Bianchi class A models in presence of a minimally coupled and massive scalar field with or without a perfect fluid. To this end, we use the Hamiltonian formalism of Arnowitt, Deser and Misner(ADM) and the dynamical systems analysis methods. Our results allow to define three kinds of isotropisation called class 1, 2 and 3. We have specifically studied the class 1 and obtained some general constraints on scalar-tensor theories which are necessary conditions for isotropisation. The asymptotical behaviors of the metric functions and potential in the neighborhood of isotropy have also been determined when the isotropic state is reached sufficiently quickly. We show that the scalar field responsible for isotropisation may be quintessent and that the presence of curvature favor a late times acceleration and quintessence. Some applications are made with the well known exponential law potential by using our theoretical results but also by help of numerical analysis. The isotropisation process with a power law potential is also explored. We think this work represents a framework able to guide some future researches on the isotropisation of homogeneous models in scalar-tensor theories and we argue by discussing briefly about some recent results we have obtained in presence of a non minimally coupled scalar field or several scalar fields.

gr-qc↗
arXiv2005

Homogeneous cosmology dynamics revealed by Hamiltonian ADM formalism

We study the homogeneous but anisotropic cosmological models of Bianchi in presence of a massive scalar field using the ADM Hamiltonian formalism. We begin to describe the main steps to find the ADM Hamiltonian of the General Relativity with a massive scalar field and then we study the dynamics of the flat Bianchi type $I$ anisotropic Universe according to initial and final values of this Hamiltonian and sign of the potential. After a brief recall of the conditions necessary to isotropise an anisotropic Bianchi class A model with such a field, we extend them to a non minimally coupled scalar field by using a conformal transformation of the metric which casts the General Relativity with a scalar field into a scalar-tensor theory. The new line element then corresponds to the so-called Brans-Dicke frame, the former one being the Einstein frame. Then, we study the isotropisation process of the Bianchi class A model when we consider the low energy form of the string theory without its antisymmetric tensor and the Brans-Dicke theory with some exponential or power laws of the scalar field for the potential. Finally, assuming an isotropic Universe such as all the metric functions behave as some power or exponential laws of the proper time, we find the conditions such that the gravitation function and the potential of the scalar field are bounded as it is observed today, and compare them with the necessary conditions for isotropy.

gr-qc↗
arXiv2005

Bianchi type IX asymptotical behaviours with a massive scalar field: chaos strikes back

We use numerical integrations to study the asymptotical behaviour of a homogeneous but anisotropic Bianchi type IX model in General Relativity with a massive scalar field. As it is well known, for a Brans-Dicke theory, the asymptotical behaviour of the metric functions is ruled only by the Brans-Dicke coupling constant with respect to the value -3/2. In this paper we examine if such a condition still exists with a massive scalar field. We also show that, contrary to what occurs for a massless scalar field, the singularity oscillatory approach may exist in presence of a massive scalar field having a positive energy density.

gr-qc↗
arXiv2005

Isotropisation of flat homogeneous Bianchi type I model with a non minimally coupled and massive scalar field

In previous works, we studied the isotropisation of Bianchi class A models with a minimally coupled scalar field. In this paper, we extend these results to the case of a non minimally coupled one. We first make the calculations in the Einstein frame where the scalar field is minimally coupled to the curvature but non minimally coupled to the perfect fluid. Then, we use a conformal transformation to generalise our results to a scalar field non minimally coupled to the curvature. Universe isotropisation for the Brans-Dicke and low energy string theories are studied.

gr-qc↗
arXiv2004

Scalar fields properties for flat galactic rotation curves

The whole class of minimally coupled and massive scalar fields which may be responsible for flattening of galactic rotation curves is found. An interesting relation with a class of scalar-tensor theories able to isotropise anisotropic models of Universe is shown. The resulting metric is found and its stability and scalar field properties are tested with respect to the presence of a second scalar field or a small perturbation of the rotation velocity at galactic outer radii.

gr-qc↗