arXiv subjects

Jean-Michel Alimi

Publications and source records attributed to Jean-Michel Alimi.

26 records · Page 2Linked to original sources

arXiv1997

Upper limit to $Ω_B$ in scalar-tensor gravity theories

In a previous paper (Serna & Alimi 1996), we have pointed out the existence of some particular scalar-tensor gravity theories able to relax the nucleosynthesis constraint on the cosmic baryonic density. In this paper, we present an exhaustive study of primordial nucleosynthesis in the framework of such theories taking into account the currently adopted observational constraints. We show that a wide class of them allows for a baryonic density very close to that needed for the universe closure. This class of theories converges soon enough towards General Relativity and, hence, is compatible with all solar-system and binary pulsar gravitational tests. In other words, we show that primordial nucleosynthesis does not always impose a very stringent bound on the baryon contribution to the density parameter.

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arXiv1997

Self Similar Spherical Collapse Revisited: a Comparison between Gas and Dark Matter Dynamics

We reconsider the collapse of cosmic structures in an Einstein-de Sitter Universe, using the self similar initial conditions of Fillmore & Goldreich (1984). We first derive a new approximation to describe the dark matter dynamics in spherical geometry, that we refer to the "fluid approach". This method enables us to recover the self-similarity solutions of Fillmore & Goldreich for dark matter. We derive also new self-similarity solutions for the gas. We thus compare directly gas and dark matter dynamics, focusing on the differences due to their different dimensionalities in velocity space. This work may have interesting consequences for gas and dark matter distributions in large galaxy clusters, allowing to explain why the total mass profile is always steeper than the X-ray gas profile. We discuss also the shape of the dark matter density profile found in N-body simulations in terms of a change of dimensionality in the dark matter velocity space. The stable clustering hypothesis has been finally considered in the light of this analytical approach.

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arXiv1997

Gas and Dark Matter Spherical Dynamics

We investigate the formation of spherical cosmological structures following both dark matter and gas components. We focus on the dynamical aspect of the collapse assuming an adiabatic, $γ= 5/3$, fully ionized primordial plasma. We use for that purpose a fully Lagrangian hydrodynamical code designed to describe highly compressible flows in spherical geometry. We investigate also a "fluid approach" to describe the mean physical quantities of the dark matter flow. We test its validity for a wide range of initial density contrast. We show that an homogeneous isentropic core forms in the gas distribution, surrounded by a self-similar hydrostatic halo, with much higher entropy generated by shock dissipation. We derive analytical expressions for the size, density and temperature of the core, as well as for the surrounding halo. We show that, unless very efficient heating processes occur in the intergalactic medium, we are unable to reproduce within adiabatic models the typical core sizes in X-ray clusters. We also show that, for dynamical reasons only, the gas distribution is naturally antibiased relative to the total mass distribution, without invoking any reheating processes. This could explain why the gas fraction increases with radius in very large X-ray clusters. As a preparation for the next study devoted to the thermodynamical aspect of the collapse, we investigate the initial entropy level required to solve the core problem in X-ray clusters.

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arXiv1997

Non-linear Dynamics and Mass Function of Cosmic Structures: I Analytical Results

We investigate some modifications to the Press & Schechter (1974) (PS) prescription resulting from shear and tidal effects. These modifications rely on more realistic treatments of the collapse process than the standard approach based on the spherical model. First, we show that the mass function resulting from a new approximate Lagrangian dynamic (Audit & Alimi 96), contains more objects at high mass, than the classical PS mass function and is well fitted by a PS-like function with a threshold density of $δ_c \simeq 1.4$. However, such a Lagrangian description can underestimate the epoch of structure formation since it defines it as the collapse of the first principal axis. We therefore suggest some analytical prescriptions, for computing the collapse time along the second and third principal axes, and we deduce the corresponding mass functions. The collapse along the third axis is delayed by the shear and the number of objects of high mass then decreases. Finally, we show that the shear also strongly affects the formation of low-mass halos. This dynamical effect implies a modification of the low-mass slope of the mass function and allows the reproduction of the observed luminosity function of field galaxies. In a companion paper, we present results of numerical simulations which complete this work.

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arXiv1996

Gravitational Dynamics of Cold Matter using the Deformation Tensor

In this paper we present a new local Lagrangian approximation to the gravitational dynamics of cold matter. We describe the dynamics of a Lagrangian fluid element through only one quantity, the deformation tensor. We show that this tensor is clearly suited to the study of gravitational dynamics and, moreover, that knowing its evolution is enough to completely describe a fluid element. Our approximate dynamics treat exactly the conservation of mass, of the velocity divergence and of the shear and is exact in the case of planar, cylindrical and spherical collapses. It also reproduces very accurately the evolution of all dynamical quantities for a very wide class of initial conditions as illustrated by a detailed comparison with the homogeneous ellipsoid model. Beside, we highlight, for the first time, the important dynamical role played by the Newtonian counterpart of the magnetic part of the Weyl tensor.

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arXiv1995

Adaptative Smooth Particle Hydrodynamics and Particle-Particle coupled codes: Energy and Entropy Conservation

We present and test a general-purpose code, called PPASPH, for evolving self-gravitating fluids in astrophysics, both with and without a collisionless component. In PPASPH, hydrodynamical properties are computed by using the SPH (Smoothed Particle Hydrodynamics) method while, unlike most previous implementations of SPH, gravitational forces are computed by a PP (Particle-Particle) approach. another important feature of this code is that hydrodynamics equations optionally include the correction terms appearing when $h(t,\br)$ is not constant. Our code has been implemented by using the data parallel programming model on CM5(CM). PPASPH has been applied to study the importance of adaptative smoothing correction terms on the entropy conservation. We confirm Hernquist's (1993) interpretation of the entropy violation observed in previous SPH simulations as a result of having neglected these terms. An improvement on the entropy conservation is not found by merely considering larger numbers of particles or different $N_S$ choices. The correct continuum description is only obtained if the $\bn h$ correction terms are included. Otherwise, the entropy conservation is always rather poor as compared to that found for the total energy.

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arXiv1995

Scalar-Tensor Cosmological Models

We analyze the qualitative behaviors of scalar-tensor cosmologies with an arbitrary monotonic $ω(Φ)$ function. In particular, we are interested on scalar-tensor theories distinguishable at early epochs from General Relativity (GR) but leading to predictions compatible with solar-system experiments. After extending the method developed by Lorentz-Petzold and Barrow, we establish the conditions required for convergence towards GR at $t\rightarrow\infty$. Then, we obtain all the asymptotic analytical solutions at early times which are possible in the framework of these theories. The subsequent qualitative evolution, from these asymptotic solutions until their later convergence towards GR, has been then analyzed by means of numerical computations. From this analysis, we have been able to establish a classification of the different qualitative behaviors of scalar-tensor cosmological models with an arbitrary monotonic $ω(Φ)$ function.

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arXiv1995

Constraints on the Scalar-Tensor theories of gravitation from Primordial Nucleosynthesis

We present a detailed calculation of the light element production in the framework of Scalar-Tensor (ST) theories of gravitation. The coupling function ωhas been described by an appropriate form which reproduces all the possible asymptotic behaviors at early times of viable ST cosmological models with a monotonic ω(Φ). This form gives an exact representation for most of the particular theories proposed in the literature, but also a first-order approximation to many other theories. In most of ST theories, the nucleosynthesis bounds lead to cosmological models which do not significantly differ from the standard FRW ones. We have found however a particular class of ST theories where the expansion rate of the universe during nucleosynthesis can be very different from that found in GR, while the present value of ωis high enough to ensure compatibility with solar-system experiments. In the framework of this class of theories, right primordial abundances can be obtained for a baryon density range much wider (2.8 \lesssim η_{10} \lesssim 58.7) than in GR. Consequently, the usual constraint on the baryon contribution to the density parameter of the universe can be drastically relaxed (0.01 \lesssim Ω_{b0} \lesssim 1.38) by considering these gravity theories. This is the first time that a ST theory is found to be compatible both with primordial nucleosynthesis and solar-system experiments while implying cosmological models significantly different from the FRW ones.

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