Jérôme Perez

Dissipationless collapse of a set of N massive particles

The formation of self-gravitating systems is studied by simulating the collapse of a set of N particles which are generated from several distribution functions. We first establish that the results of such simulations depend on N for small values of N. We complete a previous work by Aguilar & Merritt concerning the morphological segregation between spherical and elliptical equilibria. We find and interpret two new segregations: one concerns the equilibrium core size and the other the equilibrium temperature. All these features are used to explain some of the global properties of self-gravitating objects: origin of globular clusters and central black hole or shape of elliptical galaxies.

Radial orbit instability as a dissipation-induced phenomenon

This paper is devoted to Radial Orbit Instability in the context of self-gravitating dynamical systems. We present this instability in the new frame of Dissipation-Induced Instability theory. This allows us to obtain a rather simple proof based on energetics arguments and to clarify the associated physical mechanism.

The Jungle Universe: coupled cosmological models in a Lotka–Volterra framework

In this paper, we exploit the fact that the dynamics of homogeneous and isotropic Friedmann–Lemaître universes is a special case of generalized Lotka–Volterra system where the competitive species are the barotropic fluids filling the Universe. Without coupling between those fluids, Lotka–Volterra formulation offers a pedagogical and simple way to interpret usual Friedmann–Lemaître cosmological dynamics. A natural and physical coupling between cosmological fluids is proposed which preserves the structure of the dynamical equations. Using the standard tools of Lotka–Volterra dynamics, we obtain the general Lyapunov function of the system when one of the fluids is coupled to dark energy. This provides in a rigorous form a generic asymptotic behavior for cosmic expansion in presence of coupled species, beyond the standard de Sitter, Einstein-de Sitter and Milne cosmologies. Finally, we conjecture that chaos can appear for at least four interacting fluids.

Radial Orbit Instability: Review and Perspectives

This article presents elements about the radial orbit instability, which occurs in spherical self-gravitating systems with a strong anisotropy in the radial velocity direction. It contains an overview on the history of radial orbit instability. We also present the symplectic method we use to explore stability of equilibrium states directly related to the dissipation induced instability mechanism well known in theoretical mechanics and plasma physics.

Gravity, dimension, equilibrium, and thermodynamics

It is actually possible to interpret gravitation as a property of space in a purely classical way. We note that an extended self-gravitating system equilibrium depends directly on the number of dimensions of the space in which it evolves. Given these precisions, we review the principal thermodynamical knowledge in the context of classical gravity with arbitrary dimension of space. Stability analyses for bounded 3D systems, namely the Antonov instability paradigm, are then associated to some amazing properties of globular clusters and galaxies.

Stability of rotating spherical stellar systems

We study the stability of rotating collisionless self-gravitating spherical systems by using highresolution N-body experiments on a Connection Machine CM-5.We added rotation to Ossipkov±Merritt (OM) anisotropic spherical systems by using twomethods. The first method conserves the anisotropy of the distribution function defined in the OM algorithm. The second method distorts the systems in velocity-space. We then show that the stability of systems depends both on their anisotropy and on the value of the ratio of the total kinetic energy to the rotational kinetic energy. We also test the relevance of the stability parameters introduced by Perez et al. for the case of rotating systems

A symplectic approach to gravitational instability

We present a global approach of non-dissipative physics. Based on symplectic mechanics this technique allows us to obtain the solution of a very large class of problems in terms of a Taylor expand. We apply this method to the problem of gravitational instability and we obtain a general expression of the gravitational potential, solution of the Vlasov- Poisson gravitational potential, solution of the Vlasov-Poisson system, as a function of time in the context of Newtonian dust cosmology.

Dynamics of Anisotropic Universes

We present a general study of the dynamical properties of Anisotropic Bianchi Universes in the context of Einstein General Relativity. Integrability results using Kovalevskaya exponents are reported and connected to general knowledge about Bianchi dynamics. Finally, dynamics toward singularity in Bianchi type VIII and IX universes are showed to be equivalent in some precise sence.

Isochrony in 3D Radial Potentials

Revisiting and extending an old idea of Michel Hénon, we geometrically and algebraically characterize the whole set of isochrone potentials. Such potentials are fundamental in potential theory. They appear in spherically symmetrical systems formed by a large amount of charges (electrical or gravitational) of the same type considered in mean-field theory. Such potentials are defined by the fact that the radial period of a test charge in such potentials, provided that it exists, depends only on its energy and not on its angular momentum. Our characterization of the isochrone set is based on the action of a real affine subgroup on isochrone potentials related to parabolas in the