Raoul D. Viollier

Unification of dark matter and dark energy: The Inhomogeneous Chaplygin gas

Abstract We extend the world model of Kamenshchik et al. to large perturbations by formulating a Zeldovich-like approximation. We sketch how this model unifies dark matter with dark energy in a geometric setting reminiscent of M-theory.

Born?Infeld phantom gravastars

We construct new gravitational vacuum star solutions with a Born–Infeld phantom replacing the de Sitter interior. The model allows for a wide range of masses and radii required by phenomenology, and can be motivated from low energy string theory.

Dark Matter Concentration in the Galactic Center

It is shown that the matter concentration observed through stellar motion at the Galactic center is consistent with a supermassive object of 2.5 × 106 solar masses composed of self-gravitating, degenerate, heavy neutrinos. This result is opposed to the alternative black hole interpretation. According to the observational data, the lower bounds on possible neutrino masses are mν ≥ 12.0 keV/c2 for g = 2 or mν ≥ 14.3 keV/c2 for g = 1, where g is the spin degeneracy factor. The advantage of this scenario is that it could naturally explain the low X-ray and gamma-ray activity of Sgr A*, i.e., the so-called blackness problem of the Galactic center.

Gravitational phase transition of fermionic matter

Abstract We study the phase transition of a system of self-gravitating weakly interacting massive fermions in the presence of a large radiation-density background in the framework of the Thomas-Fermi model. We show that by cooling a nondegenerate gas of weakly interacting massive fermions below some critical temperature, a condensed phase emerges, consisting of quasidegenerate supermassive fermion stars. These compact dark objects could play an important role in structure formation in the early universe, as they might in fact provide seeds for galactic nuclei and quasistellar objects.

Nonlinear evolution of dark matter and dark energy in the Chaplygin-gas cosmology

The hypothesis that dark matter and dark energy are unified through the Chaplygin gas, an exotic fluid obeying p = −A/ρ, is re-examined. Using generalizations of the spherical model which incorporate effects of the acoustic horizon we show that an initially perturbative Chaplygin gas evolves into a mixed system containing cold dark matter-like gravitational condensate.

Dark Matter, Dark Energy and the Chaplygin Gas

We formulate a Zeľdovich-like approximation for the Chaplain gas equation of state P = -A/ρ, and sketch how this model unifies dark matter with dark energy in a geometric setting reminiscent of M-theory.

The Motion of Stars near the Galactic Center: A Comparison of the Black Hole and Fermion Ball Scenarios

After a discussion of the properties of degenerate fermion balls, we analyze the orbits of the stars S0-1 and S0-2, which have the smallest projected distances to Sgr A*, in the supermassive black hole as well as in the fermion ball scenarios of the Galactic center. It is shown that both scenarios are consistent with the data, as measured during the last 6 years by Genzel and coworkers and by Ghez and coworkers. The free parameters of the projected orbit of a star are the unknown components of its velocity vz and distance z to Sgr A* in 1995.4, with the z-axis being in the line of sight. We show, in the case of S0-1 and S0-2, that the z-vz phase space, which fits the data, is much larger for the fermion ball than for the black hole scenario. Future measurements of the positions or radial velocities of S0-1 and S0-2 could reduce this allowed phase space and eventually rule out one of the currently acceptable scenarios. This may shed some light on the nature of the supermassive compact dark object, or dark matter in general, at the center of our Galaxy.

Stars and halos of degenerate relativistic heavy-neutrino and neutralino matter

Rudjer Boˇskovi´c Institute, 10000 Zagreb, Croatia(February 5, 2008)Heavy-neutrino (or neutralino) stars are studied using the general relativistic equations of hy-drostatic equilibrium and the relativistic equation of state for degenerate fermionic matter. TheTolman-Oppenheimer-Volkoff equations are then generalized to include a system of degenerate neu-trino and neutralino matter that is gravitationally coupled. The properties and implications of suchan interacting astrophysical system are discussed in detail.04.40.-b, 04.40.Dg, 97.20.Rp, 95.35.+d, 97.60.Jd, 98.35.JkI. INTRODUCTION

Neutrino halos around baryonic stars and supermassive neutrino stars — Atoms of the macrocosm?

The properties and observational implications of self-gravitating degenerate heavy neutrino matter clustered around baryonic stars are investigated for neutrino masses in the range of 10 to 25 keV/c2. As a test of our ideas, we propose a new experiment aimed at observing the radiative decay of such neutrinos, which the sun might have accumulated in a halo of a few solar masses and a few light years radius. We further study the characteristics of supermassive ‘stars’ consisting of self-gravitating degenerate neutrino matter. Such compact dark objects could be as massive as 109.5 to 106.5 solar masses, with radii of about one to ten light days; they might thus mimic phenomena that are expected around the supermassive black holes recently purported at the centres of some galaxies, including our own, and quasi-stellar objects. Finally, we turn to the cosmological consequences of such a standard neutrino. Overclosure of the universe could be avoided through (i) plasma reheating due to gravitational collapse of primordial neutrino density fluctuations prior to recombination, and (ii) annihilation of the heavy neutrinos into light neutrinos in supermassive degenerate ‘neutrino stars’ after recombination. These compact dark objects could play an important role in the formation of galactic nuclei.

SAGITTARIUS A*: A SUPERMASSIVE BLACK HOLE OR A SPATIALLY EXTENDED OBJECT?

We report here on a calculation of possible orbits of the fast moving infrared source S1, which has been observed recently by Eckart & Genzel near the Galactic center. It is shown that tracking of the orbit of S1 or any other fast moving star near Sagittarius A* offers a possibility of distinguishing between the supermassive black hole and the extended object scenarios of Sgr A*. In our calculations, we assumed that the extended object at the Galactic center is a nonbaryonic ball made of degenerate, self-gravitating heavy neutrino matter, as it has been proposed recently by Tsiklauri & Viollier.