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Neven Bilic

Publications and source records attributed to Neven Bilic.

At least 55 records · Page 3Linked to original sources
arXiv2003

Supermassive fermion balls and constraints from stellar dynamics near Sgr A*

All presently known stellar-dynamical constraints on the size and mass of the supermassive compact dark object at the Galactic center are consistent with a ball of self-gravitating degenerate fermions with mass between 76 and 491 keV. Sterile neutrinos of 76 keV mass, which are mixed with at least one of the active neutrinos with a mixing angle ~10^{-7}, are produced in about the right amount in the early Universe and may be responsible for the formation of the supermassive degenerate fermion balls and black holes at the galactic centers.

astro-ph↗
arXiv2003

Inhomogeneous Chaplygin Gas Cosmology

The hypothesis that dark matter and dark energy are unified through the Chaplygin gas is reexamined. Using a generalization of the spherical model which incorporates effects of the acoustic horizon we show that an initially perturbative Chaplygin gas evolves into a mixed system containing cold dark matter in the form of gravitational condensate. Furthermore, we demonstrate that the observed CMB angular and baryonic power spectra are reproduced.

astro-ph↗
arXiv2003

Superluminal pions in a hadronic fluid

We study the propagation of pions at finite temperature and finite chemical potential in the framework of the linear sigma model with 2 quark flavors and $N_c$ colors. The velocity of massless pions in general differs from that of light. One-loop calculations show that in the chiral symmetry broken phase pions, under certain conditions, propagate faster than light.

hep-ph↗
arXiv2002

Dark Matter, Dark Energy and the Chaplygin Gas

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

astro-ph↗
arXiv2002

Comparison of the Black Hole and Fermion Ball Scenarios of the Galactic Center

After discussion of the properties of degenerate fermion balls, we analyze the orbit of the star S0-1, which has a projected distance of ~5 light-days 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. We show that the z-v_z 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, which could be orbiting within such a fermion ball, may reduce this allowed phase space and eventually rule out one of the currently acceptable scenarios.

astro-ph↗
arXiv2002

Unified description of dark matter at the center and in the halo of the Galaxy

We consider a self-gravitating ideal fermion gas at nonzero temperature as a model for the Galactic halo. The Galactic halo of mass ~ 2 x 10^12 Msol enclosed within a radius of ~ 200 kpc implies the existence of a supermassive compact dark object at the Galactic center that is in hydrostatic and thermal equilibrium with the halo. The central object has a maximal mass of ~ 2.3 x 10^6 Msol within a minimal radius of ~ 18 mpc for fermin masses ~ 15 keV.

astro-ph↗
arXiv2002

Dark matter in the Galaxy

After a brief introduction to standard cosmology and the dark matter problem in the the Universe, we consider a self-gravitating noninteracting fermion gas at nonzero temperature as a model for the dark matter halo of the Galaxy. This fermion gas model is then shown to imply the existence of a supermassive compact dark object at the Galactic center.

hep-ph↗
arXiv2001

Does Dark Matter at the Center and in the Halo of the Galaxy Consist of the Same Particles?

After a discussion of the properties od degenerate fermion balls, we analize the orbits of the star S0-1, which has the smallest projected distance 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 six yiers by Genzel ae al. and Ghez et al.. We then consider a self-gravitating ideal fermion gas at nonzero temperature as a model for the Galactic halo. The Galactic halo of mass ~2 x 10^12 Msol enclosed within a radius of ~200 kpc implies the existence of a supermassive compact dark object at the Galactic center that is in hydrostatic and thermal equilibrium with the halo. The central object has a maximal mass of ~2.3 x 10^6 Msol within a minimal radius of ~18 mpc for fermion masses ~15 keV. We thus conclude that both the supermassive compact dark object and the halo could be made of the same weakly interacting ~15 keV particle.

astro-ph↗
arXiv2001

The Dynamics of the Formation of Degenerate Heavy Neutrino Stars

Through reformulating the cold, self-gravitating fermion gas as a Bose condensate by identifying their mutual Thomas-Fermi limits, the dissipationless formation of a heavy neutrino star in gravitational collapse is numerically demonstrated. Such stars offer an alternative to supermassive black holes for the compact dark objects at the centers of galaxies.

astro-ph↗
arXiv2001

On the Formation of Degenerate Heavy Neutrino Stars

The dynamics of a self-gravitating cold Fermi gas is described using the analogy with an interacting self-gravitating Bose condensate having the same Thomas-Fermi limit. The dissipationless formation of a heavy neutrino star through gravitational collapse and ejection of matter is demonstrated numerically. Such neutrino stars offer an alternative to black holes for the supermassive compact dark objects at the centers of galaxies.

astro-ph↗
arXiv2000

Self-gravitating bosons at nonzero temperature

A system of charged bosons at finite temperature and chemical potential is studied in a general-relativistic framework. We assume that the boson fields interact only gravitationally. At sufficiently low temperature the system exists in two phases: the gas and the condensate. By studying the condensation process numerically we determine the critical temperature $T_c$ at which the condensate emerges. As the temperature decreases, the system eventually settles down in the ground state of a cold boson star.

gr-qc↗
arXiv2000

Fermion stars as gravitational lenses

We study in detail gravitational lensing caused by a supermassive fermion star and compare it with lensing by a black hole of the same mass. It is argued that lensing effects, being very distinct, may shed some light on the yet unexplained nature of the compact dark massive object at the Galactic center.

astro-ph↗
arXiv1999

Relativistic Acoustic Geometry

Sound wave propagation in a relativistic perfect fluid with a non-homogeneous isentropic flow is studied in terms of acoustic geometry. The sound wave equation turns out to be equivalent to the equation of motion for a massless scalar field propagating in a curved space-time geometry. The geometry is described by the acoustic metric tensor that depends locally on the equation of state and the four-velocity of the fluid. For a relativistic supersonic flow in curved space-time the ergosphere and acoustic horizon may be defined in a way analogous the non-relativistic case. A general-relativistic expression for the acoustic analog of surface gravity has been found.

gr-qc↗
arXiv1999

General-Relativistic Thomas-Fermi model

A system of self-gravitating massive fermions is studied in the framework of the general-relativistic Thomas-Fermi model. We study the properties of the free energy functional and its relation to Einstein's field equations. A self-gravitating fermion gas we then describe by a set of Thomas-Fermi type self-consistency equations.

gr-qc↗
arXiv1998

Thermodynamic instability of self-gravitating heavy neutrino matter

It is shown, in the framework of the Thomas-Fermi model at finite temperature, that a cooling non-degenerate gas of massive neutrinos will, at a certain temperature, become unstable and undergo a first-order phase transition in which quasi-degenerate supermassive neutrino stars are formed through gravitational collapse. For neutrinos in the mass range of 10 to 25 keV/c$^{2}$, these compact dark objects could mimic the role of supermassive black holes that are reported to exist at the centres of galaxies and quasi-stellar objects.

astro-ph↗
arXiv1998

Gravitational Phase Transition of Fermionic Matter in a General-Relativistic Framework

The Thomas-Fermi model at finite temperature is extended to describe a system of self-gravitating weakly interacting massive fermions in a general-relativistic framework. By cooling a nondegenerate gas of weakly interacting massive fermions below some critical temperature, a condensed phase emerges, consisting of quasidegenerate fermion stars. For fermion masses of 10 to 25 keV, these fermion stars may very well provide an alternative explanation for the supermassive compact dark objects that are observed at galactic centers.

hep-ph↗
arXiv1998

Gravitational phase transition of heavy neutrino matter

In the framework of the Thomas-Fermi model at finite temperature, we show that a cooling nondegenerate gas of massive neutrinos will undergo a phase transition in which quasi-degenerate supermassive neutrino stars are formed through gravitational collapse. For neutrinos in the mass range of 10 to 25 keV these compact dark objects could mimic the role of supermassive black holes that are usually assumed to exist at the centres galaxies. Astrophysical implications and constraints on the neutrino mass are discussed for this scenario.

hep-ph↗