Gary B. Tupper

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.

Supermassive neutrino stars and galactic nuclei

Abstract The characteristics of supermassive ‘stars’ consisting of self-gravitating degenerate neutrino (or neutralino) matter are studied with particular emphasis on fermion masses around 17 keV/ c 2 . Such compact dark objects could be as massive as 10 9.5 to 1o 6.5 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 and quasi-stellar 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.

On the formation of degenerate heavy neutrino stars

Abstract 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.

Chaplygin gas cosmology- : unification of dark matter and dark energy

The models that unify dark matter and dark energy based upon the Chaplygin gas fail owing to the suppression of structure formation by the adiabatic speed of sound. Including string theory effects, in particular the Kalb–Ramond field, we show how nonadiabatic perturbations allow a successful structure formation.

The dynamics of stars near Sgr A∗ and dark matter at the center and in the halo of the galaxy

Abstract After a discussion of the properties of degenerate fermion balls, we analyze 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 years by Genzel et 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 × 10 12 M ⊙ 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 × 10 6 M ⊙ within a minimal radius of ∼ 18 mpc or ∼ 21 light-days 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.

Transient acceleration from a hybrid Chaplygin gas

We extend the Chaplygin gas model for dark matter and dark energy unification by promoting the Chaplygin gas parameter A to the potential for an extra scalar with canonical kinetic energy. The hybrid model allows for accelerated Hubble expansion to be a transient effect around redshift zero.

A symbiotic scenario for the rapid formation of supermassive black holes

The most massive black holes, lurking at the centres of large galaxies, must have formed less than a billion years after the big bang, as they are visible today in the form of bright quasars at redshift larger than six. Their early appearance is mysterious, because the radiation pressure, generated by infalling ionized matter, inhibits the rapid growth of these black holes from stellar-mass black holes. It is shown that the supermassive black holes may form timeously through the accretion of predominantly degenerate sterile neutrino dark matter onto stellar-mass black holes. Our symbiotic scenario relies on the formation of, first, supermassive degenerate sterile neutrino balls through gravitational cooling and, then, stellar-mass black holes through supernova explosions of massive stars at the centre of the neutrino balls. The observed lower and upper limits of the supermassive black holes are explained by the corresponding mass limits of the preformed neutrino balls.