arXiv subjects
Publications and source records attributed to Raoul D. Viollier.
We consider a model of dark energy/matter unification based on a k-essence type of theory similar to tachyon condensate models. Using an extension of the general relativistic spherical model which incorporates the effects of both pressure and the acoustic horizon we show that an initially perturbative k-essence fluid evolves into a mixed system containing cold dark matter like gravitational condensate in significant quantities.
We consider the prospects for dark matter/energy unification in k-essence type theories. General mappings are established between the k-essence scalar field, the hydrodynamic and braneworld descriptions. We develop an extension of the general relativistic dust model that incorporates the effects of both pressure and the associated acoustic horizon. Applying this to a tachyon model, we show that this inhomogeneous "variable Chaplygin gas" does evolve into a mixed system containing cold dark matter like gravitational condensate in significant quantities. Our methods can be applied to any dark energy model as well as to mixtures of dark energy and traditional dark matter.
Recently, there are two hints arising from physics beyond the standard model. One is a possible energy loss mechanism due to emission of very weakly interacting light particles from white dwarf stars, with a coupling strength ~ 0.7x10^{-13}, and another is the high energy positrons observed by the PAMELA satellite experiment. We construct a supersymmetric flipped-SU(5) model, SU(5)xU(1)_X with appropriate additional symmetries, [U(1)_H]_{gauge}x[U(1)_RxU(1)_Γ]_{global}xZ_2, such that these are explained by a very light electrophilic axion of mass 0.5 meV from the spontaneously broken U(1)_Γand two component cold dark matters from Z_2 parity. We show that in the flipped-SU(5) there exists a basic mechanism for allowing excess positrons through the charged SU(2) singlet leptons, but not allowing anti-proton excess due to the absence of the SU(2) singlet quarks. We show the discovery potential of the charged SU(2) singlet E at the LHC experiments by observing the electron and positron spectrum. With these symmetries, we also comment on the mass hierarchy between the top and bottom quarks.
Applying the Thomas-Fermi approximation to renormalizable field theories, we construct ghost condensation models that are free of the instabilities associated with violations of the null-energy condition.
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.
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.
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.
Unified dark matter/energy models (quartessence) based upon the Chaplygin gas D-brane fail owing to the suppression of structure formation by the adiabatic speed of sound. Including string theory effects, in particular the Kalb-Ramond field which becomes massive via the brane, we show how nonadiabatic perturbations allow successful structure formation.
Extending previous work in Randall-Sundrum type models, we construct low-energy effective actions for braneworlds with a bulk scalar field, with special attention to the case of BPS branes. Holding the branes at fixed coordinate position with a general ansatz for the bulk metric, and imposing the Einstein frame as a gauge condition, we obtain a scalar-tensor theory with only one scalar degree of freedom related to the proper brane separation. The formalism is applicable even when there is direct coupling of the bulk scalar and brane matter, as in the Horava-Witten model. We further show that the usual moduli space approximation actually describes a non-BPS three-brane system.
The hypothesis that dark matter and dark energy are unified through the Chaplygin gas is reexamined. 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.
We present a general framework for nonparallel brane worlds and use it to discuss the nonlinear radion problem. By imposing the Einstein frame as a gauge condition we are able to give the effective action for both Minkowski and (A)dS$_{4}$ branes. In particular we find the nonlinear radion does not disappear in the second Randall-Sundrum model.
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, nearly non-interacting, degenerate fermions with mass between 76 and 491 keV, for degeneracy factor g=2. Sterile neutrinos of 76 keV mass, which are mixed with at least one of the active neutrinos with a mixing angele ~10^{-7}, are produced in about the right amount in the early Universe by incoherent resonant and non-resonant scattering of active neutrinos having asymmetry of ~0.01. The former process yields sterile neutrinos with a quasi-degenerate spectrum while the latter leads to a thermal spectrum. As the production mechanism of the sterile neutrino is consistent with the constraints from large scale structure formation, core collapse supernovae, and diffuse X-ray background, it could be the dark matter particle of the Universe.
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.
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.
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.
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.
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.
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.