149 research outputs found
Cosmological tachyon condensation
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.Comment: 22 pages, 3 figures, title changed, typos corrected, accepted in
Phys. Rev.
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.Comment: 7 pages, presented by N. B. at IRGAC 2006, Barcelona, 11-15 July
2006, typos corrected, concluding paragraph slightly expanded, final version,
accepted in J. Phys. A, special issu
Vacuum fluctuations in a supersymmetric model in FRW spacetime
We study a noninteracting supersymmetric model in an expanding FRW spacetime.
A soft supersymmetry breaking induces a nonzero contribution to the vacuum
energy density. A short distance cutoff of the order of Planck length provides
a scale for the vacuum energy density comparable with the observed cosmological
constant. Assuming the presence of a dark energy substance in addition to the
vacuum fluctuations of the field an effective equation of state is derived in a
selfconsistent approach. The effective equation of state is sensitive to the
choice of the cut-off but no fine tuning is needed.Comment: 19 pages, accepted for publication in Phys. Rev.
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
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.Comment: 8 pages, 3 figures included. Considerably revised, discussions
expanded, one figure added, typos corrected, results unchanged. To be
published in Phys. Rev.
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.Comment: 7 pages, LaTex, to appear in Gen. Rel. Gra
Is there a Supermassive Black Hole at the Center of the Milky Way?
This review outlines the observations that now provide an overwhelming
scientific case that the center of our Milky Way Galaxy harbors a supermassive
black hole. Observations at infrared wavelength trace stars that orbit about a
common focal position and require a central mass (M) of 4 million solar masses
within a radius of 100 Astronomical Units. Orbital speeds have been observed to
exceed 5,000 km/s. At the focal position there is an extremely compact radio
source (Sgr A*), whose apparent size is near the Schwarzschild radius
(2GM/c^2). This radio source is motionless at the ~1 km/s level at the
dynamical center of the Galaxy. The mass density required by these observations
is now approaching the ultimate limit of a supermassive black hole within the
last stable orbit for matter near the event horizon.Comment: Invited review submitted to International Journal of Modern Physics
D; 23 pages; 10 figure
Gauging the Shadow Sector with SO(3)
We examine the phenomenology of a low-energy extension of the Standard Model,
based on the gauge group SU(3) x SU(2) x U(1) x SO(3), with SO(3) operating in
the shadow sector. This model offers and oscillations as the solution of the solar and atmospheric neutrino
problems. Moreover, it provides a neutral heavy shadow lepton X that could play
the role of a cold dark matter particle.Comment: 8 page
Indications for a Detonating Quark-Gluon Plasma
We propose a mechanism which naturally contains the relation of the hadronic gas produced in heavy-ion collisions at CERN. Our
starting assumption is the existence of a sharp front separating the
quark-gluon plasma phase from the hadronic phase. Energy-momentum conservation
across the front leads to the following consequences for an adiabatic process
a) The baryon chemical potential, , is approximately continuous across
the front. b) The temperature in the hadronic gas is higher than the phase
transition temperature due to superheating. c) In the region covered by the
experiments the velocity of the hadronic gas approximately equals the speed of
sound in the hadronic gas.Comment: Latex file 9 pages + 6 figures available as postscript file
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