297 research outputs found
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
The Impact of Treated Urban Wastewaters and Flood Discharge on the Quality of Bathing Water
What do we know about the Adriatic Sea and the state of its health [...
Classification of dark energy models in the (w_0,w_a) plane
We classify dark energy models in a plane of observables that correspond to
the common parameterization of a non-constant equation of state, w(a)=w_0 +
w_a(1-a), where is the scale factor of the universe. The models fall into
four classes and only two of these classes have a region of overlap in the
observable plane. We perform a joint analysis of all Type Ia supernova (SNIa)
data compiled by the High-Z SN Search Team (HZT) and the Supernova Legacy
Survey (SNLS) and find that no class of models is excluded by current SNIa
data. However, an analysis of large scale structure, Ly forest and bias
constraints from SDSS, the Gold SNIa data and WMAP data indicates that
non-phantom barotropic models with a a positive sound speed are excluded at the
95% C.L.Comment: 11 pages, 3 figures. Classification generalized to include models for
which the equation of state crosses the phantom divide line. Published
versio
Tachyon cosmology with non-vanishing minimum potential: a unified model
We investigate the tachyon condensation process in the effective theory with
non-vanishing minimum potential and its implications to cosmology. It is shown
that the tachyon condensation on an unstable three-brane described by this
modified tachyon field theory leads to lower-dimensional branes (defects)
forming within a stable three-brane. Thus, in the cosmological background, we
can get well-behaved tachyon matter after tachyon inflation, (partially)
avoiding difficulties encountered in the original tachyon cosmological models.
This feature also implies that the tachyon inflated and reheated universe is
followed by a Chaplygin gas dark matter and dark energy universe. Hence, such
an unstable three-brane behaves quite like our universe, reproducing the key
features of the whole evolutionary history of the universe and providing a
unified description of inflaton, dark matter and dark energy in a very simple
single-scalar field model.Comment: 18 p
Fermion determinants in matrix models of QCD at nonzero chemical potential
The presence of a chemical potential completely changes the analytical
structure of the QCD partition function. In particular, the eigenvalues of the
Dirac operator are distributed over a finite area in the complex plane, whereas
the zeros of the partition function in the complex mass plane remain on a
curve. In this paper we study the effects of the fermion determinant at nonzero
chemical potential on the Dirac spectrum by means of the resolvent, G(z), of
the QCD Dirac operator. The resolvent is studied both in a one-dimensional U(1)
model (Gibbs model) and in a random matrix model with the global symmetries of
the QCD partition function. In both cases we find that, if the argument z of
the resolvent is not equal to the mass m in the fermion determinant, the
resolvent diverges in the thermodynamic limit. However, for z =m the resolvent
in both models is well defined. In particular, the nature of the limit is illuminated in the Gibbs model. The phase structure of the
random matrix model in the complex m and \mu-planes is investigated both by a
saddle point approximation and via the distribution of Yang-Lee zeros. Both
methods are in complete agreement and lead to a well-defined chiral condensate
and quark number density.Comment: 27 pages, 6 figures, Late
Stable gravastars with generalised exteriors
New spherically symmetric gravastar solutions, stable to radial
perturbations, are found by utilising the construction of Visser and Wiltshire.
The solutions possess an anti--de Sitter or de Sitter interior and a
Schwarzschild--(anti)--de Sitter or Reissner--Nordstr\"{o}m exterior. We find a
wide range of parameters which allow stable gravastar solutions, and present
the different qualitative behaviours of the equation of state for these
parameters.Comment: 14 pages, 11 figures, to appear in Classical and Quantum Gravit
Phase transition in Schwarzschild-de Sitter spacetime
Using a static massive spherically symmetric scalar field coupled to gravity
in the Schwarzschild-de Sitter (SdS) background, first we consider some
asymptotic solutions near horizon and their local equations of state(E.O.S) on
them. We show that near cosmological and event horizons our scalar field
behaves as a dust. At the next step near two pure de-Sitter or Schwarzschild
horizons we obtain a coupling dependent pressure to energy density ratio. In
the case of a minimally couplling this ratio is -1 which springs to the mind
thermodynamical behavior of dark energy. If having a negative pressure behavior
near these horizons we concluded that the coupling constant must be
>. Therefore we derive a new constraint on the value of our coupling .
These two different behaviors of unique matter in the distinct regions of
spacetime at present era can be interpreted as a phase transition from dark
matter to dark energy in the cosmic scales and construct a unified scenario.Comment: 7 pages,no figures,RevTex, Typos corrected and references adde
Matter Power Spectrum for the Generalized Chaplygin Gas Model: The Newtonian Approach
We model the cosmic medium as the mixture of a generalized Chaplygin gas and
a pressureless matter component. Within a neo-Newtonian approach (in which,
different from standard Newtonian cosmology, the pressure enters the
homogeneous and isotropic background dynamics) we compute the matter power
spectrum. The 2dFGRS data are used to discriminate between unified models of
the dark sector (a purely baryonic matter component of roughly 5 percent of the
total energy content and roughly 95 percent generalized Chaplygin gas) and
different models, for which there is separate dark matter, in addition to that
accounted for by the generalized Chaplygin gas. Leaving the corresponding
density parameters free, we find that the unified models are strongly
disfavored. On the other hand, using unified model priors, the observational
data are also well described, in particular for small and large values of the
generalized Chaplygin gas parameter . The latter result is in agreement
with a recent, more qualitative but fully relativistic, perturbation analysis
in Gorini et al.Comment: Latex file, 17 pages, 29 figures in eps forma
Linear sigma model and chiral symmetry at finite temperature
The chiral phase transition is investigated within the framework of the
linear sigma model at finite temperature. We concentrate on the meson sector of
the model and calculate the finite temperature effective potential in the
Hartree approximation by using the Cornwall-Jackiw-Tomboulis formalism of
composite operators. The effective potential is calculated for N=4 involving
the usual sigma and three pions and in the large N approximation involving N-1
pion fields. In the N=4 case we have examined the theory both in the chiral
limit and with the presence of a symmetry breaking term which generates the
pion masses. In both cases we have solved the system of the resulting gap
equations for the thermal effective masses of the particles numerically and we
have investigated the evolution of the effective potential. In the N=4 case
there is indication of a first order phase transition and the Goldstone theorem
is not satisfied. The situation is different in the general case using the
large approximation, the Goldstone theorem is satisfied and the phase
transition is of the second order. For this analysis we have ignored quantum
effects and we used the imaginary time formalism for calculations.Comment: 14 pages, 5 eps figures, RevTex, axodraw.st
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