393 research outputs found
Hybrid Chaplygin gas and phantom divide crossing
Hybrid Chaplygin gas model is put forward, in which the gases play the role
of dark energy. For this model the coincidence problem is greatly alleviated.
The effective equation of state of the dark energy may cross the phantom divide
. Furthermore, the crossing behaviour is decoupled from any gravity
theories. In the present model, is only a transient behaviour. There is
a de Sitter attractor in the future infinity. Hence, the big rip singularity,
which often afflicts the models with matter whose effective equation of state
less than -1, is naturally disappear. There exist stable scaling solutions,
both at the early universe and the late universe. We discuss the perturbation
growth of this model. We find that the index is consistent with observations.Comment: 11 pages, 4 figures, V3: discussions on the perturbation growth
added, V4: minor corrections, to match the published versio
Constraints on Exotic Matter for An Emergent Universe
We study a composition of normal and exotic matter which is required for a
flat Emergent Universe scenario permitted by the equation of state
(EOS)() and predict the range of the permissible values
for the parameters and to explore a physically viable cosmological
model. The permitted values of the parameters are determined taking into
account the data obtained from observations, a model independent BAO
peak parameter and CMB shift parameter (WMAP7 data). It is found that although
can be very close to zero, most of the observations favours a small and
negative . As a consequence, the effective Equation of State parameter for
this class of Emergent Universe solutions remains negative always. We also
compared the magnitude () vs. redshift() curve obtained in the
model with that obtained from the union compilation data. According to our
analysis the class of Emergent Universe solutions considered here is not ruled
out by the observations.Comment: 6 pages, 7 figures, 1 tabl
Pressure and non-linear susceptibilities in QCD at finite chemical potentials
When the free energy density of QCD is expanded in a series in the chemical
potential, mu, the Taylor coefficients are the non-linear quark number
susceptibilities. We show that these depend on the prescription for putting
chemical potential on the lattice, making all extrapolations in chemical
potential prescription dependent at finite lattice spacing. To put bounds on
the prescription dependence, we investigate the magnitude of the non-linear
susceptibilities over a range of temperature, T, in QCD with two degenerate
flavours of light dynamical quarks at lattice spacing 1/4T. The prescription
dependence is removed in quenched QCD through a continuum extrapolation, and
the dependence of the pressure, P, on mu is obtained.Comment: 15 pages, 2 figures. Data on chi_uuuu added, discussion enhance
Emergent Universe from A Composition of Matter, Exotic Matter and Dark Energy
A specific class of flat Emergent Universe (EU) is considered and its
viability is tested in view of the recent observations. Model parameters are
constrained from Stern data for Hubble Parameter and Redshift ( vs. )
and from a model independent measurement of BAO peak parameter. It is noted
that a composition of Exotic matter, dust and dark energy, capable of producing
an EU, can not be ruled out with present data. Evolution of other relevant
cosmological parameters, viz. density parameter (), effective equation
of state (EOS) parameter () are also shown.Comment: 5 pages, 5 figures (accepted in MNRAS
Phase transitions in self-gravitating systems. Self-gravitating fermions and hard spheres models
We discuss the nature of phase transitions in self-gravitating systems both
in the microcanonical and in the canonical ensemble. We avoid the divergence of
the gravitational potential at short distances by considering the case of
self-gravitating fermions and hard spheres models. Three kinds of phase
transitions (of zeroth, first and second order) are evidenced. They separate a
``gaseous'' phase with a smoothly varying distribution of matter from a
``condensed'' phase with a core-halo structure. We propose a simple analytical
model to describe these phase transitions. We determine the value of energy (in
the microcanonical ensemble) and temperature (in the canonical ensemble) at the
transition point and we study their dependance with the degeneracy parameter
(for fermions) or with the size of the particles (for a hard spheres gas).
Scaling laws are obtained analytically in the asymptotic limit of a small short
distance cut-off. Our analytical model captures the essential physics of the
problem and compares remarkably well with the full numerical solutions.Comment: Submitted to Phys. Rev. E. New material adde
Non-Commutativity of the Zero Chemical Potential Limit and the Thermodynamic Limit in Finite Density Systems
Monte Carlo simulations of finite density systems are often plagued by the
complex action problem. We point out that there exists certain
non-commutativity in the zero chemical potential limit and the thermodynamic
limit when one tries to study such systems by reweighting techniques. This is
demonstrated by explicit calculations in a Random Matrix Theory, which is
thought to be a simple qualitative model for finite density QCD. The
factorization method allows us to understand how the non-commutativity, which
appears at the intermediate steps, cancels in the end results for physical
observables.Comment: 7 pages, 9 figure
Can the Chaplygin gas be a plausible model for dark energy?
In this note two cosmological models representing the flat Friedmann Universe
filled with a Chaplygin fluid, with or without dust, are analyzed in terms of
the recently proposed "statefinder" parameters. Trajectories of both models in
the parameter plane are shown to be significantly different w.r.t. "quiessence"
and "tracker" models. The generalized Chaplygin gas model with an equation of
state of the form is also analyzed in terms of the
statefinder parameters.Comment: 6 pages, 2 figure
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
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
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
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