7,828 research outputs found
Exact Dissipative Cosmologies with Stiff Fluid
The general solution of the gravitational field equations in the flat
Friedmann-Robertson-Walker geometry is obtained in the framework of the full
Israel-Stewart-Hiscock theory for a bulk viscous stiff cosmological fluid, with
bulk viscosity coefficient proportional to the energy density.Comment: 7 pages, 6 figure
Full causal dissipative cosmologies with stiff matter
The general solution of the gravitational field equations for a full causal
bulk viscous stiff cosmological fluid, with bulk viscosity coefficient
proportional to the energy density to the power 1/4, is obtained in the flat
Friedmann-Robertson-Walker geometry. The solution describes a non-inflationary
Universe, which starts its evolution from a singular state. The time variation
of the scale factor, deceleration parameter, viscous pressure, viscous
pressure-thermodynamic pressure ratio, comoving entropy and Ricci and
Kretschmann invariants is considered in detail.Comment: 6 pages, 6 figures, to appear in Int. J. Mod. Phys.
Large-Scale Simulations of the Two-Dimensional Melting of Hard Disks
Large-scale computer simulations involving more than a million particles have
been performed to study the melting transition in a two-dimensional hard disk
fluid. The van der Waals loop previously observed in the pressure-density
relationship of smaller simulations is shown to be an artifact of finite-size
effects. Together with a detailed scaling analysis of the bond orientation
order, the new results provide compelling evidence for the
Halperin-Nelson-Young picture. Scaling analysis of the translational order also
yields a lower bound for the melting density that is much higher than
previously thought.Comment: 4 pages, 4 figure
Anisotropic Stars in General Relativity
We present a class of exact solutions of Einstein's gravitational field
equations describing spherically symmetric and static anisotropic stellar type
configurations. The solutions are obtained by assuming a particular form of the
anisotropy factor. The energy density and both radial and tangential pressures
are finite and positive inside the anisotropic star. Numerical results show
that the basic physical parameters (mass and radius) of the model can describe
realistic astrophysical objects like neutron stars.Comment: 12 pages, 5 figures, revised version to appear in Proc. R. Soc.
London A: Mathematical, Physical & Engineering Science
Isotropic stars in general relativity
We present a general solution of the Einstein gravitational field equations
for the static spherically symmetric gravitational interior spacetime of an
isotropic fluid sphere. The solution is obtained by transforming the pressure
isotropy condition, a second order ordinary differential equation, into a
Riccati type first order differential equation, and using a general
integrability condition for the Riccati equation. This allows us to obtain an
exact non-singular solution of the interior field equations for a fluid sphere,
expressed in the form of infinite power series. The physical features of the
solution are studied in detail numerically by cutting the infinite series
expansions, and restricting our numerical analysis by taking into account only
terms in the power series representations of the relevant astrophysical
parameters. In the present model all physical quantities (density, pressure,
speed of sound etc.) are finite at the center of the sphere. The physical
behavior of the solution essentially depends on the equation of state of the
dense matter at the center of the star. The stability properties of the model
are also analyzed in detail for a number of central equations of state, and it
is shown that it is stable with respect to the radial adiabatic perturbations.
The astrophysical analysis indicates that this solution can be used as a
realistic model for static general relativistic high density objects, like
neutron stars.Comment: 12 pages, 10 figures, accepted for publication in EPJC; references
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Brans-Dicke cosmology with a scalar field potential
Three solutions of the Brans-Dicke theory with a self-interacting quartic
potential and perfect fluid distribution are presented for a spatially flat
geometry. The physical behavior is consistent with the recent cosmological
scenario favored by type Ia supernova observations, indicating an accelerated
expansion of the Universe.Comment: 6 pages, 4 figure
A New Monte Carlo Method and Its Implications for Generalized Cluster Algorithms
We describe a novel switching algorithm based on a ``reverse'' Monte Carlo
method, in which the potential is stochastically modified before the system
configuration is moved. This new algorithm facilitates a generalized
formulation of cluster-type Monte Carlo methods, and the generalization makes
it possible to derive cluster algorithms for systems with both discrete and
continuous degrees of freedom. The roughening transition in the sine-Gordon
model has been studied with this method, and high-accuracy simulations for
system sizes up to were carried out to examine the logarithmic
divergence of the surface roughness above the transition temperature, revealing
clear evidence for universal scaling of the Kosterlitz-Thouless type.Comment: 4 pages, 2 figures. Phys. Rev. Lett. (in press
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