5,231 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.

### 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

### 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

### Relativistic Compact Objects in Isotropic Coordinates

We present a matrix method for obtaining new classes of exact solutions for
Einstein's equations representing static perfect fluid spheres. By means of a
matrix transformation, we reduce Einstein's equations to two independent
Riccati type differential equations for which three classes of solutions are
obtained. One class of the solutions corresponding to the linear barotropic
type fluid with an equation of state $p=\gamma \rho$ is discussed in detail.Comment: 9 pages, no figures, accepted for publication in Pramana-Journal of
Physic

### A Comment on "Brans-Dicke Cosmology with a scalar field potential"

We show that a recent letter claiming to present exact cosmological solutions
in Brans-Dicke theory actually uses a flawed set of equations as the starting
point for their analysis. The results presented in the letter are therefore not
valid.Comment: 2 pages, no figures. To appear in Europhysics Letter

### Viscous dissipative effects in isotropic brane cosmology

We consider the dynamics of a viscous cosmological fluid in the generalized
Randall-Sundrum model for an isotropic brane. To describe the dissipative
effects we use the Israel-Hiscock-Stewart full causal thermodynamic theory. In
the limiting case of a stiff cosmological fluid with pressure equal to the
energy density, the general solution of the field equations can be obtained in
an exact parametric form for a cosmological fluid with constant bulk viscosity
and with a bulk viscosity coefficient proportional to the square root of the
energy density, respectively. The obtained solutions describe generally
non-inflationary brane worlds, starting from a singular state. During this
phase of evolution the comoving entropy of the Universe is an increasing
function of time, and thus a large amount of entropy is created in the brane
world due to viscous dissipative processes.Comment: 15 pages, 11 figure

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