1,274 research outputs found
Constraints on Inflationary Solutions in the Presence of Shear and Bulk Viscosity
Inflationary models and their claim to solve many of the outstanding problems
in cosmology have been the subject of a great deal of debate over the last few
years. A major sticking point has been the lack of both good observational and
theoretical arguments to single out one particular model out of the many that
solve these problems. Here we examine the degree of restrictiveness on the
dynamical relationship between the cosmological scale factor and the inflation
driving self-interaction potential of a minimally coupled scalar field, imposed
by the condition that the scalar field is required to be real during a
classical regime (the reality condition). We systema\-tically look at the
effects of this constraint on many of the inflationary models found in the
literature within the FLRW framework, and also look at what happens when
physically motivated perturbations such as shear and bulk viscosity are
introduced. We find that in many cases, either the models are totally excluded
or the reality condition gives rise to constraints on the scale factor and on
the various parameters of the model.Comment: 21 pages, LaTe
On average properties of inhomogeneous fluids in general relativity II: perfect fluid cosmologies
For general relativistic spacetimes filled with an irrotational perfect fluid
a generalized form of Friedmann's equations governing the expansion factor of
spatially averaged portions of inhomogeneous cosmologies is derived. The
averaging problem for scalar quantities is condensed into the problem of
finding an `effective equation of state' including kinematical as well as
dynamical `backreaction' terms that measure the departure from a standard FLRW
cosmology. Applications of the averaged models are outlined including
radiation-dominated and scalar field cosmologies (inflationary and
dilaton/string cosmologies). In particular, the averaged equations show that
the averaged scalar curvature must generically change in the course of
structure formation, that an averaged inhomogeneous radiation cosmos does not
follow the evolution of the standard homogeneous-isotropic model, and that an
averaged inhomogeneous perfect fluid features kinematical `backreaction' terms
that, in some cases, act like a free scalar field source. The free scalar field
(dilaton) itself, modelled by a `stiff' fluid, is singled out as a special
inhomogeneous case where the averaged equations assume a simple form.Comment: TeX 21 pages, matches published version: G.R.G., in pres
A note on second-order perturbations of non-canonical scalar fields
We study second-order perturbations for a general non-canonical scalar field,
minimally coupled to gravity, on the unperturbed FRW background, where metric
fluctuations are neglected a priori. By employing different approaches to
cosmological perturbation theory, we show that, even in this simplified set-up,
the second-order perturbations to the stress tensor, the energy density and the
pressure display potential instabilities, which are not present at linear
order. The conditions on the Lagrangian under which these instabilities take
place are provided. We also discuss briefly the significance of our analysis in
light of the possible linearization instability of these fields about the FRW
background.Comment: 8 page, Revtex 4. Clarifications added, results unchanged; [v3] 10
pages, matches with the published version, Discussion for specific cases
expanded and preliminary results including the metric perturbations discusse
Lagrangian description of fluid flow with pressure in relativistic cosmology
The Lagrangian description of fluid flow in relativistic cosmology is
extended to the case of flow accelerated by pressure. In the description, the
entropy and the vorticity are obtained exactly for the barotropic equation of
state. In order to determine the metric, the Einstein equation is solved
perturbatively, when metric fluctuations are small but entropy inhomogeneities
are large. Thus, the present formalism is applicable to the case when the
inhomogeneities are small in the large scale but locally nonlinear.Comment: 11 pages (RevTeX); accepted for publication in Phys. Rev.
Bounce behaviour in Kantowski-Sachs and Bianchi Cosmologies
Many cosmological scenarios envisage either a bounce of the universe at early
times, or collapse of matter locally to form a black hole which re-expands into
a new expanding universe region. Energy conditions preclude this happening for
ordinary matter in general relativistic universes, but scalar or dilatonic
fields can violate some of these conditions, and so could possibly provide
bounce behaviour. In this paper we show that such bounces cannot occur in
Kantowski-Sachs models without violating the {\it reality condition}
. This also holds true for other isotropic spatially
homogenous Bianchi models, with the exception of closed
Friedmann-Robertson-Walker and Bianchi IX models; bounce behaviour violates the
{\em weak energy condition} and . We turn to the
Randall-Sundrum type braneworld scenario for a possible resolution of this
problem.Comment: Matches published versio
Exact Conformal Scalar Field Cosmologies
New exact solutions of Einstein's gravity coupled to a self-interacting
conformal scalar field are derived in this work. Our approach extends a
solution-generating technique originally introduced by Bekenstein for massless
conformal scalar fields. Solutions are obtained for a
Friedmann-Robertson-Walker geometry both for the cases of zero and non-zero
curvatures, and a variety of interesting features are found. It is shown that
one class of solutions tends asymptotically to a power-law inflationary
behaviour with , while another class exhibits a late time
approach to the behaviour of the coasting models. Bouncing models
which avoid an initial singularity are also obtained. A general discussion of
the asymptotic behaviour and of the possibility of occurrence of inflation is
provided.Comment: Latex, 27 pages plus 16 figures (not included, available from the
authors upon request) DFFCUL-94-01-0
Homogeneous cosmologies with cosmological constant
Spatially homogeneous cosmological models with a positive cosmological
constant are investigated, using dynamical systems methods. We focus on the
future evolution of these models. In particular, we address the question
whether there are models within this class that are de Sitter-like in the
future, but are tilted.Comment: 10 pages, 13 eps-figures. Submitted to Phys. Rev.
An Inflationary Model with an Exact Perturbation Spectrum
We present a new, exact scalar field cosmology for which the spectrum of
scalar (density) perturbations can be calculated exactly. We use this exact
result to the probe the accuracy of approximate calculations of the
perturbation spectrum.Comment: 7 pages, plus 2 figure
Dense quark matter in compact stars
The densest predicted state of matter is colour-superconducting quark matter,
in which quarks near the Fermi surface form a condensate of Cooper pairs. This
form of matter may well exist in the core of compact stars, and the search for
signatures of its presence is an ongoing enterprise. Using a bag model of quark
matter, I discuss the effects of colour superconductivity on the mass-radius
relationship of compact stars, showing that colour superconducting quark matter
can occur in compact stars at values of the bag constant where ordinary quark
matter would not be allowed. The resultant ``hybrid'' stars with colour
superconducting quark matter interior and nuclear matter surface have masses in
the range 1.3-1.6 Msolar and radii 8-11 km. Once perturbative corrections are
included, quark matter can show a mass-radius relationship very similar to that
of nuclear matter, and the mass of a hybrid star can reach 1.8 \Msolar.Comment: 11 pages, for proceedings of SQM 2003 conference; references added,
abstract reworde
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