1,004 research outputs found
Kaluza-Klein anisotropy in the CMB
We show that 5-dimensional Kaluza-Klein graviton stresses can slow the decay
of shear anisotropy on the brane to observable levels, and we use cosmic
microwave background anisotropies to place limits on the initial anisotropy
induced by these stresses. An initial shear to Hubble distortion of only \sim
10^{-3}\Omega_0h_0^2 at the 5D Planck time would allow the observed large-angle
CMB signal to be a relic mainly of KK tidal effects.Comment: 4 pages revtex; minor changes to improve clarit
Inflation driven by causal heat flux
We find a simple inflationary solution in an inhomogeneous spacetime with
heat flux. The heat flux obeys a causal transport equation, and counteracts the
inflationary decrease of energy density. At late times, the heat flux tends to
zero and the fluid approaches the equation of state .Comment: Latex 5 pages; to appear Gen. Rel. Gra
Asymmetric radiating brane-world
At high energies on a cosmological brane of Randall-Sundrum type, particle
interactions can produce gravitons that are emitted into the bulk and that can
feed a bulk black hole. We generalize previous investigations of such radiating
brane-worlds by allowing for a breaking of Z_2-symmetry, via different bulk
cosmological constants and different initial black hole masses on either side
of the brane. One of the notable features of asymmetry is a suppression of the
asymptotic level of dark radiation, which means that nucleosynthesis
constraints are easier to satisfy. There are also models where the radiation
escapes to infinity on one or both sides, rather than falling into a black
hole, but these models can have negative energy density on the brane.Comment: sign error in eq. (34) corrected; version to appear Phys. Rev.
Structure formation on the brane: A mimicry
We show how braneworld cosmology with bulk matter can explain structure
formation. In this scenario, the nonlocal corrections to the Friedmann
equations supply a Weyl fluid that can dominate over matter at late times due
to the energy exchange between the brane and the bulk. We demonstrate that the
presence of the Weyl fluid radically changes the perturbation equations, which
can take care of the fluctuations required to account for the large amount of
inhomogeneities observed in the local universe. Further, we show how this Weyl
fluid can mimic dark matter. We also investigate the bulk geometry responsible
for the scenario.Comment: 7 pages. Matches published versio
Anisotropic stresses in inhomogeneous universes
Anisotropic stress contributions to the gravitational field can arise from
magnetic fields, collisionless relativistic particles, hydrodynamic shear
viscosity, gravitational waves, skew axion fields in low-energy string
cosmologies, or topological defects. We investigate the effects of such
stresses on cosmological evolution, and in particular on the dissipation of
shear anisotropy. We generalize some previous results that were given for
homogeneous anisotropic universes, by including small inhomogeneity in the
universe. This generalization is facilitated by a covariant approach. We find
that anisotropic stress dominates the evolution of shear, slowing its decay.
The effect is strongest in radiation-dominated universes, where there is slow
logarithmic decay of shear.Comment: 7 pages Revte
Cosmological solutions with nonlinear bulk viscosity
A recently proposed nonlinear transport equation is used to model bulk
viscous cosmologies that may be far from equilibrium, as happens during viscous
fluid inflation or during reheating. The asymptotic stability of the de Sitter
and Friedmann solutions is investigated. The former is stable for bulk
viscosity index . New solutions are obtained in
the weakly nonlinear regime for . These solutions are singular and some of
them represent a late-time inflationary era.Comment: 16 pages Latex (IOP style); to appear Class. Quantum Gra
New Vistas in Braneworld Cosmology
Traditionally, higher-dimensional cosmological models have sought to provide
a description of the fundamental forces in terms of a unifying geometrical
construction. In this essay we discuss how, in their present incarnation,
higher-dimensional `braneworld' models might provide answers to a number of
cosmological puzzles including the issue of dark energy and the nature of the
big-bang singularity.Comment: Honorable mention in the 2002 Essay Competition of the Gravity
Research Foundation. 10 pages, 2 figure
Lie symmetries for equations in conformal geometries
We seek exact solutions to the Einstein field equations which arise when two
spacetime geometries are conformally related. Whilst this is a simple method to
generate new solutions to the field equations, very few such examples have been
found in practice. We use the method of Lie analysis of differential equations
to obtain new group invariant solutions to conformally related Petrov type D
spacetimes. Four cases arise depending on the nature of the Lie symmetry
generator. In three cases we are in a position to solve the master field
equation in terms of elementary functions. In the fourth case special solutions
in terms of Bessel functions are obtained. These solutions contain known models
as special cases.Comment: 19 pages, To appear in J. Phys.
Massless and massive graviton spectra in anisotropic dilatonic braneworld cosmologies
We consider a braneworld model in which an anisotropic brane is embedded in a
dilatonic background. We solve the background solutions and study the behavior
of the perturbations when the universe evolves from an inflationary Kasner
phase to a Minkowski phase. We calculate the massless mode spectrum, and find
that it does not differ from what expected in standard four-dimensional
cosmological models. We then evaluate the spectrum of both light
(ultrarelativistic) and heavy (nonrelativistic) massive modes, and find that,
at high energies, there can be a strong enhancement of the Kaluza-Klein
spectral amplitude, which can become dominant in the total spectrum. The
presence of the dilaton, on the contrary, decrease the relative importance of
the massive modes.Comment: 18 pages, 4 figures, Typos correction
Brane-world generalizations of the Einstein static universe
A static Friedmann brane in a 5-dimensional bulk (Randall-Sundrum type
scenario) can have a very different relation between the density, pressure,
curvature and cosmological constant than in the case of the general
relativistic Einstein static universe. In particular, static Friedmann branes
with zero cosmological constant and 3-curvature, but satisfying rho>0 and
rho+3p>0, are possible. Furthermore, we find static Friedmann branes in a bulk
that satisfies the Einstein equations but is not Schwarzschild-anti de Sitter
or its specializations. In the models with negative bulk cosmological constant,
a positive brane tension leads to negative density and 3-curvature.Comment: additional interpretation of new solutions; accepted by
Class.Quant.Gra
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