527 research outputs found
Preheating after N-flation
We study preheating in N-flation, assuming the Mar\v{c}enko-Pastur mass
distribution, equal energy initial conditions at the beginning of inflation and
equal axion-matter couplings, where matter is taken to be a single, massless
bosonic field. By numerical analysis we find that preheating via parametric
resonance is suppressed, indicating that the old theory of perturbative
preheating is applicable. While the tensor-to-scalar ratio, the non-Gaussianity
parameters and the scalar spectral index computed for N-flation are similar to
those in single field inflation (at least within an observationally viable
parameter region), our results suggest that the physics of preheating can
differ significantly from the single field case.Comment: 14 pages, 14 figures, references added, fixed typo
CMB Anisotropies at Second Order I
We present the computation of the full system of Boltzmann equations at
second-order describing the evolution of the photon, baryon and cold dark
matter fluids. These equations allow to follow the time evolution of the Cosmic
Microwave Background (CMB) anisotropies at second-order at all angular scales
from the early epoch, when the cosmological perturbations were generated, to
the present through the recombination era. This paper sets the stage for the
computation of the full second-order radiation transfer function at all scales
and for a a generic set of initial conditions specifying the level of
primordial non-Gaussianity. In a companion paper, we will present the
computation of the three-point correlation function at recombination which is
so relevant for the issue of non-Gaussianity in the CMB anisotropies.Comment: 26 pages, LaTeX file, typos correcte
Phase-plane analysis of Friedmann-Robertson-Walker cosmologies in Brans-Dicke gravity
We present an autonomous phase-plane describing the evolution of
Friedmann-Robertson-Walker models containing a perfect fluid (with barotropic
index gamma) in Brans-Dicke gravity (with Brans-Dicke parameter omega). We find
self-similar fixed points corresponding to Nariai's power-law solutions for
spatially flat models and curvature-scaling solutions for curved models. At
infinite values of the phase-plane variables we recover O'Hanlon and Tupper's
vacuum solutions for spatially flat models and the Milne universe for negative
spatial curvature. We find conditions for the existence and stability of these
critical points and describe the qualitative evolution in all regions of the
(omega,gamma) parameter space for 0-3/2. We show that the
condition for inflation in Brans-Dicke gravity is always stronger than the
general relativistic condition, gamma<2/3.Comment: 24 pages, including 9 figures, LaTe
Inflation on a Warped Dvali-Gabadadze-Porrati Brane
We discuss an inflation model, in which the inflation is driven by a single
scalar field with exponential potential on a warped DGP brane. In contrast to
the power law inflation in standard model, we find that the inflationary phase
can exit spontaneously without any mechanism. The running of the index of
scalar perturbation spectrum can take an enough large value to match the
observation data, while other parameters are in a reasonable region.Comment: Revtex, v3: 15 pages including 6 eps figures, some changes made and
references added, to appear in JCA
An anti-Schwarzshild solution: wormholes and scalar-tensor solutions
We investigate a static solution with an hyperbolic nature, characterised by
a pseudo-spherical foliation of space. This space-time metric can be perceived
as an anti-Schwarzschild solution, and exhibits repulsive features. It belongs
to the class of static vacuum solutions termed "a degenerate static solution of
class A". In the present work we review its fundamental features, discuss the
existence of generalised wormholes, and derive its extension to scalar-tensor
gravity theories in general.Comment: 3 pages, contribution to the proceedings of the Spanish Relativity
Meeting-ERE200
Cosmologies with Energy Exchange
We provide a simple mathematical description of the exchange of energy
between two fluids in an expanding Friedmann universe with zero spatial
curvature. The evolution can be reduced to a single non-linear differential
equation which we solve in physically relevant cases and provide an analysis of
all the possible evolutions. Particular power-law solutions exist for the
expansion scale factor and are attractors at late times under particular
conditions. We show how a number of problems studied in the literature, such as
cosmological vacuum energy decay, particle annihilation, and the evolution of a
population of evaporating black holes, correspond to simple particular cases of
our model. In all cases we can determine the effects of the energy transfer on
the expansion scale factor. We also consider the situation in the presence of
anti-decaying fluids and so called phantom fluids which violate the dominant
energy conditions.Comment: 12 pages, 1 figur
Inhomogeneous vacuum energy
Vacuum energy remains the simplest model of dark energy which could drive the
accelerated expansion of the Universe without necessarily introducing any new
degrees of freedom. Inhomogeneous vacuum energy is necessarily interacting in
general relativity. Although the four-velocity of vacuum energy is undefined,
an interacting vacuum has an energy transfer and the vacuum energy defines a
particular foliation of spacetime with spatially homogeneous vacuum energy in
cosmological solutions. It is possible to give a consistent description of
vacuum dynamics and in particular the relativistic equations of motion for
inhomogeneous perturbations given a covariant prescription for the vacuum
energy, or equivalently the energy transfer four-vector, and we construct
gauge-invariant vacuum perturbations. We show that any dark energy cosmology
can be decomposed into an interacting vacuum+matter cosmology whose
inhomogeneous perturbations obey simple first-order equations.Comment: 8 pages; v2 clarified discussion of Chaplygin gas model, references
adde
Nonlinear curvature perturbations in an exactly soluble model of multi-component slow-roll inflation
Using the nonlinear formalism, we consider a simple exactly
soluble model of multi-component slow-roll inflation in which the nonlinear
curvature perturbation can be evaluated analytically.Comment: 4 pages, no figure, typos corrected, references added, final version
to be published in CQ
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