133 research outputs found
Cosmological matching conditions
We investigate the evolution of scalar metric perturbations across a sudden
cosmological transition, allowing for an inhomogeneous surface stress at the
transition leading to a discontinuity in the local expansion rate, such as
might be expected in a big crunch/big bang event. We assume that the transition
occurs when some function of local matter variables reaches a critical value,
and that the surface stress is also a function of local matter variables. In
particular we consider the case of a single scalar field and show that a
necessary condition for the surface stress tensor to be perturbed at the
transition is the presence of a non-zero intrinsic entropy perturbation of the
scalar field. We present the matching conditions in terms of gauge-invariant
variables assuming a sudden transition to a fluid-dominated universe with
barotropic equation of state. For adiabatic perturbations the comoving
curvature perturbation is continuous at the transition, while the Newtonian
potential may be discontinuous if there is a discontinuity in the background
Hubble expansion.Comment: 12 pages, no figure
Dynamics of dark energy
In this paper we review in detail a number of approaches that have been
adopted to try and explain the remarkable observation of our accelerating
Universe. In particular we discuss the arguments for and recent progress made
towards understanding the nature of dark energy. We review the observational
evidence for the current accelerated expansion of the universe and present a
number of dark energy models in addition to the conventional cosmological
constant, paying particular attention to scalar field models such as
quintessence, K-essence, tachyon, phantom and dilatonic models. The importance
of cosmological scaling solutions is emphasized when studying the dynamical
system of scalar fields including coupled dark energy. We study the evolution
of cosmological perturbations allowing us to confront them with the observation
of the Cosmic Microwave Background and Large Scale Structure and demonstrate
how it is possible in principle to reconstruct the equation of state of dark
energy by also using Supernovae Ia observational data. We also discuss in
detail the nature of tracking solutions in cosmology, particle physics and
braneworld models of dark energy, the nature of possible future singularities,
the effect of higher order curvature terms to avoid a Big Rip singularity, and
approaches to modifying gravity which leads to a late-time accelerated
expansion without recourse to a new form of dark energy.Comment: 93 pages, 26 figures, Invited Review to be submitted to International
Journal of Modern Physics D; comments are welcome; Additional references
included in response to over 60 comments received. Rewriting of sub-sections
on anthropic principle and gravitational backreaction. New subsections adde
Cosmological parameter estimation and the spectral index from inflation
Accurate estimation of cosmological parameters from microwave background
anisotropies requires high-accuracy understanding of the cosmological model.
Normally, a power-law spectrum of density perturbations is assumed, in which
case the spectral index can be measured to around using
microwave anisotropy satellites such as MAP and Planck. However, inflationary
models generically predict that the spectral index of the density
perturbation spectrum will be scale-dependent. We carry out a detailed
investigation of the measurability of this scale dependence by Planck,
including the influence of polarization on the parameter estimation. We also
estimate the increase in the uncertainty in all other parameters if the scale
dependence has to be included. This increase applies even if the scale
dependence is too small to be measured unless it is assumed absent, but is
shown to be a small effect. We study the implications for inflation models,
beginning with a brief examination of the generic slow-roll inflation
situation, and then move to a detailed examination of a recently-devised hybrid
inflation model for which the scale dependence of may be observable.Comment: 6 pages LaTeX file with one figure incorporated (uses mn.sty and
epsf). Important modifications to result
Classical theory of radiating strings
The divergent part of the self force of a radiating string coupled to gravity, an antisymmetric tensor and a dilaton in four dimensions are calculated to first order in classical perturbation theory. While this divergence can be absorbed into a renormalization of the string tension, demanding that both it and the divergence in the energy momentum tensor vanish forces the string to have the couplings of compactified N = 1 D = 10 supergravity. In effect, supersymmetry cures the classical infinities
Cosmological Solutions of Low-Energy Heterotic M-Theory
We derive a set of exact cosmological solutions to the D=4, N=1 supergravity
description of heterotic M-theory. Having identified a new and exact SU(3) Toda
model solution, we then apply symmetry transformations to both this solution
and to a previously known SU(2) Toda model, in order to derive two further sets
of new cosmological solutions. In the symmetry-transformed SU(3) Toda case we
find an unusual "bouncing" motion for the M5 brane, such that this brane can be
made to reverse direction part way through its evolution. This bounce occurs
purely through the interaction of non-standard kinetic terms, as there are no
explicit potentials in the action. We also present a perturbation calculation
which demonstrates that, in a simple static limit, heterotic M-theory possesses
a scale-invariant isocurvature mode. This mode persists in certain asymptotic
limits of all the solutions we have derived, including the bouncing solution.Comment: 24 pages, 2 tables, 9 eps figures; minor corrections to conten
Using atom interferometry to detect dark energy
We review the tantalising prospect that the first evidence for the dark energy driving the observed acceleration of the Universe on giga-parsec scales may be found through metre scale laboratory based atom interferometry experiments. To do that, we first introduce the idea that scalar fields could be responsible for dark energy and show that in order to be compatible with fifth force constraints these fields must have a screening mechanism which hides their effects from us within the solar system. Particular emphasis is placed on one such screening mechanism known as the chameleon effect where the field's mass becomes dependent on the environment. The way the field behaves in the presence of a spherical source is determined and we then go on to show how in the presence of the kind of high vacuum associated with atom interferometry experiments, and when the test particle is an atom, it is possible to use the associated interference pattern to place constraints on the acceleration due to the fifth force of the chameleon field - this has already been used to rule out large regions of the chameleon parameter space and maybe one day will be able to detect the force due to the dark energy field in the laboratory
Self-tuning and the derivation of the Fab Four
We have recently proposed a special class of scalar tensor theories known as
the Fab Four. These arose from attempts to analyse the cosmological constant
problem within the context of Horndeski's most general scalar tensor theory.
The Fab Four together give rise to a model of self-tuning, with the relevant
solutions evading Weinberg's no-go theorem by relaxing the condition of
Poincare invariance in the scalar sector. The Fab Four are made up of four
geometric terms in the action with each term containing a free potential
function of the scalar field. In this paper we rigorously derive this model
from the general model of Horndeski, proving that the Fab Four represents the
only classical scalar tensor theory of this type that has any hope of tackling
the cosmological constant problem. We present the full equations of motion for
this theory, and give an heuristic argument to suggest that one might be able
to keep radiative corrections under control. We also give the Fab Four in terms
of the potentials presented in Deffayet et al's version of Horndeski.Comment: 25 pages, 1 figur
Reheating and gravitino production in braneworld inflation
We consider the constraints that can be imposed on a wide class of Inflation
models in modified gravity scenarios in which the Friedmann equation is
modified by the inclusion of terms, where is the total energy
density. In particular we obtain the reheating temperature and gravitino
abundance associated with the end of inflation. Whereas models of chaotic
inflation and natural inflation can easily avoid the conventional gravitino
overproduction problem, we show that supersymmetric hybrid inflation models
(driven by both F and D-terms) do not work in the dominated era. We
also study inflation driven by exponetial potentials in this modified
background, and show that the gravitino production is suppressed enough to
avoid there being a problem, although other conditions severely constrain these
models.Comment: 24page
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