316 research outputs found

### Brane-world cosmology

Brane-world models, where observers are restricted to a brane in a
higher-dimensional spacetime, offer a novel perspective on cosmology. I discuss
some approaches to cosmology in extra dimensions and some interesting aspects
of gravity and cosmology in brane-world models.Comment: 16 pages, 4 figures, to appear in proceedings of ERE2005, the XXVIII
Spanish Relativity Meeting, Oviedo, Spai

### Cosmological perturbations through the big bang

Several scenarios have been proposed in which primordial perturbations could
originate from quantum vacuum fluctuations in a phase corresponding to a
collapse phase (in an Einstein frame) preceding the Big Bang. I briefly review
three models which could produce scale-invariant spectra during collapse: (1)
curvature perturbations during pressureless collapse, (2) axion field
perturbations in a pre big bang scenario, and (3) tachyonic fields during
multiple-field ekpyrotic collapse. In the separate universes picture one can
derive generalised perturbation equations to describe the evolution of large
scale perturbations through a semi-classical bounce, assuming a large-scale
limit in which inhomogeneous perturbations can be described by locally
homogeneous patches. For adiabatic perturbations there exists a conserved
curvature perturbation on large scales, but isocurvature perturbations can
change the curvature perturbation through the non-adiabatic pressure
perturbation on large scales. Different models for the origin of large scale
structure lead to different observational predictions, including gravitational
waves and non-Gaussianity.Comment: 13 pages, latex, no figures. To appear in Adv Sci Lett, special issue
on Quantum Gravity, Cosmology amd Black Hole

### Primordial non-Gaussianity from mixed inflaton-curvaton perturbations

We characterise the primordial perturbations produced due to both inflaton
and curvaton fluctuations in models where the curvaton has a quadratic, cosine
or hyperbolic potential, and the inflaton potential is characterised by the
usual slow-roll parameters. Isocurvature curvaton field perturbations can
produce significant non-Gaussianity in the primordial density field, in
contrast with adiabatic inflaton field perturbations which produce negligible
non-Gaussianity for canonical scalar fields. A non-self-interacting curvaton
with quadratic potential produces a local-type non-Gaussianity that is well
described by the non-linearity parameter fNL, which may be scale-dependent when
the inflaton perturbations dominate the power spectrum. We show how
observational bounds on non-linearity parameters and the tensor-scalar ratio
can be used to constrain curvaton and inflaton parameters. We find a
consistency relation between the bispectrum and trispectrum parameters in a
mixed inflaton-curvaton model for a quadratic curvaton potential.
Self-interaction terms in the curvaton potential can lead to both a large
trispectrum parameter, gNL, and scale-dependence of the non-linearity
parameters.Comment: 17 pages, 8 figures, (v2 references added

### Tilted Ekpyrosis

We consider a simple model of cosmological collapse driven by canonical
fields with exponential potentials. We generalise the two-field ekpyrotic
collapse to consider non-orthogonal or tilted potentials and give the general
condition for isocurvature field fluctuations to have a scale-invariant
spectrum in this model. In particular we show that tilted potentials allow for
a slightly red spectrum of perturbations as required by current observations.
However a red spectrum of fluctuations implies that the two-field ekpyrotic
phase must have a finite duration and requires a preceding phase which sets the
initial conditions for what otherwise appears to be a fine-tuned trajectory in
the phase space.Comment: 5 pages, references adde

### 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

### Cosmological perturbations

We review the study of inhomogeneous perturbations about a homogeneous and
isotropic background cosmology. We adopt a coordinate based approach, but give
geometrical interpretations of metric perturbations in terms of the expansion,
shear and curvature of constant-time hypersurfaces and the orthogonal timelike
vector field. We give the gauge transformation rules for metric and matter
variables at first and second order. We show how gauge invariant variables are
constructed by identifying geometric or matter variables in physically-defined
coordinate systems, and give the relations between many commonly used
gauge-invariant variables. In particular we show how the Einstein equations or
energy-momentum conservation can be used to obtain simple evolution equations
at linear order, and discuss extensions to non-linear order. We present
evolution equations for systems with multiple interacting fluids and scalar
fields, identifying adiabatic and entropy perturbations. As an application we
consider the origin of primordial curvature and isocurvature perturbations from
field perturbations during inflation in the very early universe.Comment: 96 pages, submitted to Phys. Rep; v2: minor changes, typos corrected,
references added, 1 figure added, corresponds to published versio

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