11 research outputs found
Primordial perturbations from slow-roll inflation on a brane
In this paper we quantise scalar perturbations in a Randall-Sundrum-type model of inflation where the inflaton field is confined to a single brane embedded in five-dimensional anti-de Sitter space-time. In the high energy regime, small-scale inflaton fluctuations are strongly coupled to metric perturbations in the bulk and gravitational back-reaction has a dramatic effect on the behaviour of inflaton perturbations on sub-horizon scales. This is in contrast to the standard four-dimensional result where gravitational back-reaction can be neglected on small scales. Nevertheless, this does not give rise to significant particle production, and the correction to the power spectrum of the curvature perturbations on super-horizon scales is shown to be suppressed by a slow-roll parameter. We calculate the complete first order slow-roll corrections to the spectrum of primordial curvature perturbations
Scalar perturbations in braneworld cosmology
We study the behaviour of scalar perturbations in the radiation-dominated era
of Randall-Sundrum braneworld cosmology by numerically solving the coupled bulk
and brane master wave equations. We find that density perturbations with
wavelengths less than a critical value (set by the bulk curvature length) are
amplified during horizon re-entry. This means that the radiation era matter
power spectrum will be at least an order of magnitude larger than the
predictions of general relativity (GR) on small scales. Conversely, we
explicitly confirm from simulations that the spectrum is identical to GR on
large scales. Although this magnification is not relevant for the cosmic
microwave background or measurements of large scale structure, it will have
some bearing on the formation of primordial black holes in Randall-Sundrum
models.Comment: 17 pages, 7 figure
Slow-roll corrections to inflaton fluctuations on a brane
Quantum fluctuations of an inflaton field, slow-rolling during inflation are
coupled to metric fluctuations. In conventional four dimensional cosmology one
can calculate the effect of scalar metric perturbations as slow-roll
corrections to the evolution of a massless free field in de Sitter spacetime.
This gives the well-known first-order corrections to the field perturbations
after horizon-exit. If inflaton fluctuations on a four dimensional brane
embedded in a five dimensional bulk spacetime are studied to first-order in
slow-roll then we recover the usual conserved curvature perturbation on
super-horizon scales. But on small scales, at high energies, we find that the
coupling to the bulk metric perturbations cannot be neglected, leading to a
modified amplitude of vacuum oscillations on small scales. This is a large
effect which casts doubt on the reliability of the usual calculation of
inflaton fluctuations on the brane neglecting their gravitational coupling.Comment: 18 pages, 4 figure
Slow-roll corrections to inflaton fluctuations on a brane
Quantum fluctuations of an inflaton field, slow-rolling during inflation are
coupled to metric fluctuations. In conventional four dimensional cosmology one
can calculate the effect of scalar metric perturbations as slow-roll
corrections to the evolution of a massless free field in de Sitter spacetime.
This gives the well-known first-order corrections to the field perturbations
after horizon-exit. If inflaton fluctuations on a four dimensional brane
embedded in a five dimensional bulk spacetime are studied to first-order in
slow-roll then we recover the usual conserved curvature perturbation on
super-horizon scales. But on small scales, at high energies, we find that the
coupling to the bulk metric perturbations cannot be neglected, leading to a
modified amplitude of vacuum oscillations on small scales. This is a large
effect which casts doubt on the reliability of the usual calculation of
inflaton fluctuations on the brane neglecting their gravitational coupling.Comment: 18 pages, 4 figure
Primordial perturbations from slow-roll inflation on a brane
In this paper we quantise scalar perturbations in a Randall-Sundrum-type
model of inflation where the inflaton field is confined to a single brane
embedded in five-dimensional anti-de Sitter space-time. In the high energy
regime, small-scale inflaton fluctuations are strongly coupled to metric
perturbations in the bulk and gravitational back-reaction has a dramatic effect
on the behaviour of inflaton perturbations on sub-horizon scales. This is in
contrast to the standard four-dimensional result where gravitational
back-reaction can be neglected on small scales. Nevertheless, this does not
give rise to significant particle production, and the correction to the power
spectrum of the curvature perturbations on super-horizon scales is shown to be
suppressed by a slow-roll parameter. We calculate the complete first order
slow-roll corrections to the spectrum of primordial curvature perturbations.Comment: 23 pages, 10 figure
Living on a dS brane: Effects of KK modes on inflation
We develop a formalism to study non-local higher-dimensional effects in
braneworld scenarios from a four-dimensional effective theory point of view and
check it against the well-known Garriga-Tanaka result in the appropriate limit.
We then use this formalism to study the spectrum of density perturbations
during inflation as seen from the lower-dimensional effective theory. In
particular, we find that the gravitational potential is greatly enhanced at
short wavelengths. The consequences to the curvature perturbations are
nonetheless very weak and will lead to no characteristic signatures on the
power spectrum.Comment: 21 pages, no figure
String-inspired cosmology
I discuss cosmological models either derived from, or inspired by, string
theory or M-theory. In particular I discuss solutions in the low-energy
effective theory and the role of the dilaton, moduli and antisymmetric form
fields in the dimensionally reduced effective action. The pre big bang model is
an attempt to use cosmological solutions to make observational predictions. I
then discuss the effective theory of gravity found in recent brane-world models
where we live on a 3-brane embedded in a five-dimensional spacetime and how the
study of cosmological perturbations may enable us to test these ideas.Comment: 15 pages, 5 figures, latex with iopart, invited talk at `The Early
Universe and Cosmological Observations: a Critical Review', Cape Town, July
200
Cosmological perturbations in the bulk and on the brane
We study cosmological perturbations in a brane-world scenario where the
matter fields live on a four-dimensional brane and gravity propagates in the
five-dimensional bulk. We present the equations of motion in an arbitrary gauge
for metric perturbations in the bulk and matter perturbations on the brane.
Gauge-invariant perturbations are then constructed corresponding to
perturbations in longitudinal and Gaussian normal gauges. Longitudinal gauge
metric perturbations may be directly derived from three master variables
(separately describing scalar, vector and tensor metric perturbations) which
obey five-dimensional wave-equations. Gaussian normal gauge perturbations are
directly related to the induced metric perturbations on the brane with the
additional bulk degrees of freedom interpreted as an effective Weyl
energy-momentum tensor on the brane. We construct gauge-invariant perturbations
describing the effective density, momentum and pressures of this Weyl fluid at
the brane and throughout the bulk. We show that there exist gauge-invariant
curvature perturbations on the brane and in the bulk that are conserved on
large-scales when three-dimensional spatial gradients are negligible.Comment: 26 pages, 1 figure; equations for 5D longitudinal gauge perturbations
in terms of master variables corrected. Published versio
Cosmology and Brane Worlds: A Review
Cosmological consequences of the brane world scenario are reviewed in a
pedagogical manner. According to the brane world idea, the standard model
particles are confined on a hyper--surface (a so--called brane), which is
embedded in a higher--dimensional spacetime (the so--called bulk). We begin our
review with the simplest consistent brane world model: a single brane embedded
in a five--dimensional Anti-de Sitter space--time. Then we include a scalar
field in the bulk and discuss in detail the difference with the Anti-de Sitter
case. The geometry of the bulk space--time is also analysed in some depth.
Finally, we investigate the cosmology of a system with two branes and a bulk
scalar field. We comment on brane collisions and summarize some open problems
of brane world cosmology.Comment: 37 pages; invited topical review for Classical and Quantum Gravity;
to appea