328 research outputs found
Coupled boundary and bulk fields in anti-de Sitter
We investigate the dynamics of a boundary field coupled to a bulk field with
a linear coupling in an anti-de Sitter bulk spacetime bounded by a Minkowski
(Randall-Sundrum) brane. An instability criterion for the coupled boundary and
bulk system is found. There exists a tachyonic bound state when the coupling is
above a critical value, determined by the masses of the brane and bulk fields
and AdS curvature scale. This bound state is normalizable and localised near
the brane, and leads to a tachonic instability of the system on large scales.
Below the critical coupling, there is no tachyonic state and no bound state.
Instead, we find quasi-normal modes which describe stable oscillations, but
with a finite decay time. Only if the coupling is tuned to the critical value
does there exist a massless stable bound state, as in the case of zero coupling
for massless fields. We discuss the relation to gravitational perturbations in
the Randall-Sundrum brane-world.Comment: 12 pages, 4 figures, revtex
The cosmological gravitational wave background from primordial density perturbations
We discuss the gravitational wave background generated by primordial density
perturbations evolving during the radiation era. At second-order in a
perturbative expansion, density fluctuations produce gravitational waves. We
calculate the power spectra of gravitational waves from this mechanism, and
show that, in principle, future gravitational wave detectors could be used to
constrain the primordial power spectrum on scales vastly different from those
currently being probed by large-scale structure. As examples we compute the
gravitational wave background generated by both a power-law spectrum on all
scales, and a delta-function power spectrum on a single scale.Comment: 8 Page
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
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
Fully relativistic predictions in Horndeski gravity from standard Newtonian N-body simulations
The N-body gauge allows the introduction of relativistic effects in Newtonian cosmological simulations. Here we extend this framework to general Horndeski gravity theories, and investigate the relativistic effects that the scalar field introduces in the matter power spectrum at intermediate and large scales. In particular, we show that the kineticity function at these scales enhances the amplitude of the signal of contributions coming from the extra degree of freedom. Using the Quasi-Static Approximation (QSA), we separate modified gravity effects into two parts: one that only affects small-scale physics, and one that is due to relativistic effects. This allows our formalism to be readily implemented in modified gravity N-body codes in a straightforward manner, e.g., relativistic effects can be included as an additional linear density field in simulations. We identify the emergence of gravity acoustic oscillations (GAOs) in the matter power spectrum at large scales, Mpc. GAO features have a purely relativistic origin, coming from the dynamical nature of the scalar field. GAOs may be enhanced to detectable levels by the rapid evolution of the dark energy sound horizon in certain modified gravity models and can be seen as a new test of gravity at scales probed by future galaxy and intensity-mapping surveys
The Genesis of Cosmological Tracker Fields
The role of the quintessence field as a probable candidate for the repulsive
dark energy, the conditions for tracking and the requisites for tracker fields
are examined. The concept of `integrated tracking' is introduced and a new
criterion for the existence of tracker potentials is derived assuming monotonic
increase in the scalar energy density parameter \Omega_\phi with the evolution
of the universe as suggested by the astrophysical constraints. It provides a
technique to investigate generic potentials of the tracker fields. The general
properties of the tracker fields are discussed and their behaviour with respect
to tracking parameter \epsilon is analyzed. It is shown that the tracker fields
around the limiting value \epsilon \simeq \frac 23 give the best fit with the
observational constraints.Comment: 8 pages, Latex file, 1 figure, comments adde
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
Non-Gaussian perturbations from multi-field inflation
We show how the primordial bispectrum of density perturbations from inflation
may be characterised in terms of manifestly gauge-invariant cosmological
perturbations at second order. The primordial metric perturbation, zeta,
describing the perturbed expansion of uniform-density hypersurfaces on large
scales is related to scalar field perturbations on unperturbed (spatially-flat)
hypersurfaces at first- and second-order. The bispectrum of the metric
perturbation is thus composed of (i) a local contribution due to the
second-order gauge-transformation, and (ii) the instrinsic bispectrum of the
field perturbations on spatially flat hypersurfaces. We generalise previous
results to allow for scale-dependence of the scalar field power spectra and
correlations that can develop between fields on super-Hubble scales.Comment: 11 pages, RevTex; minor changes to text; conclusions unchanged;
version to appear in JCA
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
Scalar tilt from broken conformal invariance
Within recently proposed scenario which explains flatness of the spectrum of
scalar cosmological perturbations by a combination of conformal and global
symmetries, we discuss the effect of weak breaking of conformal invariance. We
find that the scalar power spectrum obtains a small tilt which depends on both
the strength of conformal symmetry breaking and the law of evolution of the
scale factor
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