781 research outputs found
Dynamics of entropy perturbations in assisted dark energy with mixed kinetic terms
We study dynamics of entropy perturbations in the two-field assisted dark
energy model. Based on the scenario of assisted dark energy, in which one
scalar field is subdominant compared with the other in the early epoch, we show
that the entropy perturbations in this two-field system tend to be constant on
large scales in the early epoch and hence survive until the present era for a
generic evolution of both fields during the radiation and matter eras. This
behaviour of the entropy perturbations is preserved even when the fields are
coupled via kinetic interaction. Since, for assisted dark energy, the
subdominant field in the early epoch becomes dominant at late time, the entropy
perturbations can significantly influence the dynamics of density perturbations
in the universe. Assuming correlations between the entropy and curvature
perturbations, the entropy perturbations can enhance the integrated Sachs-Wolfe
(ISW) effect if the signs of the contributions from entropy perturbations and
curvature perturbations are opposite after the matter era, otherwise the ISW
contribution is suppressed. For canonical scalar field the effect of entropy
perturbations on ISW effect is small because the initial value of the entropy
perturbations estimated during inflation cannot be sufficiently large. However,
in the case of k-essence, the initial value of the entropy perturbations can be
large enough to affect the ISW effect to leave a significant imprint on the CMB
power spectrum.Comment: 25 pages, 8 figures, revised version, accepted for publication in
JCA
The curvature perturbation at second order
We give an explicit relation, up to second-order terms, between scalar-field fluctuations defined on spatially-flat slices and the curvature perturbation on uniform-density slices. This expression is a necessary ingredient for calculating observable quantities at second-order and beyond in multiple-field inflation. We show that traditional cosmological perturbation theory and the `separate universe' approach yield equivalent expressions for superhorizon wavenumbers, and in particular that all nonlocal terms can be eliminated from the perturbation-theory expressions
Practical tools for third order cosmological perturbations
We discuss cosmological perturbation theory at third order, deriving the
gauge transformation rules for metric and matter perturbations, and
constructing third order gauge invariant quantities. We present the Einstein
tensor components, the evolution equations for a perfect fluid, and the
Klein-Gordon equation at third order, including scalar, vector and tensor
perturbations. In doing so, we also give all second order tensor components and
evolution equations in full, exhilarating generality.Comment: 17 pages, revtex4; v2: corresponds to version published in JCA
A note on second-order perturbations of non-canonical scalar fields
We study second-order perturbations for a general non-canonical scalar field,
minimally coupled to gravity, on the unperturbed FRW background, where metric
fluctuations are neglected a priori. By employing different approaches to
cosmological perturbation theory, we show that, even in this simplified set-up,
the second-order perturbations to the stress tensor, the energy density and the
pressure display potential instabilities, which are not present at linear
order. The conditions on the Lagrangian under which these instabilities take
place are provided. We also discuss briefly the significance of our analysis in
light of the possible linearization instability of these fields about the FRW
background.Comment: 8 page, Revtex 4. Clarifications added, results unchanged; [v3] 10
pages, matches with the published version, Discussion for specific cases
expanded and preliminary results including the metric perturbations discusse
Gauge invariant averages for the cosmological backreaction
We show how to provide suitable gauge invariant prescriptions for the
classical spatial averages (resp. quantum expectation values) that are needed
in the evaluation of classical (resp. quantum) backreaction effects. We also
present examples illustrating how the use of gauge invariant prescriptions can
avoid interpretation problems and prevent misleading conclusions.Comment: 21 pages, no figures. Comments and references added, typos corrected.
Small corrections and reference added, matches version published in JCA
Gauge-invariant perturbations at second order in two-field inflation
We study the second-order gauge-invariant adiabatic and isocurvature
perturbations in terms of the scalar fields present during inflation, along
with the related fully non-linear space gradient of these quantities. We
discuss the relation with other perturbation quantities defined in the
literature. We also construct the exact cubic action of the second-order
perturbations (beyond any slow-roll or super-horizon approximations and
including tensor perturbations), both in the uniform energy density gauge and
the flat gauge in order to settle various gauge-related issues. We thus provide
the tool to calculate the exact non-Gaussianity beyond slow-roll and at any
scale.Comment: 28 pages, no figures. v2: Added a summary subsection 4.3 with further
discussion of the results. Generalized all super-horizon results of section 4
and appendix A to exact ones. Other minor textual changes and references
added. Conclusions unchanged. Matches published versio
Modelling non-dust fluids in cosmology
Currently, most of the numerical simulations of structure formation use
Newtonian gravity. When modelling pressureless dark matter, or `dust', this
approach gives the correct results for scales much smaller than the
cosmological horizon, but for scenarios in which the fluid has pressure this is
no longer the case. In this article, we present the correspondence of
perturbations in Newtonian and cosmological perturbation theory, showing exact
mathematical equivalence for pressureless matter, and giving the relativistic
corrections for matter with pressure. As an example, we study the case of
scalar field dark matter which features non-zero pressure perturbations. We
discuss some problems which may arise when evolving the perturbations in this
model with Newtonian numerical simulations and with CMB Boltzmann codes.Comment: 5 pages; v2: typos corrected and refs added, submitted version; v3:
version to appear in JCA
Vector and tensor contributions to the curvature perturbation at second order
We derive the evolution equation for the second order curvature perturbation
using standard techniques of cosmological perturbation theory. We do this for
different definitions of the gauge invariant curvature perturbation, arising
from different splits of the spatial metric, and compare the expressions. The
results are valid at all scales and include all contributions from scalar,
vector and tensor perturbations, as well as anisotropic stress, with all our
results written purely in terms of gauge invariant quantities. Taking the
large-scale approximation, we find that a conserved quantity exists only if, in
addition to the non-adiabatic pressure, the transverse traceless part of the
anisotropic stress tensor is also negligible. We also find that the version of
the gauge invariant curvature perturbation which is exactly conserved is the
one defined with the determinant of the spatial part of the inverse metric.Comment: 21 pages. Appendix added and conclusions extended. Updated to match
version published in JCA
Nonlinear superhorizon perturbations of non-canonical scalar field
We develop a theory of non-linear cosmological perturbations at superhorizon
scales for a scalar field with a Lagrangian of the form , where
and is the scalar field. We
employ the ADM formalism and the spatial gradient expansion approach to obtain
general solutions valid up to the second order in the gradient expansion. This
formulation can be applied to, for example, DBI inflation models to investigate
superhorizon evolution of non-Gaussianities. With slight modification, we also
obtain general solutions valid up to the same order for a perfect fluid with a
general equation of state .Comment: 14 page
Multiple field inflation
Inflation offers a simple model for very early evolution of our Universe and
the origin of primordial perturbations on large scales. Over the last 25 years
we have become familiar with the predictions of single-field models, but
inflation with more than one light scalar field can alter preconceptions about
the inflationary dynamics and our predictions for the primordial perturbations.
I will discuss how future observational data could distinguish between
inflation driven by one field, or many fields. As an example, I briefly review
the curvaton as an alternative to the inflaton scenario for the origin of
structure.Comment: 27 pages, no figures. To appear in proceedings of 22nd IAP
Colloquium, Inflation +25, Paris, June 200
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