97 research outputs found
Graviton production in anti-de Sitter braneworld cosmology: A fully consistent treatment of the boundary condition
In recent work by two of us, [Durrer & Ruser, PRL 99, 071601 (2007); Ruser &
Durrer PRD 76, 104014 (2007)], graviton production due to a moving spacetime
boundary (braneworld) in a five dimensional bulk has been considered. In the
same way as the presence of a conducting plate modifies the electromagnetic
vacuum, the presence of a brane modifies the graviton vacuum. As the brane
moves, the time dependence of the vacuum leads to particle creation via the so
called 'dynamical Casimir effect'. In our previous work a term in the boundary
condition which is linear in the brane velocity has been neglected. In this
work we develop a new approach which overcomes this approximation. We show that
the previous results are not modified if the brane velocity is low.Comment: 13 pages, 6 figures, added a clarifying paragraph about the setup,
the brane motion adapted w.r.t the version published in PR
Gauge invariant Boltzmann equation and the fluid limit
This article investigates the collisionless Boltzmann equation up to second
order in the cosmological perturbations. It describes the gauge dependence of
the distribution function and the construction of a gauge invariant
distribution function and brightness, and then derives the gauge invariant
fluid limit.Comment: 36 page
A fully covariant description of CMB anisotropies
Starting from the exact non-linear description of matter and radiation, a
fully covariant and gauge-invariant formula for the observed temperature
anisotropy of the cosmic microwave background (CBR) radiation, expressed in
terms of the electric () and magnetic () parts of the Weyl
tensor, is obtained by integrating photon geodesics from last scattering to the
point of observation today. This improves and extends earlier work by Russ et
al where a similar formula was obtained by taking first order variations of the
redshift. In the case of scalar (density) perturbations, is related to
the harmonic components of the gravitational potential and the usual
dominant Sachs-Wolfe contribution to the
temperature anisotropy is recovered, together with contributions due to the
time variation of the potential (Rees-Sciama effect), entropy and velocity
perturbations at last scattering and a pressure suppression term important in
low density universes. We also explicitly demonstrate the validity of assuming
that the perturbations are adiabatic at decoupling and show that if the surface
of last scattering is correctly placed and the background universe model is
taken to be a flat dust dominated Friedmann-Robertson-Walker model (FRW), then
the large scale temperature anisotropy can be interpreted as being due to the
motion of the matter relative to the surface of constant temperature which
defines the surface of last scattering on those scales.Comment: 18 pages LaTeX, 1 figure. Submitted to Classical and Quantum Gravity.
Also available at http://shiva.mth.uct.ac.za/preprints/9705.htm
Cold, warm, and composite (cool) cosmic string models
The dynamical behaviour of a cosmic string is strongly affected by any
reduction of the effective string tension below the constant value
say that characterizes the simple, longitudinally Lorentz invariant, Goto Nambu
string model in terms of a fixed mass scale whose magnitude depends on that
of the Higgs field responsible for the existence of the string. Such a
reduction occurs in the standard "hot" cosmic string model in which the effect
of thermal perturbations of a simple Goto Nambu model is expressed by the
formula , where is the string
temperature. A qualitatively similar though analytically more complicated
tension reduction phenomenon occurs in "cold" conducting cosmic string models
where the role of the temperature is played by an effective chemical potential
that is constructed as the magnitude of the phase of a bosonic
condensate of the kind whose existence was first proposed by Witten. The
present article describes the construction and essential mechanical properties
of a category of "warm" cosmic string models that are intermediate between
these "hot" and "cold" extremes. These "warm" models are the string analogues
of the standard Landau model for a 2-constituent finite temperature superfluid,
and as such involve two independent currents interpretable as that of the
entropy on one hand and that of the bosonic condensate on the other. It is
surmised that the stationary (in particular ring) equilibrium states of such
"warm" cosmic strings may be of cosmological significance.Comment: 31 pages, Tex preprint version of manuscript subsequently published
(with editorial modifications) in Nuclear Physics
Identification of perturbation modes and controversies in ekpyrotic perturbations
If the linear perturbation theory is valid through the bounce, the surviving
fluctuations from the ekpyrotic scenario (cyclic one as well) should have very
blue spectra with suppressed amplitude for the scalar-type structure. We derive
the same (and consistent) result using the curvature perturbation in the
uniform-field (comoving) gauge and in the zero-shear gauge. Previously, Khoury
et al. interpreted results from the latter gauge condition incorrectly and
claimed the scale-invariant spectrum, thus generating controversy in the
literature. We also correct similar errors in the literature based on wrong
mode identification and joining condition. No joining condition is needed for
the derivation.Comment: 5 pages, substantially revised, match with PLB versio
Adiabatic perturbations in pre big bang models: matching conditions and scale invariance
At low energy, the four-dimensional effective action of the ekpyrotic model
of the universe is equivalent to a slightly modified version of the pre big
bang model. We discuss cosmological perturbations in these models. In
particular we address the issue of matching the perturbations from a collapsing
to an expanding phase in full generality. We show that, generically, one
obtains for the spectrum of scalar perturbations in the original pre big
model (with vanishing potential). When an exponential potential for the dilaton
is included, a scale invariant spectrum () of adiabatic scalar
perturbations is produced under very generic matching conditions, both in a
modified pre big bang and ekpyrotic scenario. We also derive general results
valid for power law scale factors matched to a radiation dominated era.Comment: 11 pages, 1 figure, revised version with small corrections to match
version in print. Results and conclusions unchange
The scalar bi-spectrum during preheating in single field inflationary models
In single field inflationary models, preheating refers to the phase that
immediately follows inflation, but precedes the epoch of reheating. During this
phase, the inflaton typically oscillates at the bottom of its potential and
gradually transfers its energy to radiation. At the same time, the amplitude of
the fields coupled to the inflaton may undergo parametric resonance and, as a
consequence, explosive particle production can take place. A priori, these
phenomena could lead to an amplification of the super-Hubble scale curvature
perturbations which, in turn, would modify the standard inflationary
predictions. However, remarkably, it has been shown that, although the
Mukhanov-Sasaki variable does undergo narrow parametric instability during
preheating, the amplitude of the corresponding super-Hubble curvature
perturbations remain constant. Therefore, in single field models, metric
preheating does not affect the power spectrum of the large scale perturbations.
In this article, we investigate the corresponding effect on the scalar
bi-spectrum. Using the Maldacena's formalism, we analytically show that, for
modes of cosmological interest, the contributions to the scalar bi-spectrum as
the curvature perturbations evolve on super-Hubble scales during preheating is
completely negligible. Specifically, we illustrate that, certain terms in the
third order action governing the curvature perturbations which may naively be
expected to contribute significantly are exactly canceled by other
contributions to the bi-spectrum. We corroborate selected analytical results by
numerical investigations. We conclude with a brief discussion of the results we
have obtained.Comment: v1: 15 pages, 4 figures; v2: 15 pages, 4 figures, discussion and
references added, to appear in Phys. Rev.
Assisted Contraction
We consider the dynamics of a contracting universe ruled by two minimally
coupled scalar fields with general exponential potentials. This model describes
string-inspired scenarios in the Einstein frame. Both background and
perturbations can be solved analytically in this model. Curvature perturbations
are generated with a scale invariant spectrum only for a dust-like collapse, as
happens for a single field model with an exponential potential. We find the
conditions for which a scale invariant spectrum for isocurvature perturbation
is generated.Comment: 13 pages, 3 figures, revised version, conclusions unchange
Parametric amplification of metric fluctuations through a bouncing phase
We clarify the properties of the behavior of classical cosmological
perturbations when the Universe experiences a bounce. This is done in the
simplest possible case for which gravity is described by general relativity and
the matter content has a single component, namely a scalar field in a closed
geometry. We show in particular that the spectrum of scalar perturbations can
be affected by the bounce in a way that may depend on the wave number, even in
the large scale limit. This may have important implications for string
motivated models of the early Universe.Comment: 17 pages, 12 figures, LaTeX-ReVTeX format, version to match Phys.
Rev.
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