82 research outputs found
On the properties of the transition matrix in bouncing cosmologies
We elaborate further on the evolution properties of cosmological fluctuations
through a bounce. We show this evolution to be describable either by
``transmission'' and ``reflection'' coefficients or by an effective unitary
S-matrix. We also show that they behave in a time reversal invariant way.
Therefore, earlier results are now interpreted in a different perspective and
put on a firmer basis.Comment: 4 pages, 1 figure, to appear in PR
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
Particle Physics Models of Inflation and the Cosmological Density Perturbation
This is a review of particle-theory models of inflation, and of their
predictions for the primordial density perturbation that is thought to be the
origin of structure in the Universe. It contains mini-reviews of the relevant
observational cosmology, of elementary field theory and of supersymmetry, that
may be of interest in their own right. The spectral index , specifying
the scale-dependence of the spectrum of the curvature perturbation, will be a
powerful discriminator between models, when it is measured by Planck with
accuracy . The usual formula for is derived, as well as
its less familiar extension to the case of a multi-component inflaton; in both
cases the key ingredient is the separate evolution of causally disconnected
regions of the Universe. Primordial gravitational waves will be an even more
powerful discriminator if they are observed, since most models of inflation
predict that they are completely negligible. We treat in detail the new wave of
models, which are firmly rooted in modern particle theory and have
supersymmetry as a crucial ingredient. The review is addressed to both
astrophysicists and particle physicists, and each section is fairly homogeneous
regarding the assumed background knowledge.Comment: 156 pages, after final proof corrections and addition
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
Quantum Fields in a Big Crunch/Big Bang Spacetime
We consider quantum field theory on a spacetime representing the Big
Crunch/Big Bang transition postulated in the ekpyrotic or cyclic cosmologies.
We show via several independent methods that an essentially unique matching
rule holds connecting the incoming state, in which a single extra dimension
shrinks to zero, to the outgoing state in which it re-expands at the same rate.
For free fields in our construction there is no particle production from the
incoming adiabatic vacuum. When interactions are included the total particle
production for fixed external momentum is finite at tree level. We discuss a
formal correspondence between our construction and quantum field theory on de
Sitter spacetime.Comment: 30 pages, RevTex file, five postscript figure file
Zonal Modes of Cosmic Microwave Background Temperature Maps
All-sky maps of the cosmic microwave background temperature fluctuations are
usually represented by a spherical harmonic decomposition involving modes
labelled by their degree l and order m (where -l < m < +l). The zonal modes
(i.e those with m = 0) are of particular interest because they vary only with
galactic latitude; any anomalous behaviour in them might therefore be an
indication of erroneous foreground substraction. We perform a simple
statistical analysis of the modes with low l for sky maps derived via different
cleaning procedures from the Wilkinson Microwave Anisotropy Probe (WMAP) and
show that the zonal modes provide a useful diagnostic of possible systematics.Comment: 5 pages, 4 tables, 2 figures, accepted for publication in MNRA
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.
Design of a Cyclic Multiverse
Recently, it has been noticed that the amplification of the amplitude of
curvature perturbation cycle by cycle can lead to a cyclic multiverse scenario,
in which the number of universes increases cycle by cycle. However, this
amplification will also inevitably induce either the ultimate end of
corresponding cycle, or the resulting spectrum of perturbations inside
corresponding universe is not scale invariant, which baffles the existence of
observable universes. In this paper, we propose a design of a cyclic
multiverse, in which the observable universe can emerges naturally. The
significance of a long period of dark energy before the turnaround of each
cycle for this implementing is shown.Comment: 6 pages, 3 eps figures, v2: 1 eps figure added and some arguments
added, to published in PL
The Primordial Perturbation Spectrum from Various Expanding and Contracting Phases
In this paper, focusing on the case of single scalar field, we discuss
various expanding and contracting phases generating primordial perturbations,
and study the relation between the primordial perturbation spectrum from these
phases and the parameter w of state equation in details. Furthermore, we offer
an interesting classification for the primordial perturbation spectrum from
various phases, which may have important implications for building an early
universe scenario embedded in possible high energy theories.Comment: 5 pages, 3 eps figure
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.
- …