32 research outputs found
Perturbations in a Bouncing Brane Model
The question of how perturbations evolve through a bounce in the Cyclic and
Ekpyrotic models of the Universe is still a matter of ongoing debate. In this
report we show that the collision between boundary branes is in most cases
singular even in the full 5-D formalism, and that first order perturbation
theory breaks down for at least one perturbation variable. Only in the case
that the boundary branes approach each other with constant velocity shortly
before the bounce, can a consistent, non singular solution be found. It is then
possible to follow the perturbations explicitly until the actual collision. In
this case, we find that if a scale invariant spectrum developed on the hidden
brane, it will get transferred to the visible brane during the bounce.Comment: 15 pages, minor modifications, a few typos correcte
The Cosmology of Massless String Modes
We consider the spacetime dynamics of a gas of closed strings in the context
of General Relativity in a background of arbitrary spatial dimensions. Our
motivation is primarily late time String Gas Cosmology, where such a spacetime
picture has to emerge after the dilaton has stabilized. We find that after
accounting for the thermodynamics of a gas of strings, only string modes which
are massless at the self-dual radius are relevant, and that they lead to a
dynamics which is qualitatively different from that induced by the modes
usually considered in the literature. In the context of an ansatz with three
large spatial dimensions and an arbitrary number of small extra dimensions, we
obtain isotropic stabilization of these extra dimensions at the self-dual
radius. This stabilization occurs for fixed dilaton, and is induced by the
special string states we focus on. The three large dimensions undergo a regular
Friedmann-Robertson-Walker expansion. We also show that this framework for
late-time cosmology is consistent with observational bounds.Comment: 15 pages, no figures, references added (again
Radion Stabilization by Stringy Effects in General Relativity
We consider the effects of a gas of closed strings (treated quantum
mechanically) on a background where one dimension is compactified on a circle.
After we address the effects of a time dependent background on aspects of the
string spectrum that concern us, we derive the energy-momentum tensor for a
string gas and investigate the resulting space-time dynamics. We show that a
variety of trajectories are possible for the radius of the compactified
dimension, depending on the nature of the string gas, including a demonstration
within the context of General Relativity (i.e. without a dilaton) of a solution
where the radius of the extra dimension oscillates about the self-dual radius,
without invoking matter that violates the various energy conditions. In
particular, we find that in the case where the string gas is in thermal
equilibrium, the radius of the compactified dimension dynamically stabilizes at
the self-dual radius, after which a period of usual Friedmann-Robertson-Walker
cosmology of the three uncompactified dimensions can set in. We show that our
radion stabilization mechanism requires a stringy realization of inflation as
scalar field driven inflation invalidates our mechanism. We also show that our
stabilization mechanism is consistent with observational bounds.Comment: New section on the phenomonological implications of and bound on the
model has been adde
String Gas Cosmology and Structure Formation
It has recently been shown that a Hagedorn phase of string gas cosmology may
provide a causal mechanism for generating a nearly scale-invariant spectrum of
scalar metric fluctuations, without the need for an intervening period of de
Sitter expansion. A distinctive signature of this structure formation scenario
would be a slight blue tilt of the spectrum of gravitational waves. In this
paper we give more details of the computations leading to these results.Comment: 12 pages, 3 figure
On the new string theory inspired mechanism of generation of cosmological perturbations
Recently a non-inflationary mechanism of generation of scale-free
cosmological perturbations of metric was proposed by Brandenberger, Nayeri, and
Vafa in the context of the string gas cosmology. We discuss various problems of
their model and argue that the cosmological perturbations of metric produced in
this model have blue spectrum with a spectral index n = 5, which strongly
disagrees with observations. We conclude that this model in its present form is
not a viable alternative to inflationary cosmology.Comment: 11 pages, 1 figur
Observational Constraints on Theories with a Blue Spectrum of Tensor Modes
Motivated by the string gas cosmological model, which predicts a blue tilt of
the primordial gravitational wave spectrum, we examine the constraints imposed
by current and planned observations on a blue tilted tensor spectrum. Starting
from an expression for the primordial gravitational wave spectrum normalized
using cosmic microwave background observations, pulsar timing, direct detection
and nucleosynthesis bounds are examined. If we assume a tensor to scalar ratio
on scales of the CMB which equals the current observational upper bound, we
obtain from these current observations constraints on the tensor spectral index
of , , and
respectively.Comment: 12 pages, 1 figure, 2 references added, relationship of this work
with Ref. 20 adde
Cosmological Perturbations in Non-Commutative Inflation
We compute the spectrum of cosmological perturbations in a scenario in which
inflation is driven by radiation in a non-commutative space-time. In this
scenario, the non-commutativity of space and time leads to a modified
dispersion relation for radiation with two branches, which allows for
inflation. The initial conditions for the cosmological fluctuations are
thermal. This is to be contrasted with the situation in models of inflation in
which the accelerated expansion of space is driven by the potential energy of a
scalar field, and in which the fluctuations are of quantum vacuum type. We find
that, in the limit that the expansion of space is almost exponential, the
spectrum of fluctuations is scale-invariant with a slight red tilt. The
magnitude of the tilt is different from what is obtained in a usual
inflationary model with the same expansion rate during the period of inflation.
The amplitude also differs, and can easily be adjusted to agree with
observations.Comment: 7 pages, 1 figur
Looking Beyond Inflationary Cosmology
In spite of the phenomenological successes of the inflationary universe
scenario, the current realizations of inflation making use of scalar fields
lead to serious conceptual problems which are reviewed in this lecture. String
theory may provide an avenue towards addressing these problems. One particular
approach to combining string theory and cosmology is String Gas Cosmology. The
basic principles of this approach are summarized.Comment: invited talk at "Theory Canada 1" (Univ. of British Columbia,
Vancouver, Canada, June 2 - 4, 2005) (references updated
Moduli Stabilization in Brane Gas Cosmology with Superpotentials
In the context of brane gas cosmology in superstring theory, we show why it
is impossible to simultaneously stabilize the dilaton and the radion with a
general gas of strings (including massless modes) and D-branes. Although this
requires invoking a different mechanism to stabilize these moduli fields, we
find that the brane gas can still play a crucial role in the early universe in
assisting moduli stabilization. We show that a modest energy density of
specific types of brane gas can solve the overshoot problem that typically
afflicts potentials arising from gaugino condensation.Comment: minor changes to match the journal versio