997 research outputs found
A class of non-singular gravi-dilaton backgrounds
We present a class of static, spherically symmetric, non-singular solutions
of the tree-level string effective action, truncated to first order in
. In the string frame the solutions approach asymptotically (at and ) two different anti-de Sitter configurations, thus
interpolating between two maximally symmetric states of different constant
curvature. The radial-dependent dilaton defines a string coupling which is
everywhere finite, with a peak value that can be chosen arbitrarily small so as
to neglect quantum-loop corrections. This example stresses the possible
importance of finite-size corrections, typical of string theory, in
avoiding space-time singularities.Comment: 9 pages, LATEX, four figure included using EPSFIG. Essay written for
the 1997 Awards of the Gravity Research Foundation, and selected for
"Honorable Mention". To appear in Mod. Phys. Lett.
String cosmology versus standard and inflationary cosmology
This paper presents a review of the basic, model-independent differences
between the pre-big bang scenario, arising naturally in a string cosmology
context, and the standard inflationary scenario. We use an unconventional
approach in which the introduction of technical details is avoided as much as
possible, trying to focus the reader's attention on the main conceptual aspects
of both scenarios. The aim of the paper is not to conclude in favour either of
one or of the other scenario, but to raise questions that are left to the
reader's meditation. Warnings: the paper does not contain equations, and is not
intended as a complete review of all aspects of string cosmology.Comment: 22 pages, Latex (IOP Style), three figures included using epsfig. To
appear in Class. Quantum Grav. (Topical Review Section). Two misprints
correcte
Electromagnetic Origin of the CMB Anisotropy in String Cosmology
In the inflationary scenarios suggested by string theory, the vacuum
fluctuations of the electromagnetic field can be amplified by the
time-evolution of the dilaton background, and can grow large enough to explain
both the origin of the cosmic magnetic fields and of the observed CMB
anisotropy. The normalization of the perturbation spectrum is fixed, and
implies a relation between the perturbation amplitude at the COBE scale and the
spectral index . Working within a generic two-parameter family of
backgrounds, a large scale anisotropy is found to
correspond to a spectral index in the range .Comment: 11 pages, LATE
Peak and end point of the relic graviton background in string cosmology
Using general arguments we determine the allowed region for the end point
frequency and the peak energy density of the stochastic background of gravity
waves expected in string cosmology. We provide an accurate estimate of the
minimal experimental sensitivity required to detect a signal in the Hz to GHz
range.Comment: 11 pages, LATEX, one figure included using eps. A complete collection
of papers and references on the pre-big-bang scenario in string cosmology is
available at http://www.to.infn.it/teorici/gasperini
Time Gauge Fixing and Hilbert Space in Quantum String Cosmology
Recently the low-energy effective string theory has been used by Gasperini
and Veneziano to elaborate a very interesting scenario for the early history of
the universe (``birth of the universe as quantum scattering''). Here we
investigate the gauge fixing and the problem of the definition of a global time
parameter for this model, and we obtain the positive norm Hilbert space of
states.Comment: 13 pages, Plain TEX, no figure
Seeds of large-scale anisotropy in string cosmology
Pre-big bang cosmology predicts tiny first-order dilaton and metric
perturbations at very large scales. Here we discuss the possibility that other
-- more copiously generated -- perturbations may act, at second order, as
scalar seeds of large-scale structure and CMB anisotropies. We study, in
particular, the cases of electromagnetic and axionic seeds. We compute the
stochastic fluctuations of their energy-momentum tensor and determine the
resulting contributions to the multipole expansion of the temperature
anisotropy. In the axion case it is possible to obtain a flat or slightly
tilted blue spectrum that fits present data consistently, both for massless and
for massive (but very light) axions.Comment: 27 pages, LATEX, one figure included using eps
Constraints on pre-big bang models for seeding large-scale anisotropy by massive Kalb-Ramond axions
We discuss the conditions under which pre-big bang models can fit the
observed large-scale anisotropy with a primordial spectrum of massive
(Kalb--Ramond) axion fluctuations.
The primordial spectrum must be sufficiently flat at low frequency and
sufficiently steeper at high frequency. For a steep and/or long enough
high-frequency branch of the spectrum the bounds imposed by COBE's
normalization allow axion masses of the typical order for a
Peccei--Quinn--Weinberg--Wilczek axion. We provide a particular example in
which an appropriate axion spectrum is obtained from a class of backgrounds
satisfying the low-energy string cosmology equations.Comment: 11 pages, revtex, two figures included using epsfig. An updated
collection of papers on the pre-big bang scenario is available at
http://www.to.infn.it/~gasperi
Gauge-invariant gravitational wave modes in pre-big bang cosmology
The t<0 branch of pre-big bang cosmological scenarios is subject to a
gravitational wave instability. The unstable behaviour of tensor perturbations
is derived in a very simple way in Hwang's covariant and gauge-invariant
formalism developed for extended theories of gravity. A simple interpretation
of this instability as the effect of an "antifriction" is given, and it is
argued that a universe must eventually enter the expanding phase.Comment: 4 pages, latex, to appear in Eur. Phys. J.
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