1,322 research outputs found
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
Inflation and initial conditions in the pre-big bang scenario
The pre-big bang scenario describes the evolution of the Universe from an
initial state approaching the flat, cold, empty, string perturbative vacuum.
The choice of such an initial state is suggested by the present state of our
Universe if we accept that the cosmological evolution is (at least partially)
duality-symmetric. Recently, the initial conditions of the pre-big bang
scenario have been criticized as they introduce large dimensionless parameters
allowing the Universe to be "exponentially large from the very beginning". We
agree that a set of initial parameters (such as the initial homogeneity scale,
the initial entropy) larger than those determined by the initial horizon scale,
H^{-1}, would be somewhat unnatural to start with. However, in the pre-big bang
scenario, the initial parameters are all bounded by the size of the initial
horizon. The basic question thus becomes: is a maximal homogeneity scale of
order H^{-1} necessarily unnatural if the initial curvature is small and,
consequently, H^{-1} is very large in Planck (or string) units? In the
impossibility of experimental information one could exclude "a priori", for
large horizons, the maximal homogeneity scale H^{-1} as a natural initial
condition. In the pre-big bang scenario, however, pre-Planckian initial
conditions are not necessarily washed out by inflation and are accessible (in
principle) to observational tests, so that their naturalness could be also
analyzed with a Bayesan approach, in terms of "a posteriori" probabilities.Comment: 4 pages, Latex, one figure. Many references added. The text has been
improved in many points. To appear in Phys. Rev.
Particle production in string cosmology models
We compute spectra of particles produced during a dilaton-driven kinetic
inflation phase within string cosmology models. The resulting spectra depend on
the parameters of the model and on the type of particle and are quite varied,
some increasing and some decreasing with frequency. We use an approximation
scheme in which all spectra can be expressed in a nice symmetric form, perhaps
hinting at a deeper symmetry of the underlying physics. Our results may serve
as a starting point for detailed studies of relic abundances, dark matter
candidates, and possible sources of large scale anisotropy.Comment: 20 pages, no figures, latex, RevTe
Homogeneous magnetic fields in fully anisotropic string cosmological backgrounds
We present new solutions of the string cosmological effective action in the
presence of a homogeneous Maxwell field with pure magnetic component. Exact
solutions are derived in the case of space-independent dilaton and vanishing
torsion background. In our examples the four dimensional metric is either of
Bianchi-type III and VI or Kantowski-Sachs.Comment: 4 page
Singularity structure in Veneziano's model
We consider the structure of the cosmological singularity in Veneziano's
inflationary model. The problem of choosing initial data in the model is shown
to be unsolved -- the spacetime in the asymptotically flat limit can be filled
with an arbitrary number of gravitational and scalar field quanta. As a result,
the universe acquires a domain structure near the singularity, with an
anisotropic expansion of its own being realized in each domain.Comment: 16 pages, 2 figures, shorter then journal version; references added,
discussion slightly expande
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
Fully Anisotropic String Cosmologies, Maxwell Fields and Primordial Shear
We present a class of exact cosmological solutions of the low energy string
effective action in the presence of a homogeneous magnetic fields. We discuss
the physical properties of the obtained (fully anisotropic) cosmologies paying
particular attention to their vacuum limit and to the possible isotropization
mechanisms. We argue that quadratic curvature corrections are able to
isotropize fully anisotropic solutions whose scale factors describe accelerated
expansion. Moreover, the degree of isotropization grows with the duration of
the string phase. We follow the fate of the shear parameter in a decelerated
phase where, dilaton, magnetic fields and radiation fluid are simultaneously
present. In the absence of any magnetic field a long string phase immediately
followed by radiation is able to erase large anisotropies. Conversely, if a
short string phase is followed by a long dilaton dominated phase the
anisotropies can be present, in principle, also at later times. The presence of
magnetic seeds after the end of the string phase can induce further
anisotropies which can be studied within the formalism reported in this paper.Comment: 19 pages in Revtex style, 14 Encapsulated figure
Friedmann Universes and Exact Solutions in String Cosmology
We show that the classical null strings generate the Hilbert-Einstein gravity
corresponding to D-dimensional Friedmann universes.Comment: 8 pages, LATE
Low-energy quantum string cosmology
We introduce a Wheeler-De Witt approach to quantum cosmology based on the
low-energy string effective action, with an effective dilaton potential
included to account for non-perturbative effects and, possibly, higher-order
corrections. We classify, in particular, four different classes of scattering
processes in minisuperspace, and discuss their relevance for the solution of
the graceful exit problem.Comment: Extended version to appear in the Review section of Int. J. Theor.
Phys. A 13 (1998). The new permanent address is added. An updated collection
of papers on the pre-big bang scenario is available at
http://www.to.infn.it/~gasperi
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