5,155 research outputs found
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
Microscopic unitary description of tidal excitations in high-energy string-brane collisions
The eikonal operator was originally introduced to describe the effect of
tidal excitations on higher-genus elastic string amplitudes at high energy. In
this paper we provide a precise interpretation for this operator through the
explicit tree-level calculation of generic inelastic transitions between closed
strings as they scatter off a stack of parallel Dp-branes. We perform this
analysis both in the light-cone gauge, using the Green-Schwarz vertex, and in
the covariant formalism, using the Reggeon vertex operator. We also present a
detailed discussion of the high energy behaviour of the covariant string
amplitudes, showing how to take into account the energy factors that enhance
the contribution of the longitudinally polarized massive states in a simple
way.Comment: 58 page
A Causal Entropy Bound
The identification of a causal-connection scale motivates us to propose a new
covariant bound on entropy within a generic space-like region. This "causal
entropy bound", scaling as the square root of EV, and thus lying around the
geometric mean of Bekenstein's S/ER and holographic S/A bounds, is checked in
various "critical" situations. In the case of limited gravity, Bekenstein's
bound is the strongest while naive holography is the weakest. In the case of
strong gravity, our bound and Bousso's holographic bound are stronger than
Bekenstein's, while naive holography is too tight, and hence typically wrong.Comment: 12 pages, no figures, a reference added and minor typos 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
Spherical Curvature Inhomogeneities in String Cosmology
We study the evolution of non-linear spherically symmetric inhomogeneities in
string cosmology. Friedmann solutions of different spatial curvature are
matched to produce solutions which describe the evolution of non-linear density
and curvature inhomogeneities. The evolution of bound and unbound
inhomogeneities are studied. The problem of primordial black hole formation is
discussed in the string cosmological context and the pattern of evolution is
determined in the pre- and post-big-bang phases of evolution.Comment: 19 pages, Latex, 4 figure
Improved lattice operators: the case of the topological charge density
We analyze the properties of a class of improved lattice topological charge
density operators, constructed by a smearing-like procedure. By optimizing the
choice of the parameters introduced in their definition, we find operators
having (i) a better statistical behavior as estimators of the topological
charge density on the lattice, i.e. less noisy; (ii) a multiplicative
renormalization much closer to one; (iii) a large suppression of the
perturbative tail (and other unphysical mixings) in the corresponding lattice
topological susceptibility.Comment: 11 pages (REVTEX) + 4 (uuencoded) figure
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
String Cosmology: The Pre-Big Bang Scenario
A review is attempted of physical motivations, theoretical and
phenomenological aspects, as well as outstanding problems, of the pre-big bang
scenario in string cosmology.Comment: 46 pages, 8 Figures, Latex, Lectures delivered in Les Houches, July
199
Pre-Big-Bang Requires the Universe to be Exponentially Large From the Very Beginning
We show that in a generic case of the pre-big-bang scenario, inflation will
solve cosmological problems only if the universe at the onset of inflation is
extremely large and homogeneous from the very beginning. The size of a
homogeneous part of the universe at the beginning of the stage of pre-big-bang
(PBB) inflation must be greater than , where is the
stringy length. The total mass of an inflationary domain must be greater than
, where . If the universe is initially
radiation dominated, then its total entropy at that time must be greater than
. If the universe is closed, then at the moment of its formation it
must be uniform over causally disconnected domains. The natural
duration of the PBB stage in this scenario is . We argue that the
initial state of the open PBB universe could not be homogeneous because of
quantum fluctuations. Independently of the issue of homogeneity, one must
introduce two large dimensionless parameters, , and , in order to solve the flatness problem in the PBB cosmology. A regime
of eternal inflation does not occur in the PBB scenario. This should be
compared with the simplest versions of the chaotic inflation scenario, where
the regime of eternal inflation may begin in a universe of size
with vanishing initial radiation entropy, mass , and geometric entropy
O(1). We conclude that the current version of the PBB scenario cannot replace
usual inflation even if one solves the graceful exit problem in this scenario.Comment: 14 pages, a discussion of the flatness problem in the PBB cosmology
is adde
Pre-big bang model has Planck problem
The pre-big bang's kinetic driven inflationary mechanism is not an adequate
form of inflation: the Planck length grows more rapidly than the scale factor.
In order to explain our large universe, the resulting post-big bang universe
requires the same unnatural constants (Planck problem) as those of any other
non-inflationary big bang model.Comment: figures not include
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