164 research outputs found
Detecting relic gravitational radiation from string cosmology with LIGO
A characteristic spectrum of relic gravitational radiation is produced by a
period of ``stringy inflation" in the early universe. This spectrum is unusual,
because the energy-density rises rapidly with frequency. We show that
correlation experiments with the two gravitational wave detectors being built
for the Laser Interferometric Gravitational Observatory (LIGO) could detect
this relic radiation, for certain ranges of the parameters that characterize
the underlying string cosmology model.Comment: 6 pages, 5 eps figures, Revte
Graviton Spectra in String Cosmology
We propose to uncover the signature of a stringy era in the primordial
Universe by searching for a prominent peak in the relic graviton spectrum. This
feature, which in our specific model terminates an increase and
initiates an decrease, is induced during the so far overlooked
bounce of the scale factor between the collapsing deflationary era (or pre-Big
Bang) and the expanding inflationary era (or post-Big Bang). We evaluate both
analytically and numerically the frequency and the intensity of the peak and we
show that they may likely fall in the realm of the new generation of
interferometric detectors. The existence of a peak is at variance with
ordinarily monotonic (either increasing or decreasing) graviton spectra of
canonical cosmologies; its detection would therefore offer strong support to
string cosmology.Comment: 14 pages, RevTex source and 6 figures.p
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
Moduli potentials in string compactifications with fluxes: mapping the Discretuum
We find de Sitter and flat space solutions with all moduli stabilized in four
dimensional supergravity theories derived from the heterotic and type II string
theories, and explain how all the previously known obstacles to finding such
solutions can be removed. Further, we argue that if the compact manifold allows
a large enough space of discrete topological choices then it is possible to
tune the parameters of the four dimensional supergravity such that a hierarchy
is created and the solutions lie in the outer region of moduli space in which
the compact volume is large in string units, the string coupling is weak, and
string perturbation theory is valid. We show that at least two light chiral
superfields are required for this scenario to work, however, one field is
sufficient to obtain a minimum with an acceptably small and negative
cosmological constant. We discuss cosmological issues of the scenario and the
possible role of anthropic considerations in choosing the vacuum of the theory.
We conclude that the most likely stable vacuua are in or near the central
region of moduli space where string perturbation theory is not strictly valid,
and that anthropic considerations cannot help much in choosing a vacuum.Comment: 34 pages, no figure
Cold and Hot Dark Matter from a Single Nonthermal Relic
The origin of dark matter in the universe may be scalar particles produced by
amplification of quantum fluctuations during a period of dilaton-driven
inflation. We show, for the first time, that a single species of particles,
depending on its mass and interactions, can be a source of both cold and hot
dark matter simultaneously. Detection of such weakly interacting particles with
masses below a fraction of an eV presents a new challenge for dark matter
searches.Comment: 10 pages, 1 figur
Tensor perturbations in high-curvature string backgrounds
We derive a generalized equation for the evolution of tensor perturbations in
a cosmological background, taking into account higher-curvature contributions
and a tree-level coupling to the dilaton in the string frame. The equation is
obtained by perturbing the gravi-dilaton string effective action, expanded up
to first order in . The corrections can modify the
low-energy perturbation spectrum, but the modifications are shown to be small
when the background curvature keeps constant in the string frame.Comment: 9 pages, REVTEX, three figures included using EPSFIG. An updated
collection of papers on the pre-big bang scenario in string cosmology is a
available at http://www.to.infn.it/teorici/gasperin
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
Has the Universe always expanded ?
We consider a cosmological setting for which the currently expanding era is
preceded by a contracting phase, that is, we assume the Universe experienced at
least one bounce. We show that scalar hydrodynamic perturbations lead to a
singular behavior of the Bardeen potential and/or its derivatives (i.e. the
curvature) for whatever Universe model for which the last bounce epoch can be
smoothly and causally joined to the radiation dominated era. Such a Universe
would be filled with non-linear perturbations long before nucleosynthesis, and
would thus be incompatible with observations. We therefore conclude that no
observable bounce could possibly have taken place in the early universe if
Einstein gravity together with hydrodynamical fluids is to describe its
evolution, and hence, under these conditions, that the Universe has always
expanded.Comment: 11 pages, LaTeX-ReVTeX, no figures, submitted to PR
A tachyonic extension of the stringy no-go theorem
We investigate the tachyon-dilaton-metric system to study the "graceful exit"
problem in string theoretic inflation, where tachyon plays the role of the
scalar field. From the phase space analysis, we find that the inflationary
phase does not smoothly connect to a Friedmann-Robertson-Walker (FRW) expanding
universe, thereby providing a simple tachyonic extension of the recently proved
stringy no-go theorem.Comment: TeX file (PHYZZX), 10 pages, change in the title, many changes in the
text (the version to appear in Phys. Rev. D
On the origin of the large scale structures of the universe
We revise the statistical properties of the primordial cosmological density
anisotropies that, at the time of matter radiation equality, seeded the
gravitational development of large scale structures in the, otherwise,
homogeneous and isotropic Friedmann-Robertson-Walker flat universe. Our
analysis shows that random fluctuations of the density field at the same
instant of equality and with comoving wavelength shorter than the causal
horizon at that time can naturally account, when globally constrained to
conserve the total mass (energy) of the system, for the observed scale
invariance of the anisotropies over cosmologically large comoving volumes.
Statistical systems with similar features are generically known as glass-like
or lattice-like. Obviously, these conclusions conflict with the widely accepted
understanding of the primordial structures reported in the literature, which
requires an epoch of inflationary cosmology to precede the standard expansion
of the universe. The origin of the conflict must be found in the widespread,
but unjustified, claim that scale invariant mass (energy) anisotropies at the
instant of equality over comoving volumes of cosmological size, larger than the
causal horizon at the time, must be generated by fluctuations in the density
field with comparably large comoving wavelength.Comment: New section added; final version to appear in Physical Review D;
discussion extended and detailed with new calculations to support the claims
of the paper; statistical properties of vacuum fluctuations now discussed in
the context of FRW flat universe; new important conclussions adde
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