1,364 research outputs found
Towards a future singularity?
We discuss whether the future extrapolation of the present cosmological state
may lead to a singularity even in case of "conventional" (negative) pressure of
the dark energy field, namely . The discussion is based on an
often neglected aspect of scalar-tensor models of gravity: the fact that
different test particles may follow the geodesics of different metric frames,
and the need for a frame-independent regularization of curvature singularities.Comment: 8 pages. Essay written for the "2004 Awards for Essays on
Gravitation" (Gravity Research Foundation, Wellesley Hills, MA, USA), and
selected for "Honorable Mention
Repulsive gravity in the very early Universe
I present two examples in which the curvature singularity of a
radiation-dominated Universe is regularized by (a) the repulsive effects of
spin interactions, and (b) the repulsive effects arising from a breaking of the
local gravitational gauge symmetry. In both cases the collapse of an initial,
asymptotically flat state is stopped, and the Universe bounces towards a state
of decelerated expansion. The emerging picture is typical of the pre-big bang
scenario, with the main difference that the string cosmology dilaton is
replaced by a classical radiation fluid, and the solutions are not
duality-invariant.Comment: 9 pages, LATEX, one figure included using epsf. Awarded the Fourth
Prize in the 1998 Awards for Essays on Gravitation (Gravity Research
Foundation, Wellesley Hills, Ma). To appear in Gen. Rel. Grav. An updated
collection of papers on the pre-big bang scenario is available at
http://www.to.infn.it/~gasperi
Relic Gravitational Waves from String Cosmology
A large class of string-cosmology backgrounds leads to a spectrum of relic
stochastic gravitational waves, strongly tilted towards high frequencies, and
characterized by two basic parameters of the cosmological model. We estimate
the required sensitivity for detection of the predicted gravitational radiation
and show that a region of our parameter space is within reach for some of the
planned gravitational-wave detectors.Comment: 14 pages, latex, 2 figures included as epsf file
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
Quantum Squeezing and Cosmological Entropy Production
The entropy growth in a cosmological process of pair production is completely
determined by the associated squeezing parameter, and is insensitive to the
number of particles in the initial state. The total produced entropy may
represent a significant fraction of the entropy stored today in the cosmic
black-body radiation, provided pair production originates from a change in the
background metric at a curvature scale of the Planck order.Comment: 7 pages, plain TEX, to appear in Class.Quantum Grav., CERN-TH.6954/9
String Driven Cosmology and its Predictions
We present a minimal model for the Universe evolution fully extracted from
effective String Theory. This model is by its construction close to the
standard cosmological evolution, and it is driven selfconsistently by the
evolution of the string equation of state itself. The inflationary String
Driven stage is able to reach enough inflation, describing a Big Bang like
evolution for the metric. By linking this model to a minimal but well
established observational information, (the transition times of the different
cosmological epochs), we prove that it gives realistic predictions on early and
current energy density and its results are compatible with General Relativity.
Interestingly enough, the predicted current energy density is found Omega = 1
and a lower limit Omega \geq 4/9 is also found. The energy density at the exit
of the inflationary stage also gives | Omega |_{inf}=1. This result shows an
agreement with General Relativity (spatially flat metric gives critical energy
density) within an inequivalent Non-Einstenian context (string low energy
effective equations). The order of magnitude of the energy density-dilaton
coupled term at the beginning of the radiation dominated stage agrees with the
GUT scale. The predicted graviton spectrum is computed and analyzed without any
free parameters. Peaks and asymptotic behaviours of the spectrum are a direct
consequence of the dilaton involved and not only of the scale factor evolution.
Drastic changes are found at high frequencies: the dilaton produces an
increasing spectrum (in no string cosmologies the spectrum is decreasing).
Without solving the known problems about higher order corrections and graceful
exit of inflation, we find this model closer to the observational Universe than
the current available string cosmology scenarii.Comment: LaTex, 22 pages, Lectures delivered at the Chalonge School, Nato ASI:
Phase Transitions in the Early Universe: Theory and Observations. To appear
in the Proceedings, Editors H. J. de Vega, I. Khalatnikov, N. Sanchez.
(Kluwer Pub
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
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
Duality in String Cosmology
Scale factor duality, a truncated form of time dependent T-duality, is a
symmetry of string effective action in cosmological backgrounds interchanging
small and large scale factors. The symmetry suggests a cosmological scenario
("pre-big-bang") in which two duality related branches, an inflationary branch
and a decelerated branch are smoothly joined into one non-singular cosmology.
The use of scale factor duality in the analysis of the higher derivative
corrections to the effective action, and consequences for the nature of exit
transition, between the inflationary and decelerated branches, are outlined. A
new duality symmetry is obeyed by the lowest order equations for inhomogeneity
perturbations which always exist on top of the homogeneous and isotropic
background. In some cases it corresponds to a time dependent version of
S-duality, interchanging weak and strong coupling and electric and magnetic
degrees of freedom, and in most cases it corresponds to a time dependent
mixture of both S-, and T-duality.
The energy spectra obtained by using the new symmetry reproduce known results
of produced particle spectra, and can provide a useful lower bound on particle
production when our knowledge of the detailed dynamical history of the
background is approximate or incomplete.Comment: 6 pages, no figures, latex2e using ltwol2e.sty. Based on talks at the
44'th annual meeting of the Israel Physical Society, Apr 8, 1998, Rehovot,
Israel, and ICHEP98, 23-29 July, Vancouver, BC, Canada, and second conf. on
Quantum Aspects of Gauge Theories, Supersymmetry and Unification, Sept 21-26,
1998, Corfu, Greece. To be published in the proceeding
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