2,244 research outputs found
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
Non-local dilaton coupling to dark matter: cosmic acceleration and pressure backreaction
A model of non-local dilaton interactions, motivated by string duality
symmetries, is applied to a scenario of "coupled quintessence" in which the
dilaton dark energy is non-locally coupled to the dark-matter sources. It is
shown that the non-local effects tend to generate a backreaction which -- for
strong enough coupling -- can automatically compensate the acceleration due to
the negative pressure of the dilaton potential, thus asymptotically restoring
the standard (dust-dominated) decelerated regime. This result is illustrated by
analytical computations and numerical examples.Comment: 11 pages, 1 figure ep
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
Low-energy graceful exit in anisotropic string cosmology backgrounds
We discuss the possibility of a smooth transition from the pre- to the
post-big bang regime, in the context of the lowest-order string effective
action (without higher-derivative corrections), taking into account with a
phenomenological model of source the repulsive gravitational effects due to the
back-reaction of the quantum fluctuations outside the horizon. We determine a
set of necessary conditions for a successful and realistic transition, and we
find that such conditions can be satisfied (by an appropriate model of source),
provided the background is higher-dimensional and anisotropic.Comment: 13 pages, latex, 2 figures included using epsfi
Sensitivity of spherical gravitational-wave detectors to a stochastic background of non-relativistic scalar radiation
We analyze the signal-to-noise ratio for a relic background of scalar
gravitational radiation composed of massive, non-relativistic particles,
interacting with the monopole mode of two resonant spherical detectors. We find
that the possible signal is enhanced with respect to the differential mode of
the interferometric detectors. This enhancement is due to: {\rm (a)} the
absence of the signal suppression, for non-relativistic scalars, with respect
to a background of massless particles, and {\rm (b)} for flat enough spectra, a
growth of the signal with the observation time faster than for a massless
stochastic background.Comment: four pages, late
Late-time effects of Planck-scale cosmology: dilatonic interpretation of the dark energy field
We present a model of dark energy based on the string effective action, and
on the assumption that the dilaton is strongly coupled to dark matter. We
discuss the main differences between this class of models and more conventional
models of quintessence,uncoupled to dark matter. This paper is based on talks
presented at the "VII Congresso Nazionale di Cosmologia" (Osservatorio
Astronomico di Roma, Monte Porzio Catone, November 2002), and at the Meeting
"Dark Energy Day" (University of Milano-Bicocca, November 2002). To appear in
the Proc. of the International Conference on ``Thinking, Observing and Mining
the Universe" (Sorrento, September 2003), eds. G. Longo and G. Miele (World
Scientific, Singapore).Comment: 8 pages, latex, 4 figures included using eps
Why supersymmetry should be restored at the TeV scale
It is explained why the curvature associated to the vacuum energy density
arising from SUSY breaking cannot be completely transferred to the extra
spatial dimensions of a bulk space-time manifold, and it is shown -- without
using hierarchy arguments but only the results of current large-scale
observations -- why the Tev scale should correspond to the maximal allowed
SUSY-breaking scale.Comment: 6 pages. Essay written for the 2008 Awards for Essays on Gravitation
(Gravity Research Foundation, USA), and awarded with "Honorable Mention
Birth of the Universe in string cosmology
The decay of the string perturbative vacuum into our present cosmological
state is associated to the transition from a phase of growing curvature and
growing dilaton, to a phase of decreasing curvature and frozen dilaton. The
possible approaches to a classical and quantum description of such a transition
are introduced and briefly discussed.Comment: 17 pages, LATEX, seven figures included using EPSF, based on a talk
given at the "Fourth Paris Cosmology Colloquium" (Observatoire de Paris, June
1997). To appear in the Proceedings (ed. by H. J. De Vega and N. Sanchez). An
updated collection of papers on the pre-big bang scenario is available at
http://www.to.infn.it/teorici/gasperin
Response of the common mode of interferometric detectors to a stochastic background of massive scalar radiation
We compute the angular pattern and the overlap reduction functions for the
geodesic and non-geodesic response of the common mode of two interferometers
interacting with a stochastic, massive scalar background. We also discuss the
possible overlap between common and differential modes. We find that the
cross-correlated response of two common modes to a non-relativistic background
may be higher than the response of two differential modes to the same
background.Comment: 8 pages, 4 figures, revte
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
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