4,889 research outputs found
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
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
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
Gravitational waves in non-singular string cosmologies
We study the evolution of tensor metric fluctuations in a class of
non-singular models based on the string effective action, by including in the
perturbation equation the higher-derivative and loop corrections needed to
regularise the background solutions. We discuss the effects of such
higher-order corrections on the final graviton spectrum, and we compare the
results of analytical and numerical computations.Comment: 24 pages, 7 figure
Recent developments in quantum string cosmology
In this talk we discuss the quantisation of a class of string cosmology
models characterised by scale factor duality invariance. The amplitudes for the
full set of classically allowed and forbidden transitions are computed by
applying the reduced phase space and path integral methods. In particular, the
path integral calculation clarifies the meaning of the instanton-like behaviour
of the transition amplitudes that has been first pointed out in previous
investigations.Comment: 4 pages, one eps figures, uses espcrc2.sty. Talk given at the Third
Conference on Constrained Dynamics and Quantum Gravit
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
Relaxed Bounds on the Dilaton Mass in a String Cosmology Scenario
We discuss bounds on the dilaton mass, following from the cosmological
amplification of the quantum fluctuations of the dilaton background, under the
assumption that such fluctuations are dominant with respect to the classical
background oscillations. We show that if the fluctuation spectrum grows with
the frequency the bounds are relaxed with respect to the more conventional case
of a flat or decreasing spectrum. As a consequence, the allowed range of masses
may become compatible with models of supersymmetry breaking, and with a
universe presently dominated by a relic background of dilaton dark matter.Comment: 12 pages, plain tex, 2 figures (available on request) DFTT-03/9
On the response of gravitational antennas to dilatonic waves
It is pointed out that the coupling of macroscopic test masses to the
gravi-dilaton background of string theory is non geodesic, in general, and
cannot be parametrized by a Brans-Dicke model of scalar-tensor gravity. The
response of gravitational antennas to dilatonic waves should be analyzed
through a generalized equation of geodesic deviation, taking into account the
possible direct coupling of the background to the (composition-dependent)
dilatonic charge of the antenna.Comment: 10 pages, Latex, no figure
Inhomogeneous Pre-Big Bang String Cosmology
An inhomogeneous version of pre--Big Bang cosmology emerges, within string
theory, from quite generic initial conditions, provided they lie deeply inside
the weak-coupling, low-curvature regime. Large-scale homogeneity, flatness, and
isotropy appear naturally as late-time outcomes of such an evolution.Comment: 13 pages, Latex, references adde
S-duality Invariant Perturbations in String Cosmology
We investigate the generation of curvature and isocurvature (dilaton, moduli
and axion) perturbations in a general class of axion-dilaton-moduli
models,including the pre-big bang scenario. Allowing for an arbitrary coupling
constant between the dilaton field and the axion field, we exploit the SL(2,R)
symmetry of the theory to obtain the spectral indices of the field
perturbations in a pre-big bang type scenario. Axion field fluctuations about a
homogeneous background field can yield a scale-invariant (Harrison-Zel'dovich)
spectrum. As an example we present a string-motivated case with SL(2,R)xSL(2,R)
symmetry, where a second axion field arises from the compactification of the
ten-dimensional theory to four dimensions.Comment: 15 pages, no figures, plain LaTe
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