63 research outputs found
Predictive Power of Strong Coupling in Theories with Large Distance Modified Gravity
We consider theories that modify gravity at cosmological distances, and show
that any such theory must exhibit a strong coupling phenomenon, or else it is
either inconsistent or is already ruled out by the solar system observations.
We show that all the ghost-free theories that modify dynamics of spin-2
graviton on asymptotically flat backgrounds, automatically have this property.
Due to the strong coupling effect, modification of the gravitational force is
source-dependent, and for lighter sources sets in at shorter distances. This
universal feature makes modified gravity theories predictive and potentially
testable not only by cosmological observations, but also by precision
gravitational measurements at scales much shorter than the current cosmological
horizon. We give a simple parametrization of consistent large distance modified
gravity theories and their predicted deviations from the Einsteinian metric
near the gravitating sources.Comment: 12 pages, Latex, to be published in New Journal of Physic
Initial Conditions for Supersymmetric Inflation
We perform a numerical investigation of the fields evolution in the
supersymmetric inflationary model based on radiative corrections. Supergravity
corrections are also included. We find that, out of all the examined initial
data, only about 10% give an adequate amount of inflation and can be considered
as ''natural''. Moreover, these successful initial conditions appear scattered
and more or less isolated.Comment: 15 pages RevTeX 4 eps figure
Is there a monopole problem?
We investigate the high temperature behavior of SU(5) in its minimal version.
We show that there exists a range of parameters of the Higgs potential for
which the symmetry remains broken at high temperature, thus avoiding the phase
transition that gives rise to the overproduction of monopoles . We also show
that in such scenario the thermal production of monopoles can be suppressed in
a wide range of parameters, keeping their number density below the cosmological
limits.Comment: Latex, 12 pages, revised version as appeared in Physical Review
Letters. Minor corrections, comments and two references adde
Duality extended Chaplygin cosmologies with a big rip
We consider modifications to the Friedmann equation motivated by recent
proposals along these lines pursuing an explanation to the observed late time
acceleration. Here we show those modifications can be framed within a theory
with self-interacting gravity, where the term self-interaction refers here to
the presence of functions of and in the right hand side of the
Einstein equations. We then discuss the construction of the duals of the
cosmologies generated within that framework. After that we investigate the
modifications required to generate generalized and modified Chaplygin
cosmologies and show that their duals belong to a larger family of cosmologies
we call extended Chaplygin cosmologies. Finally, by letting the parameters of
those models take values not earlier considered in the literature we show some
representatives of that family of cosmologies display sudden future
singularities, which indicates their behavior is rather different from
generalized or modified Chaplygin gas cosmologies. This reinforces the idea
that modifications of gravity can be responsible for unexpected evolutionary
features in the universe.Comment: 5 pages, revtex
No cosmological domain wall problem for weakly coupled fields
After inflation occurs, a weakly coupled scalar field will in general not be
in thermal equilibrium but have a distribution of values determined by the
inflationary Hubble parameter. If such a field subsequently undergoes discrete
symmetry breaking, then the different degenerate vacua may not be equally
populated so the domain walls which form will be `biased' and the wall network
will subsequently collapse. Thus the cosmological domain wall problem may be
solved for sufficiently weakly coupled fields in a post-inflationary universe.
We quantify the criteria for determining whether this does happen, using a
Higgs-like potential with a spontaneously broken symmetry.Comment: 17 pages, 4 figures (Revtex), clarifying Comments added in
Introduction; to appear in Phys. Rev
Gravitinos from Gravitational Collapse
We reanalyse the limits on the gravitino mass in superlight
gravitino scenarios derived from arguments on energy-loss during gravitational
collapse. We conclude that the mass range is excluded by SN1987A data. In terms of the
scale of supersymmetry breaking , the range is not allowed.Comment: 6 pages, latex, no figures. Numerical typo correcte
Brane World Cosmologies and Statistical Properties of Gravitational Lenses
Brane world cosmologies seem to provide an alternative explanation for the
present accelerated stage of the Universe with no need to invoke either a
cosmological constant or an exotic \emph{quintessence} component. In this paper
we investigate statistical properties of gravitational lenses for some
particular scenarios based on this large scale modification of gravity. We show
that a large class of such models are compatible with the current lensing data
for values of the matter density parameter
(). If one fixes to be , as suggested by
most of the dynamical estimates of the quantity of matter in the Universe, the
predicted number of lensed quasars requires a slightly open universe with a
crossover distance between the 4 and 5-dimensional gravities of the order of
.Comment: 6 pages, 3 figures, revte
Astrophysical constraints on superlight gravitinos
I review the constraints on the mass of gravitinos that follow from
considerations on energy loss in stars and from Big Bang Nucleosynthesis
arguments.Comment: Invited talk at the 5th Workshop on High Energy Physics
Phenomenology(WHEPP-5), Pune, India, 12-26 January 199
Bayesian analysis of Friedmannless cosmologies
Assuming only a homogeneous and isotropic universe and using both the 'Gold'
Supernova Type Ia sample of Riess et al. and the results from the Supernova
Legacy Survey, we calculate the Bayesian evidence of a range of different
parameterizations of the deceleration parameter. We consider both spatially
flat and curved models. Our results show that although there is strong evidence
in the data for an accelerating universe, there is little evidence that the
deceleration parameter varies with redshift.Comment: 7 pages, 3 figure
Constraints on alternative models to dark energy
The recent observations of type Ia supernovae strongly support that the
universe is accelerating now and decelerated in the recent past. This may be
the evidence of the breakdown of the standard Friemann equation. We consider a
general modified Friedmann equation. Three different models are analyzed in
detail. The current supernovae data and the Wilkinson microwave anisotropy
probe data are used to constrain these models. A detailed analysis of the
transition from the deceleration phase to the acceleration phase is also
performed.Comment: 10 pages, 1 figure, revtex
- …