50 research outputs found
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
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
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
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
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
Radiation-dominated area metric cosmology
We provide further crucial support for a refined, area metric structure of
spacetime. Based on the solution of conceptual issues, such as the consistent
coupling of fermions and the covariant identification of radiation fields on
area metric backgrounds, we show that the radiation-dominated epoch of area
metric cosmology is equivalent to that epoch in standard Einstein cosmology.
This ensures, in particular, successful nucleosynthesis. This surprising result
complements the previously derived prediction of a small late-time acceleration
of an area metric universe.Comment: 23 pages, no figures; references adde
Testing the DGP model with ESSENCE
We use the recent supernova data set from the ESSENCE collaboration combined
with data from the Supernova Legacy Survey and nearby supernovae to test the
DGP brane world model and its generalisations. Combination of this data with a
flatness prior and the position of the peak of the CMB disfavours the DGP model
slightly. Inclusion of the baryon acoustic peak from the Sloan Digital Sky
Survey increase the tension of the DGP model with the data, although it is not
clear how self consistent this procedure would be without a re-analysis of the
survey data in the framework of the DGP cosmology. Generalisations of the DGP
model are tested and constraints on relevant parameters obtained.Comment: Minor corrections, clarifications and references added. Published in
JCA
Holographic Dark Energy Like in Gravity
We investigate the corresponding relation between gravity and
holographic dark energy. We introduce a kind of energy density from
which has role of the same as holographic dark energy.
We obtain the differential equation that specify the evolution of the
introduced energy density parameter based on varying gravitational constant. We
find out a relation for the equation of state parameter to low redshifts which
containing varying correction.Comment: 10 page
Constraining f(R) gravity in the Palatini formalism
Although several models of theories of gravity within the Palatini
approach have been studied already, the interest was concentrated on those that
have an effect on the late-time evolution of the universe, by the inclusion for
example of terms inversely proportional to the scalar curvature in the
gravitational action. However, additional positive powers of the curvature also
provide interesting early-time phenomenology, like inflation, and the presence
of such terms in the action is equally, if not more, probable. In the present
paper models with both additional positive and negative powers of the scalar
curvature are studied. Their effect on the evolution of the universe is
investigated for all cosmological eras, and various constraints are put on the
extra terms in the actions. Additionally, we examine the extent to which the
new terms in positive powers affect the late-time evolution of the universe and
the related observables, which also determines our ability to probe their
presence in the gravitational action.Comment: reference update and minor changes to match published versio
Cosmic Acceleration, Dark Energy and Fundamental Physics
A web of interlocking observations has established that the expansion of the
Universe is speeding up and not slowing, revealing the presence of some form of
repulsive gravity. Within the context of general relativity the cause of cosmic
acceleration is a highly elastic (p\sim -rho), very smooth form of energy
called ``dark energy'' accounting for about 75% of the Universe. The
``simplest'' explanation for dark energy is the zero-point energy density
associated with the quantum vacuum; however, all estimates for its value are
many orders-of-magnitude too large. Other ideas for dark energy include a very
light scalar field or a tangled network of topological defects. An alternate
explanation invokes gravitational physics beyond general relativity.
Observations and experiments underway and more precise cosmological
measurements and laboratory experiments planned for the next decade will test
whether or not dark energy is the quantum energy of the vacuum or something
more exotic, and whether or not general relativity can self consistently
explain cosmic acceleration. Dark energy is the most conspicuous example of
physics beyond the standard model and perhaps the most profound mystery in all
of science.Comment: 10 pages, 8 figures, invited review for Journal of the Physical
Society of Japan, in pres