14 research outputs found
Time Evolution of the Fine Structure Constant in a Two-Field Quintessence Model
We examine the variation of the fine structure constant in the context of a
two-field quintessence model. We find that, for solutions that lead to a
transient late period of accelerated expansion, it is possible to fit the data
arising from quasar spectra and comply with the bounds on the variation of
from the Oklo reactor, meteorite analysis, atomic clock measurements,
Cosmic Microwave Background Radiation and Big Bang Nucleosynthesis. That is
more difficult if we consider solutions corresponding to a late period of
permanent accelerated expansion.Comment: 6 pages, 5 figures, RevTex
Supernovae constraints on dark energy and modified gravity models
We use the Type Ia Supernova gold sample to constrain the parameters of dark
energy models namely the Cardassian, Dvali-Turner (DT) and generalized
Chaplygin gas (GCG) models. In our best fit analysis for these dark energy
proposals we consider flat and the non-flat priors. For all models, we find
that relaxing the flatness condition implies that data favors a positive
curvature; moreover, the GCG model is nearly flat, as required by Cosmic
Microwave Background (CMB) observations.Comment: 6 pages, Latex file + 9 eps figures + (jpconf.cls,jpconf11.clo), to
appear in the Proceedings of the Fourth Meeting on Constrained Dynamics and
Quantum Gravity (QG05), Cala Gonone (Italy) September 12-16 200
Supernovae constraints on models of dark energy revisited
We use the Type Ia Supernova gold sample data of Riess {\it et al} in order
to constrain three models of dark energy. We study the Cardassian model, the
Dvali-Turner gravity modified model and the generalized Chaplygin gas model of
dark energy - dark matter unification. In our best fit analysis for these three
dark energy proposals we consider flat model and the non-flat model priors. We
also discuss the degeneracy of the models with the XCDM model through the
computation of the so-called jerk parameter.Comment: Revtex4, 11 pages, 6 sets of figures, 3 tables. Version published at
Physical Review
Observational constraints on modified gravity models and the Poincar\'e dodecahedral topology
We study the constraints that spatial topology may place on the parameters of
models that account for the accelerated expansion of the universe via infrared
modifications to general relativity, namely the Dvali-Gabadadze-Porrati
braneworld model as well as the Dvali-Turner and Cardassian models. By
considering the Poincar\'e dodecahedral space as the circles-in-the-sky
observable spatial topology, we examine the constraints that can be placed on
the parameters of each model using type Ia supernovae data together with the
baryon acoustic peak in the large scale correlation function of the Sloan
Digital Sky Survey of luminous red galaxies and the Cosmic Microwave Background
Radiation shift parameter data. We show that knowledge of spatial topology does
provide relevant constraints, particularly on the curvature parameter, for all
models.Comment: Revtex4, 10 pages, 1 table, 12 figures; version to match the one to
be published in Physical Review
A Two-Field Quintessence Model
We study the dynamics of a quintessence model based on two interacting scalar
fields. The model can account for the (recent) accelerated expansion of the
Universe suggested by astronomical observations. Acceleration can be permanent
or temporary and, for both scenarios, it is possible to obtain suitable values
for the cosmological parameters while satisfying the nucleosynthesis constraint
on the quintessence energy density. We argue that the model dynamics can be
made consistent with a stable zero-energy relaxing supersymmetric vacuum.Comment: 4 pages, 3 eps figures, to be published in Phys. Rev.
Problems with Time-Varying Extra Dimensions or "Cardassian Expansion" as Alternatives to Dark Energy
It has recently been proposed that the Universe might be accelerating as a
consequence of extra dimensions with time varying size. We show that although
these scenarios can lead to acceleration, they run into serious difficulty when
taking into account limits on the time variation of the four dimensional
Newton's constant. On the other hand, models of ``Cardassian'' expansion based
on extra dimensions which have been constructed so far violate the weak energy
condition for the bulk stress energy, for parameters that give an accelerating
universe.Comment: 8 pages, minor changes. To appear in Physical Review
WMAP and Supergravity Inflationary Models
We study a class of N=1 Supergravity inflationary models in which the
evolution of the inflaton dynamics is controlled by a single power in the
inflaton field at the point where the observed density fluctuations are
produced, in the context of the braneworld scenario, in light of WMAP results.
In particular, we find that the bounds on the spectral index and its running
constrain the parameter space both for models where the inflationary potential
is dominated by a quadratic term and by a cubic term in the inflaton field. We
also find that is required for the quadratic model whereas
for the cubic model. Moreover, we have determined an upper bound
on the five-dimensional Planck scale, M_5 \lsim 0.019 M, for the quadratic
model. On the other hand, a running spectral index with on large scales
and on small scales is not possible in either case.Comment: 7 pages, 4 eps figures, references corrected, version to appear in
Phys. Rev.
Accelerating Cold Dark Matter Cosmology ()
A new kind of accelerating flat model with no dark energy that is fully
dominated by cold dark matter (CDM) is investigated. The number of CDM
particles is not conserved and the present accelerating stage is a consequence
of the negative pressure describing the irreversible process of gravitational
particle creation. A related work involving accelerating CDM cosmology has been
discussed before the SNe observations [Lima, Abramo & Germano, Phys. Rev. D53,
4287 (1996)]. However, in order to have a transition from a decelerating to an
accelerating regime at low redshifts, the matter creation rate proposed here
includes a constant term of the order of the Hubble parameter. In this case,
does not need to be small in order to solve the age problem and the
transition happens even if the matter creation is negligible during the
radiation and part of the matter dominated phase. Therefore, instead of the
vacuum dominance at redshifts of the order of a few, the present accelerating
stage in this sort of Einstein-de Sitter CDM cosmology is a consequence of the
gravitational particle creation process. As an extra bonus, in the present
scenario does not exist the coincidence problem that plagues models with
dominance of dark energy. The model is able to harmonize a CDM picture with the
present age of the universe, the latest measurements of the Hubble parameter
and the Supernovae observations.Comment: 9 pages, 6 figures, typos corrected, references added, discussion in
Appendix B extende
Coupled dark energy: Towards a general description of the dynamics
In dark energy models of scalar-field coupled to a barotropic perfect fluid,
the existence of cosmological scaling solutions restricts the Lagrangian of the
field \vp to p=X g(Xe^{\lambda \vp}), where X=-g^{\mu\nu} \partial_\mu \vp
\partial_\nu \vp /2, is a constant and is an arbitrary function.
We derive general evolution equations in an autonomous form for this Lagrangian
and investigate the stability of fixed points for several different dark energy
models--(i) ordinary (phantom) field, (ii) dilatonic ghost condensate, and
(iii) (phantom) tachyon. We find the existence of scalar-field dominant fixed
points (\Omega_\vp=1) with an accelerated expansion in all models
irrespective of the presence of the coupling between dark energy and dark
matter. These fixed points are always classically stable for a phantom field,
implying that the universe is eventually dominated by the energy density of a
scalar field if phantom is responsible for dark energy. When the equation of
state w_\vp for the field \vp is larger than -1, we find that scaling
solutions are stable if the scalar-field dominant solution is unstable, and
vice versa. Therefore in this case the final attractor is either a scaling
solution with constant \Omega_\vp satisfying 0<\Omega_\vp<1 or a
scalar-field dominant solution with \Omega_\vp=1.Comment: 21 pages, 5 figures; minor clarifications added, typos corrected and
references updated; final version to appear in JCA