4,395 research outputs found
Constraints on Dirac-Born-Infeld type dark energy models from varying alpha
We study the variation of the effective fine structure constant alpha for
Dirac-Born-Infeld (DBI) type dark energy models. The DBI action based on string
theory naturally gives rise to a coupling between gauge fields and a scalar
field responsible for accelerated expansion of the universe. This leads to the
change of alpha due to a dynamical evolution of the scalar field, which can be
compatible with the recently observed cosmological data around the redshift
. We place constraints on several different DBI models
including exponential, inverse power-law and rolling massive scalar potentials.
We find that these models can satisfy the varying alpha constraint provided
that mass scales of the potentials are fine-tuned. When we adopt the mass
scales which are motivated by string theory, both exponential and inverse
power-law potentials give unacceptably large change of alpha, thus ruled out
from observations. On the other hand the rolling massive scalar potential is
compatible with the observationally allowed variation of alpha. Therefore the
information of varying alpha provides a powerful way to distinguish between a
number of string-inspired DBI dark energy models.Comment: 11 pages, 6 figure
Generation of electromagnetic fields in string cosmology with a massive scalar field on the anti D-brane
We study the generation of electromagnetic fields in a string-inspired
scenario associated with a rolling massive scalar field on the anti-D3
branes of KKLT de Sitter vacua. The 4-dimensional DBI type effective action
naturally gives rise to the coupling between the gauge fields and the inflaton
, which leads to the production of cosmological magnetic fields during
inflation due to the breaking of conformal invariance. We find that the
amplitude of magnetic fields at decoupling epoch can be larger than the
limiting seed value required for the galactic dynamo. We also discuss the
mechanism of reheating in our scenario and show that gauge fields are
sufficiently enhanced for the modes deep inside the Hubble radius with an
energy density greater than that of the inflaton.Comment: 4 pages and 2 eps figures, minor clarifications added and typos
correcte
Prospects of inflation with perturbed throat geometry
We study brane inflation in a warped deformed conifold background that
includes general possible corrections to the throat geometry sourced by
coupling to the bulk of a compact Calabi-Yau space. We focus specifically, on
the perturbation by chiral operator of dimension 3/2 in the CFT. We find that
the effective potential in this case can give rise to required number of
e-foldings and the spectral index consistent with observation. The tensor
to scalar ratio of perturbations is generally very low in this scenario. The
COBE normalization, however, poses certain difficulties which can be
circumvented provided model parameters are properly fine tuned. We find the
numerical values of parameters which can give rise to enough inflation,
observationally consistent values of density perturbations, scalar to tensor
ratio of perturbations and the spectral index .Comment: 7 pages and nine figures; typos corrected, minor comments and
clarifications added, revised version to appear in PL
Constraining gravity models with disappearing cosmological constant
The gravity models proposed by Hu-Sawicki and Starobinsky are generic
for local gravity constraints to be evaded. The large deviations from these
models either result into violation of local gravity constraints or the
modifications are not distinguishable from cosmological constant. The curvature
singularity in these models is generic but can be avoided provided that proper
fine tuning is imposed on the evolution of scalaron in the high curvature
regime. In principle, the problem can be circumvented by incorporating
quadratic curvature correction in the Lagrangian though it might be quite
challenging to probe the relevant region numerically.Comment: 9 pages and 4 figures, minor clarifications and corrections added,
final version to appear in PR
Holographic field theory models of dark energy in interaction with dark matter
We discuss two lagrangian interacting dark energy models in the context of
the holographic principle. The potentials of the interacting fields are
constructed. The models are compared with CMB distance information, baryonic
acoustic oscilations, lookback time and the Constitution supernovae sample. For
both models the results are consistent with a non vanishing interaction between
dark sectors - with more than three standard deviations of confidence for one
of them. Moreover, in both cases, the sign of coupling is consistent with dark
energy decaying into dark matter, alleviating the coincidence problem.Comment: arXiv admin note: substantial text overlap with arXiv:0912.399
Quintessential Inflation in a thawing realization
We study quintessential inflation with an inverse hyperbolic type potential
, where ,
and "n" are parameters of the theory. We obtain a bound on
for different values of the parameter n. The spectral index and the
tensor-to-scalar-ratio fall in the bound given by the Planck 2015
data for for certain values of . However for
there exist values of for which the spectral index and the
tensor-to-scalar-ratio fall only within the bound of the Planck
data. Furthermore, we show that the scalar field with the given potential can
also give rise to late time acceleration if we invoke the coupling to massive
neutrino matter. We also consider the instant preheating mechanism with Yukawa
interaction and put bounds on the coupling constants for our model using the
nucleosynthesis constraint on relic gravity waves produced during inflation.Comment: 11 page
- …