1,952 research outputs found
Observational constraints on braneworld inflation: the effect of a Gauss-Bonnet term
High-energy modifications to general relativity introduce changes to the
perturbations generated during inflation, and the latest high-precision
cosmological data can be used to place constraints on such modified inflation
models. Recently it was shown that Randall-Sundrum type braneworld inflation
leads to tighter constraints on quadratic and quartic potentials than in
general relativity. We investigate how this changes with a Gauss-Bonnet
correction term, which can be motivated by string theory. Randall-Sundrum
models preserve the standard consistency relation between the tensor spectral
index and the tensor-to-scalar ratio. The Gauss-Bonnet term breaks this
relation, and also modifies the dynamics and perturbation amplitudes at high
energies. We find that the Gauss-Bonnet term tends to soften the
Randall-Sundrum constraints. The observational compatibility of the quadratic
potential is strongly improved. For a broad range of energy scales, the quartic
potential is rescued from marginal rejection. Steep inflation driven by an
exponential potential is excluded in the Randall-Sundrum case, but the
Gauss-Bonnet term leads to marginal compatibility for sufficient e-folds.Comment: 10 pages, 10 figures, version to appear in Physical Review
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
Dynamics of dark energy
In this paper we review in detail a number of approaches that have been
adopted to try and explain the remarkable observation of our accelerating
Universe. In particular we discuss the arguments for and recent progress made
towards understanding the nature of dark energy. We review the observational
evidence for the current accelerated expansion of the universe and present a
number of dark energy models in addition to the conventional cosmological
constant, paying particular attention to scalar field models such as
quintessence, K-essence, tachyon, phantom and dilatonic models. The importance
of cosmological scaling solutions is emphasized when studying the dynamical
system of scalar fields including coupled dark energy. We study the evolution
of cosmological perturbations allowing us to confront them with the observation
of the Cosmic Microwave Background and Large Scale Structure and demonstrate
how it is possible in principle to reconstruct the equation of state of dark
energy by also using Supernovae Ia observational data. We also discuss in
detail the nature of tracking solutions in cosmology, particle physics and
braneworld models of dark energy, the nature of possible future singularities,
the effect of higher order curvature terms to avoid a Big Rip singularity, and
approaches to modifying gravity which leads to a late-time accelerated
expansion without recourse to a new form of dark energy.Comment: 93 pages, 26 figures, Invited Review to be submitted to International
Journal of Modern Physics D; comments are welcome; Additional references
included in response to over 60 comments received. Rewriting of sub-sections
on anthropic principle and gravitational backreaction. New subsections adde
Inflation and dark energy arising from geometrical tachyons
We study the motion of a BPS D3-brane in the NS5-brane ring background. The
radion field becomes tachyonic in this geometrical set up. We investigate the
potential of this geometrical tachyon in the cosmological scenario for
inflation as well as dark energy. We evaluate the spectra of scalar and tensor
perturbations generated during tachyon inflation and show that this model is
compatible with recent observations of Cosmic Microwave Background (CMB) due to
an extra freedom of the number of NS5-branes. It is not possible to explain the
origin of both inflation and dark energy by using a single tachyon field, since
the energy density at the potential minimum is not negligibly small because of
the amplitude of scalar perturbations set by CMB anisotropies. However
geometrical tachyon can account for dark energy when the number of NS5-branes
is large, provided that inflation is realized by another scalar field.Comment: 11 pages, 8 figure
Prospects of inflation in delicate D-brane cosmology
We study D-brane inflation in a warped conifold background that includes
brane-position dependent corrections for the nonperturbative superpotential.
Instead of stabilizing the volume modulus chi at instantaneous minima of the
potential and studying the inflation dynamics with an effective single field
(radial distance between a brane and an anti-brane) phi, we investigate the
multi-field inflation scenario involving these two fields. The two-field
dynamics with the potential V(phi,chi) in this model is significantly different
from the effective single-field description in terms of the field phi when the
field chi is integrated out. The latter picture underestimates the total number
of e-foldings even by one order of magnitude. We show that a correct
single-field description is provided by a field psi obtained from a rotation in
the two-field space along the background trajectory. This model can give a
large number of e-foldings required to solve flatness and horizon problems at
the expense of fine-tunings of model parameters. We also estimate the spectra
of density perturbations and show that the slow-roll parameter eta_{psi
psi}=M_{pl}^2 V_{,psi psi}/V in terms of the rotated field psi determines the
spectral index of scalar metric perturbations. We find that it is generally
difficult to satisfy, simultaneously, both constraints of the spectral index
and the COBE normalization, while the tensor to scalar ratio is sufficiently
small to match with observations.Comment: 12 pages, 8 figures, version to appear in Physical Review
Solar system and equivalence principle constraints on f(R) gravity by chameleon approach
We study constraints on f(R) dark energy models from solar system experiments
combined with experiments on the violation of equivalence principle. When the
mass of an equivalent scalar field degree of freedom is heavy in a region with
high density, a spherically symmetric body has a thin-shell so that an
effective coupling of the fifth force is suppressed through a chameleon
mechanism. We place experimental bounds on the cosmologically viable models
recently proposed in literature which have an asymptotic form f(R)=R-lambda R_c
[1-(R_c/R)^{2n}] in the regime R >> R_c. From the solar-system constraints on
the post-Newtonian parameter gamma, we derive the bound n>0.5, whereas the
constraints from the violations of weak and strong equivalence principles give
the bound n>0.9. This allows a possibility to find the deviation from the
LambdaCDM cosmological model. For the model f(R)=R-lambda R_c(R/R_c)^p with
0<p<1 the severest constraint is found to be p<10^{-10}, which shows that this
model is hardly distinguishable from the LambdaCDM cosmology.Comment: 5 pages, no figures, version to appear in Physical Review
Generic estimates for magnetic fields generated during inflation including Dirac-Born-Infeld theories
We estimate the strength of large-scale magnetic fields produced during
inflation in the framework of Dirac-Born-Infeld (DBI) theories. This analysis
is sufficiently general in the sense that it covers most of conformal symmetry
breaking theories in which the electromagnetic field is coupled to a scalar
field. In DBI theories there is an additional factor associated with the speed
of sound, which allows a possibility to lead to an extra amplification of the
magnetic field in a ultra-relativistic region. We clarify the conditions under
which seed magnetic fields to feed the galactic dynamo mechanism at a
decoupling epoch as well as present magnetic fields on galactic scales are
sufficiently generated to satisfy observational bounds.Comment: 7 pages, no figure, accepted in Phys. Rev.
Observational constraints on patch inflation in noncommutative spacetime
We study constraints on a number of patch inflationary models in
noncommutative spacetime using a compilation of recent high-precision
observational data. In particular, the four-dimensional General Relativistic
(GR) case, the Randall-Sundrum (RS) and Gauss-Bonnet (GB) braneworld scenarios
are investigated by extending previous commutative analyses to the infrared
limit of a maximally symmetric realization of the stringy uncertainty
principle. The effect of spacetime noncommutativity modifies the standard
consistency relation between the tensor spectral index and the tensor-to-scalar
ratio. We perform likelihood analyses in terms of inflationary observables
using new consistency relations and confront them with large-field inflationary
models with potential V \propto \vp^p in two classes of noncommutative
scenarios. We find a number of interesting results: (i) the quartic potential
(p=4) is rescued from marginal rejection in the class 2 GR case, and (ii) steep
inflation driven by an exponential potential (p \to \infty) is allowed in the
class 1 RS case. Spacetime noncommutativity can lead to blue-tilted scalar and
tensor spectra even for monomial potentials, thus opening up a possibility to
explain the loss of power observed in the cosmic microwave background
anisotropies. We also explore patch inflation with a Dirac-Born-Infeld tachyon
field and explicitly show that the associated likelihood analysis is equivalent
to the one in the ordinary scalar field case by using horizon-flow parameters.
It turns out that tachyon inflation is compatible with observations in all
patch cosmologies even for large p.Comment: 16 pages, 11 figures; v2: updated references, minor corrections to
match the Phys. Rev. D versio
Generalized Galileon cosmology
We study the cosmology of a generalized Galileon field with five
covariant Lagrangians in which is replaced by general scalar functions
(i=1,...,5). For these theories, the equations of motion remain
at second-order in time derivatives. We restrict the functional forms of
from the demand to obtain de Sitter solutions responsible for
dark energy. There are two possible choices for power-law functions
, depending on whether the coupling with the Ricci
scalar is independent of or depends on . The former
corresponds to the covariant Galileon theory that respects the Galilean
symmetry in the Minkowski space-time. For generalized Galileon theories we
derive the conditions for the avoidance of ghosts and Laplacian instabilities
associated with scalar and tensor perturbations as well as the condition for
the stability of de Sitter solutions. We also carry out detailed analytic and
numerical study for the cosmological dynamics in those theories.Comment: 24 pages, 10 figures, version to appear in Physical Review
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