1,502 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
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
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
Power-law inflation with a nonminimally coupled scalar field
We consider the dynamics of power-law inflation with a nonminimally coupled
scalar field . It is well known that multiple scalar fields with
exponential potentials lead to an inflationary solution even if the each scalar field is not
capable to sustain inflation. In this paper, we show that inflation can be
assisted even in the one-field case by the effect of nonminimal coupling. When
is positive, since an effective potential which arises by a conformal
transformation becomes flatter compared with the case of for ,
we have an inflationary solution even when the universe evolves as
non-inflationary in the minimally coupled case. For the negative , the
assisted inflation can take place when evolves in the region of
\.Comment: 12 pages, 6 figures, to appear in Phys. Rev.
Observational tests of inflation with a field derivative coupling to gravity
A field kinetic coupling with the Einstein tensor leads to a gravitationally
enhanced friction during inflation, by which even steep potentials with
theoretically natural model parameters can drive cosmic acceleration. In the
presence of this non-minimal derivative coupling we place observational
constraints on a number of representative inflationary models such as chaotic
inflation, inflation with exponential potentials, natural inflation, and hybrid
inflation. We show that most of the models can be made compatible with the
current observational data mainly due to the suppressed tensor-to-scalar ratio.Comment: 11 pages, 5 figure
Reconstruction of general scalar-field dark energy models
The reconstruction of scalar-field dark energy models is studied for a
general Lagrangian density , where is a kinematic term of a
scalar field . We implement the coupling between dark energy and dark
matter and express reconstruction equations using two observables: the Hubble
parameter and the matter density perturbation . This allows us to
determine the structure of corresponding theoretical Lagrangian together with
the coupling from observations. We apply our formula to several forms of
Lagrangian and present concrete examples of reconstruction by using the recent
Gold dataset of supernovae measurements. This analysis includes a generalized
ghost condensate model as a way to cross a cosmological-constant boundary even
for a single-field case.Comment: 8 pages, 2 figure
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