1,730 research outputs found
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
Chaotic dynamics in preheating after inflation
We study chaotic dynamics in preheating after inflation in which an inflaton
is coupled to another scalar field through an interaction
. We first estimate the size of the quasi-homogeneous
field at the beginning of reheating for large-field inflaton potentials
by evaluating the amplitude of the fluctuations on
scales larger than the Hubble radius at the end of inflation. Parametric
excitations of the field during preheating can give rise to chaos
between two dynamical scalar fields. For the quartic potential (,
) chaos actually occurs for in a
linear regime before which the backreaction of created particles becomes
important. This analysis is supported by several different criteria for the
existence of chaos. For the quadratic potential () the signature of chaos
is not found by the time at which the backreaction begins to work, similar to
the case of the quartic potential with .Comment: 12 pages, 10 figures, Version to appear in Classical and Quantum
Gravit
Properties of singularities in (phantom) dark energy universe
The properties of future singularities are investigated in the universe
dominated by dark energy including the phantom-type fluid. We classify the
finite-time singularities into four classes and explicitly present the models
which give rise to these singularities by assuming the form of the equation of
state of dark energy. We show the existence of a stable fixed point with an
equation of state and numerically confirm that this is actually a
late-time attractor in the phantom-dominated universe. We also construct a
phantom dark energy scenario coupled to dark matter that reproduces singular
behaviors of the Big Rip type for the energy density and the curvature of the
universe. The effect of quantum corrections coming from conformal anomaly can
be important when the curvature grows large, which typically moderates the
finite-time singularities.Comment: 17 pages, 6 figures, references are added, version to appear in
Physical Review
Testing for double inflation with WMAP
With the WMAP data we can now begin to test realistic models of inflation
involving multiple scalar fields. These naturally lead to correlated adiabatic
and isocurvature (entropy) perturbations with a running spectral index. We
present the first full (9 parameter) likelihood analysis of double inflation
with WMAP data and find that despite the extra freedom, supersymmetric hybrid
potentials are strongly constrained with less than 7% correlated isocurvature
component allowed when standard priors are imposed on the cosomological
parameters. As a result we also find that Akaike & Bayesian model selection
criteria rather strongly prefer single-field inflation, just as equivalent
analysis prefers a cosmological constant over dynamical dark energy in the late
universe. It appears that simplicity is the best guide to our universe.Comment: 7 pages, 6 figure
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.
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
New constraints on multi-field inflation with nonminimal coupling
We study the dynamics and perturbations during inflation and reheating in a
multi-field model where a second scalar field is nonminimally coupled to
the scalar curvature ). When is positive, the usual
inflationary prediction for large-scale anisotropies is hardly altered while
the fluctuation in sub-Hubble modes can be amplified during preheating
for large . For negative values of , however, long-wave modes of the
fluctuation exhibit exponential increase during inflation, leading to
the strong enhancement of super-Hubble metric perturbations even when
is less than unity. This is because the effective mass becomes negative
during inflation. We constrain the strength of and the initial by
the amplitude of produced density perturbations. One way to avoid nonadiabatic
growth of super-Hubble curvature perturbations is to stabilize the mass
through a coupling to the inflaton. Preheating may thus be necessary in these
models to protect the stability of the inflationary phase.Comment: 20 pages, 8 figures, submitted to Physical Review
Realizing Scale-invariant Density Perturbations in Low-energy Effective String Theory
We discuss the realization of inflation and resulting cosmological
perturbations in the low-energy effective string theory. In order to obtain
nearly scale-invariant spectra of density perturbations and a suppressed
tensor-to-scalar ratio, it is generally necessary that the dilaton field
is effectively decoupled from gravity together with the existence of a slowly
varying dilaton potential. We also study the effect of second-order corrections
to the tree-level action which are the sum of a Gauss-Bonnet term coupled to
and a kinetic term . We find that it is possible to
realize observationally supported spectra of scalar and tensor perturbations
provided that the correction is dominated by the term even in
the absence of the dilaton potential. When the Gauss-Bonnet term is dominant,
tensor perturbations exhibit violent negative instabilities on small-scales
about a de Sitter background in spite of the fact that scale-invariant scalar
perturbations can be achieved.Comment: 13 pages; v2: minor corrections, refs. added, version to appear in
PR
Cosmological constraints from Gauss-Bonnet braneworld with large-field potentials
We calculate the spectral index and tensor-to-scalar ratio for patch
inflation defined by and ,
using the slow-roll expansion. The patch cosmology arisen from the Gauss-Bonnet
braneworld consists of Gauss-Bonnet (GB), Randall-Sundrum (RS), and 4D general
relativistic (GR) cosmological models. In this work, we choose large-field
potentials of to compare with the observational data. Since
second-order corrections are rather small in the slow-roll limit, the
leading-order calculation is sufficient to compare with the data. Finally, we
show that it is easier to discriminate between quadratic potential and quartic
potential in the GB cosmological model rather than the GR or RS cosmological
models.Comment: 13 pages, title changed, version to appear in JCA
A new twist to preheating
Metric perturbations typically strengthen field resonances during preheating.
In contrast we present a model in which the super-Hubble field resonances are
completely {\em suppressed} when metric perturbations are included. The model
is the nonminimal Fakir-Unruh scenario which is exactly solvable in the
long-wavelength limit when metric perturbations are included, but exhibits
exponential growth of super-Hubble modes in their absence. This gravitationally
enhanced integrability is exceptional, both for its rarity and for the power
with which it illustrates the importance of including metric perturbations in
consistent studies of preheating. We conjecture a no-go result - there exists
no {\em single-field} model with growth of cosmologically-relevant metric
perturbations during preheating.Comment: 6 pages, 3 figures, Version to appear in Physical Review
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