1,917 research outputs found
Preheating with extra dimensions
We investigate preheating in a higher-dimensional generalized Kaluza-Klein
theory with a quadratic inflaton potential
including metric perturbations explicitly. The system we consider is the
multi-field model where there exists a dilaton field which corresponds
to the scale of compactifications and another scalar field coupled to
inflaton with the interaction .
In the case of , we find that the perturbation of dilaton does not
undergo parametric amplification while the field fluctuation can be
enhanced in the usual manner by parametric resonance. In the presence of the
coupling, the dilaton fluctuation in sub-Hubble scales
is modestly amplified by the growth of metric perturbations for the large
coupling . In super-Hubble scales, the enhancement of the dilaton
fluctuation as well as metric perturbations is weak, taking into account the
backreaction effect of created particles. We argue that not only is it
possible to predict the ordinary inflationary spectrum in large scales but
extra dimensions can be held static during preheating in our scenario.Comment: 21 pages, 7 figures, submitted to JHE
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
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
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
Matter instabilities in general Gauss-Bonnet gravity
We study the evolution of cosmological perturbations in f(G) gravity, where
the Lagrangian is the sum of a Ricci scalar R and an arbitrary function f in
terms of a Gauss-Bonnet term G. We derive the equations for perturbations
assuming matter to be described by a perfect fluid with a constant equation of
state w. We show that density perturbations in perfect fluids exhibit negative
instabilities during both the radiation and the matter domination, irrespective
of the form of f(G). This growth of perturbations gets stronger on smaller
scales, which is difficult to be compatible with the observed galaxy spectrum
unless the deviation from General Relativity is very small. Thus f(G)
cosmological models are effectively ruled out from this Ultra-Violet
instability, even though they can be compatible with the late-time cosmic
acceleration and local gravity constraints.Comment: 9 pages, 2 figures, published PRD 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
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
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
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
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