13 research outputs found
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
Brane preheating
We study brane-world preheating in massive chaotic inflationary scenario
where scalar fields are confined on the 3-brane. Assuming that quadratic
contribution in energy densities dominates the Hubble expansion rate during
preheating, the amplitude of inflaton decreases slowly relative to the standard
dust-dominated case. This leads to an efficient production of particles
via nonperturbative decay of inflaton even if its coupling is of order
. We also discuss massive particle creation heavier than inflaton,
which may play important roles for the baryo- and lepto-genesis scenarios.Comment: 6 pages, 2 figures, submitted to Physical Review
Enhancing the tensor-to-scalar ratio in simple inflation
We show that in theories with a nontrivial kinetic term the contribution of
the gravitational waves to the CMB fluctuations can be substantially larger
than that is naively expected in simple inflationary models. This increase of
the tensor-to-scalar perturbation ratio leads to a larger B-component of the
CMB polarization, thus making the prospects for future detection much more
promising. The other important consequence of the considered model is a higher
energy scale of inflation and hence higher reheating temperature compared to a
simple inflation.Comment: 9 pages, 1 figure and references are added, discussion is slightly
extended, published versio
Non-Gaussian perturbations from multi-field inflation
We show how the primordial bispectrum of density perturbations from inflation
may be characterised in terms of manifestly gauge-invariant cosmological
perturbations at second order. The primordial metric perturbation, zeta,
describing the perturbed expansion of uniform-density hypersurfaces on large
scales is related to scalar field perturbations on unperturbed (spatially-flat)
hypersurfaces at first- and second-order. The bispectrum of the metric
perturbation is thus composed of (i) a local contribution due to the
second-order gauge-transformation, and (ii) the instrinsic bispectrum of the
field perturbations on spatially flat hypersurfaces. We generalise previous
results to allow for scale-dependence of the scalar field power spectra and
correlations that can develop between fields on super-Hubble scales.Comment: 11 pages, RevTex; minor changes to text; conclusions unchanged;
version to appear in JCA
Multiple-field inflation and the CMB
In this paper, we investigate some consequences of multiple-field inflation
for the cosmic microwave background radiation (CMB). We derive expressions for
the amplitudes, the spectral indices and the derivatives of the indices of the
CMB power spectrum in the context of a very general multiple-field theory of
slow-roll inflation, where the field metric can be non-trivial. Both scalar
(adiabatic, isocurvature and mixing) and tensor perturbations are treated and
the differences with single-field inflation are discussed. From these
expressions, several relations are derived that can be used to determine the
importance of multiple-field effects observationally from the CMB. We also
study the evolution of the total entropy perturbation during radiation and
matter domination and the influence of this on the isocurvature spectral
quantities.Comment: 24 pages. References added, some very minor textual changes, matches
version to be published in CQ
Correlation-consistency cartography of the double-inflation landscape
We show explicitly some exciting features of double inflation: (i) it can often lead to strongly correlated adiabatic and entropy (isocurvature) power spectra; (ii) the two-field slow-roll consistency relations can be violated when the correlation is large at the Hubble crossing; (iii) the spectra of adiabatic and entropy perturbations can be strongly scale dependent and tilted toward either the red or blue. These effects are typically due to a light or time-dependent entropy mass and a non-negligible angular velocity in field space during inflation. They are illustrated via a multiparameter numerical search for correlations in two concrete models. The correlation is found to be particularly strong in a supersymmetric scenario due to the rapid growth of entropy perturbations in the tachyonic region separating the two inflationary stages. Our analysis suggests that realistic double-inflation models will provide a rich and fruitful arena for the application of future cosmic data sets and new approximation schemes which go beyond slow roll
A Quintessentially Geometric Model
We consider string inspired cosmology on a solitary -brane moving in the
background of a ring of branes located on a circle of radius . The motion of
the -brane transverse to the plane of the ring gives rise to a radion field
which can be mapped to a massive non-BPS Born-Infeld type field with a cosh
potential. For certain bounds of the brane tension we find an inflationary
phase is possible, with the string scale relatively close to the Planck scale.
The relevant perturbations and spectral indices are all well within the
expected observational bounds. The evolution of the universe eventually comes
to be dominated by dark energy, which we show is a late time attractor of the
model. However we also find that the equation of state is time dependent, and
will lead to late time Quintessence.Comment: 11 pages, 3 figures. References and comments adde
Stochastic Gravitational Wave Production After Inflation
In many models of inflation, the period of accelerated expansion ends with
preheating, a highly non-thermal phase of evolution during which the inflaton
pumps energy into a specific set of momentum modes of field(s) to which it is
coupled. This necessarily induces large, transient density inhomogeneities
which can source a significant spectrum of gravitational waves. In this paper,
we consider the generic properties of gravitational waves produced during
preheating, perform detailed calculations of the spectrum for several specific
inflationary models, and identify problems that require further study. In
particular, we argue that if these gravitational waves exist they will
necessarily fall within the frequency range that is feasible for direct
detection experiments -- from laboratory through to solar system scales. We
extract the gravitational wave spectrum from numerical simulations of
preheating after and inflation, and find
that they lead to a gravitational wave amplitude of around . This is considerably higher than the amplitude of the primordial
gravitational waves produced during inflation. However, the typical wavelength
of these gravitational waves is considerably shorter than LIGO scales, although
in extreme cases they may be visible at scales accessible to the proposed BBO
mission. We survey possible experimental approaches to detecting any
gravitational wave background generated during preheating.Comment: 11 pages. Updated references. Minor clarification
f(R) theories
Over the past decade, f(R) theories have been extensively studied as one of
the simplest modifications to General Relativity. In this article we review
various applications of f(R) theories to cosmology and gravity - such as
inflation, dark energy, local gravity constraints, cosmological perturbations,
and spherically symmetric solutions in weak and strong gravitational
backgrounds. We present a number of ways to distinguish those theories from
General Relativity observationally and experimentally. We also discuss the
extension to other modified gravity theories such as Brans-Dicke theory and
Gauss-Bonnet gravity, and address models that can satisfy both cosmological and
local gravity constraints.Comment: 156 pages, 14 figures, Invited review article in Living Reviews in
Relativity, Published version, Comments are welcom
Inflation models and observation
We consider small-field models, which invoke the usual framework for the effective field theory, and large-field models, which go beyond this. Present and future possibilities for discriminating between the models are assessed, on the assumption that the primordial curvature perturbation is generated during inflation. With PLANCK data, the theoretical and observational uncertainties on the spectral index will be comparable, providing useful discrimination between small-field models. Further discrimination between models may come later through the tensor fraction, the running of the spectral index and non-Gaussianity