9 research outputs found
de Sitter limit of inflation and nonlinear perturbation theory
We study the fourth order action of the comoving curvature perturbation in an
inflationary universe in order to understand more systematically the de Sitter
limit in nonlinear cosmological perturbation theory. We derive the action of
the curvature perturbation to fourth order in the comoving gauge, and show that
it vanishes sufficiently fast in the de Sitter limit. By studying the de Sitter
limit, we then extrapolate to the n'th order action of the comoving curvature
perturbation and discuss the slow-roll order of the n-point correlation
function.Comment: 14 pages, 1 figure; typos corrected and discussion of tensor modes
adde
Non-gaussianity from the inflationary trispectrum
We present an estimate for the non-linear parameter \tau_NL, which measures
the non-gaussianity imprinted in the trispectrum of the comoving curvature
perturbation, \zeta. Our estimate is valid throughout the inflationary era,
until the slow-roll approximation breaks down, and takes into account the
evolution of perturbations on superhorizon scales. We find that the
non-gaussianity is always small if the field values at the end of inflation are
negligible when compared to their values at horizon crossing. Under the same
assumption, we show that in Nflation-type scenarios, where the potential is a
sum of monomials, the non-gaussianity measured by \tau_NL is independent of the
couplings and initial conditions.Comment: 15 pages, uses iopart.sty. Replaced with version accepted by JCAP;
journal reference adde
The inflationary trispectrum
We calculate the trispectrum of the primordial curvature perturbation
generated by an epoch of slow-roll inflation in the early universe, and
demonstrate that the non-gaussian signature imprinted at horizon crossing is
unobservably small, of order tau_NL < r/50, where r < 1 is the tensor-to-scalar
ratio. Therefore any primordial non-gaussianity observed in future microwave
background experiments is likely to have been synthesized by gravitational
effects on superhorizon scales. We discuss the application of Maldacena's
consistency condition to the trispectrum.Comment: 23 pages, 2 diagrams drawn with feynmp.sty, uses iopart.cls. v2,
replaced with version accepted by JCAP. Estimate of maximal tau_NL refined in
Section 5, resulting in smaller numerical value. Sign errors in Eq. (44) and
Eq. (48) corrected. Some minor notational change
Large Non-Gaussianities in Single Field Inflation
We compute the 3-point correlation function for a general model of inflation
driven by a single, minimally coupled scalar field. Our approach is based on
the numerical evaluation of both the perturbation equations and the integrals
which contribute to the 3-point function. Consequently, we can analyze models
where the potential has a "feature", in the vicinity of which the slow roll
parameters may take on large, transient values. This introduces both scale and
shape dependent non-Gaussianities into the primordial perturbations. As an
example of our methodology, we examine the ``step'' potentials which have been
invoked to improve the fit to the glitch in the for ,
present in both the one and three year WMAP data sets. We show that for the
typical parameter values, the non-Gaussianities associated with the step are
far larger than those in standard slow roll inflation, and may even be within
reach of a next generation CMB experiment such as Planck. More generally, we
use this example to explain that while adding features to potential can improve
the fit to the 2-point function, these are generically associated with a
greatly enhanced signal at the 3-point level. Moreover, this 3-point signal
will have a very nontrivial shape and scale dependence, which is correlated
with the form of the 2-point function, and may thus lead to a consistency check
on the models of inflation with non-smooth potentials.Comment: 23 pages JHEP-style, 7 Figures. Updated with improved results.
Accepted for publication by JCA
Large Nongaussianity from Nonlocal Inflation
We study the possibility of obtaining large nongaussian signatures in the
Cosmic Microwave Background in a general class of single-field nonlocal
hill-top inflation models. We estimate the nonlinearity parameter f_{NL} which
characterizes nongaussianity in such models and show that large nongaussianity
is possible. For the recently proposed p-adic inflation model we find that
f_{NL} ~ 120 when the string coupling is order unity. We show that large
nongaussianity is also possible in a toy model with an action similar to those
which arise in string field theory.Comment: 27 pages, no figures. Added references and some clarifying remark
Predictions for Nongaussianity from Nonlocal Inflation
In our previous work the nonlinearity parameter f_NL, which characterizes
nongaussianity in the cosmic microwave background, was estimated for a class of
inflationary models based on nonlocal field theory. These models include p-adic
inflation and generically have the remarkable property that slow roll inflation
can proceed even with an extremely steep potential. Previous calculations found
that large nongaussianity is possible; however, the technical complications
associated with studying perturbations in theories with infinitely many
derivatives forced us to provide only an order of magnitude estimate for f_NL.
We reconsider the problem of computing f_NL in nonlocal inflation models,
showing that a particular choice of field basis and recent progress in
cosmological perturbation theory makes an exact computation possible. We
provide the first quantitatively accurate computation of the bispectrum in
nonlocal inflation, confirming our previous claim that it can be observably
large. We show that the shape of the bispectrum in this class of models makes
it observationally distinguishable from Dirac-Born-Infeld inflation models.Comment: 26 pages, 5 figures; references added, sign convention for f_NL
clarified, minor correction
Inflation in a conformally invariant two-scalar-field theory with an extra R 2 term
We explore inflationary cosmology in a theory where there are two scalar fields which non-minimally couple to the Ricci scalar and an additional term, which breaks the conformal invariance. Particularly, we investigate the slow-roll inflation in the case of one dynamical scalar field and that of two dynamical scalar fields. It is explicitly demonstrated that the spectral index of the scalar mode of the density perturbations and the tensor-to-scalar ratio can be consistent with the observations obtaind by the recent Planck satellite. The graceful exit from the inflationary stage is achieved as in convenient gravity. We also propose the generalization of the model under discussion with three scalar fields