6 research outputs found
Non-Gaussianities in two-field inflation
We study the bispectrum of the curvature perturbation on uniform energy
density hypersurfaces in models of inflation with two scalar fields evolving
simultaneously. In the case of a separable potential, it is possible to compute
the curvature perturbation up to second order in the perturbations, generated
on large scales due to the presence of non-adiabatic perturbations, by
employing the -formalism, in the slow-roll approximation. In this
case, we provide an analytic formula for the nonlinear parameter . We
apply this formula to double inflation with two massive fields, showing that it
does not generate significant non-Gaussianity; the nonlinear parameter at the
end of inflation is slow-roll suppressed. Finally, we develop a numerical
method for generic two-field models of inflation, which allows us to go beyond
the slow-roll approximation and confirms our analytic results for double
inflation.Comment: 29 pages, 6 figures. v2, comparison with previous estimates. v3, JCAP
version; Revisions based on Referee's comment, corrected typos, added few eqs
and refs, conclusions unchange
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
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
Combined local and equilateral non-Gaussianities from multifield DBI inflation
We study multifield aspects of Dirac-Born-Infeld (DBI) inflation. More
specifically, we consider an inflationary phase driven by the radial motion of
a D-brane in a conical throat and determine how the D-brane fluctuations in the
angular directions can be converted into curvature perturbations when the
tachyonic instability arises at the end of inflation. The simultaneous presence
of multiple fields and non-standard kinetic terms gives both local and
equilateral shapes for non-Gaussianities in the bispectrum. We also study the
trispectrum, pointing out that it acquires a particular momentum dependent
component whose amplitude is given by . We show that
this relation is valid in every multifield DBI model, in particular for any
brane trajectory, and thus constitutes an interesting observational signature
of such scenarios.Comment: 38 pages, 11 figures. Typos corrected; references added. This version
matches the one in press by JCA
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