451 research outputs found
Quantitative bispectra from multifield inflation
After simplifying and improving the non-Gaussian formalism we developed in
previous work, we derive a quantitative expression for the three-point
correlator (bispectrum) of the curvature perturbation in general multiple-field
inflation models. Our result describes the evolution of non-Gaussianity on
superhorizon scales caused by the nonlinear influence of isocurvature
perturbations on the adiabatic perturbation during inflation. We then study a
simple quadratic two-field potential and find that when slow roll breaks down
and the field trajectory changes direction in field space, the non-Gaussianity
can become large. However, for the simple models studied to date, the magnitude
of this non-Gaussianity decays away after the isocurvature mode is converted
into the adiabatic mode.Comment: 7 pages, 1 figure. v4: Added remarks on momentum dependence, minor
textual changes, matches published versio
Simple route to non-Gaussianity in inflation
We present a simple way to calculate non-Gaussianity in inflation using fully
non-linear equations on long wavelengths with stochastic sources to take into
account the short-wavelength quantum fluctuations. Our formalism includes both
scalar metric and matter perturbations, combining them into variables which are
invariant under changes of time slicing in the long-wavelength limit. We
illustrate this method with a perturbative calculation in the single-field
slow-roll case. We also introduce a convenient choice of variables to
graphically present the full momentum dependence of the three-point correlator.Comment: 6 pages, 2 figures. v2: Updated formalism to version described in
astro-ph/0504508, leading to dropping of one unnecessary approximation. Final
results not significantly changed. Extended discussion of calculation and
added graphical presentation of full momentum dependence. References
corrected and added. v3: Final version, only small textual change
Non-linear inflationary perturbations
We present a method by which cosmological perturbations can be quantitatively
studied in single and multi-field inflationary models beyond linear
perturbation theory. A non-linear generalization of the gauge-invariant
Sasaki-Mukhanov variables is used in a long-wavelength approximation. These
generalized variables remain invariant under time slicing changes on long
wavelengths. The equations they obey are relatively simple and can be
formulated for a number of time slicing choices. Initial conditions are set
after horizon crossing and the subsequent evolution is fully non-linear. We
briefly discuss how these methods can be implemented numerically in the study
of non-Gaussian signatures from specific inflationary models.Comment: 10 pages, replaced to match JCAP versio
Hunting for Isocurvature Modes in the CMB non-Gaussianities
We investigate new shapes of local primordial non-Gaussianities in the CMB.
Allowing for a primordial isocurvature mode along with the main adiabatic one,
the angular bispectrum is in general a superposition of six distinct shapes:
the usual adiabatic term, a purely isocurvature component and four additional
components that arise from correlations between the adiabatic and isocurvature
modes. We present a class of early Universe models in which various hierarchies
between these six components can be obtained, while satisfying the present
upper bound on the isocurvature fraction in the power spectrum. Remarkably,
even with this constraint, detectable non-Gaussianity could be produced by
isocurvature modes. We finally discuss the prospects of detecting these new
shapes with the Planck satellite.Comment: 9 pages, 2 figure
Tree indiscernibilities, revisited
We give definitions that distinguish between two notions of indiscernibility
for a set \{a_\eta \mid \eta \in \W\} that saw original use in \cite{sh90},
which we name \textit{\s-} and \textit{\n-indiscernibility}. Using these
definitions and detailed proofs, we prove \s- and \n-modeling theorems and
give applications of these theorems. In particular, we verify a step in the
argument that TP is equivalent to TP or TP that has not seen
explication in the literature. In the Appendix, we exposit the proofs of
\citep[{App. 2.6, 2.7}]{sh90}, expanding on the details.Comment: submitte
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
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