80 research outputs found
Non-Gaussianity constrains hybrid inflation
26pp, 2 figs v2: note added (see abstract)In hybrid inflationary models, inflation ends by a sudden instability associated with a steep ridge in the potential. Here we argue that this feature can generate a large contribution to the curvature perturbation on observable scales. This contribution is almost scale-invariant but highly non-Gaussian. The degree of non-Gaussianity can exceed current observational bounds, unless the inflationary scale is extremely low or the hybrid potential contains very large coupling constants. Non-linear effects on small scales may quench the non-Gaussian signal, and while we find no compelling evidence that this occurs, full lattice simulations are required to definitively address this issue. Note added: We now believe that nonlinear effects will invalidate the original computation in this paper essentially instantaneously after the short-wavelength modes reach the minimum of their potential. This means that the mechanism described in this paper will not lead to appreciatable curvature perturbations on long wavelengths, and no useful constraints on hybrid inflation will result. We have inserted a brief calculation on p2 of this manuscript to explain this fact, but have otherwise left the manuscript unchanged
Non-Gaussianity after many-field reheating
International audienceWe numerically investigate reheating after quadratic inflation with up to 65 fields, focusing on the production of non-Gaussianity. We consider several sets of initial conditions, masses, and decay rates. As expected, we find that the reheating phase can have a significant effect on the non-Gaussian signal, but that for this number of fields a detectable level of non-Gaussianity requires the initial conditions, mass range, and decay rates to be ordered in a particular way. We speculate on whether this might change in the N-flation limit
Attractor behaviour in multifield inflation
We study multifield inflation in scenarios where the fields are coupled
non-minimally to gravity via , where
are coupling constants, the fields driving inflation,
the space-time metric, the Ricci tensor, and .
We consider the so-called -attractor models in two formulations of
gravity: in the usual metric case where ,
and in the Palatini formulation where is an independent variable.
As the main result, we show that, regardless of the underlying theory of
gravity, the field-space curvature in the Einstein frame has no influence on
the inflationary dynamics at the limit of large , and one effectively
retains the single-field case. However, the gravity formulation does play an
important role: in the metric case the result means that multifield models
approach the single-field -attractor limit, whereas in the Palatini
case the attractor behaviour is lost also in the case of multifield inflation.
We discuss what this means for distinguishing between different models of
inflation.Comment: 20 pages, 6 figures. Typos corrected and references added. This is an
author-created, un-copyedited version of an article published in JCAP. IOP
Publishing Ltd is not responsible for any errors or omissions in this version
of the manuscript or any version derived from it. The Version of Record is
available online at
https://iopscience.iop.org/article/10.1088/1475-7516/2018/06/032/pd
Transport equations for the inflationary trispectrum
We use transport techniques to calculate the trispectrum produced in
multiple-field inflationary models with canonical kinetic terms. Our method
allows the time evolution of the local trispectrum parameters, tauNL and gNL,
to be tracked throughout the inflationary phase. We illustrate our approach
using examples. We give a simplified method to calculate the superhorizon part
of the relation between field fluctuations on spatially flat hypersurfaces and
the curvature perturbation on uniform density slices, and obtain its
third-order part for the first time. We clarify how the 'backwards' formalism
of Yokoyama et al. relates to our analysis and other recent work. We supply
explicit formulae which enable each inflationary observable to be computed in
any canonical model of interest, using a suitable first-order ODE solver.Comment: 24 pages, plus references and appendix. v2: matches version published
in JCAP; typo fixed in Eq. (54
Evolution of fNL to the adiabatic limit
We study inflationary perturbations in multiple-field models, for which zeta
typically evolves until all isocurvature modes decay--the "adiabatic limit". We
use numerical methods to explore the sensitivity of the nonlinear parameter fNL
to the process by which this limit is achieved, finding an appreciable
dependence on model-specific data such as the time at which slow-roll breaks
down or the timescale of reheating. In models with a sum-separable potential
where the isocurvature modes decay before the end of the slow-roll phase we
give an analytic criterion for the asymptotic value of fNL to be large. Other
examples can be constructed using a waterfall field to terminate inflation
while fNL is transiently large, caused by descent from a ridge or convergence
into a valley. We show that these two types of evolution are distinguished by
the sign of the bispectrum, and give approximate expressions for the peak fNL.Comment: v1: 25 pages, plus Appendix and bibliography, 6 figures. v2: minor
edits to match published version in JCA
Non-linear non-local Cosmology
Non-local equations of motion contain an infinite number of derivatives and
commonly appear in a number of string theory models. We review how these
equations can be rewritten in the form of a diffusion-like equation with
non-linear boundary conditions. Moreover, we show that this equation can be
solved as an initial value problem once a set of non-trivial initial conditions
that satisfy the boundary conditions is found. We find these initial conditions
by looking at the linear approximation to the boundary conditions. We then
numerically solve the diffusion-like equation, and hence the non-local
equations, as an initial value problem for the full non-linear potential and
subsequently identify the cases when inflation is attained.Comment: 6 pages, 7 figures. Prepared for 4th International Workshop on The
Dark Side of the Universe, BUE, 1-5 June 200
Moment transport equations for the primordial curvature perturbation
In a recent publication, we proposed that inflationary perturbation theory
can be reformulated in terms of a probability transport equation, whose moments
determine the correlation properties of the primordial curvature perturbation.
In this paper we generalize this formulation to an arbitrary number of fields.
We deduce ordinary differential equations for the evolution of the moments of
zeta on superhorizon scales, which can be used to obtain an evolution equation
for the dimensionless bispectrum, fNL. Our equations are covariant in field
space and allow identification of the source terms responsible for evolution of
fNL. In a model with M scalar fields, the number of numerical integrations
required to obtain solutions of these equations scales like O(M^3). The
performance of the moment transport algorithm means that numerical calculations
with M >> 1 fields are straightforward. We illustrate this performance with a
numerical calculation of fNL in Nflation models containing M ~ 10^2 fields,
finding agreement with existing analytic calculations. We comment briefly on
extensions of the method beyond the slow-roll approximation, or to calculate
higher order parameters such as gNL.Comment: 23 pages, plus appendices and references; 4 figures. v2: incorrect
statements regarding numerical delta N removed from Sec. 4.3. Minor
modifications elsewher
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