Thorsten Battefeld, Jan Grieb
We discuss bispectra of single-field inflationary models described by general
Lorentz invariant Lagrangians that are at most first order in field
derivatives, including the fast-roll models investigated by Noller and
Magueijo. Based on a factor analysis, we identify the least correlated basic
contributions to the general shape and show quantitatively which templates
provide a good approximation. We compute how relative contributions of basic
shapes to the total bispectrum scale as slow roll is relaxed. To enable future
comparison with CMB observations, we provide a modal expansion of these
non-separable bispectra in Fourier space, employing the formalism by Shellard
et al. Convergence is rapid, usually better than ninety-five percent with less
than thirty modes, due to the smoothness of these primordial shapes.
Truncated polynomial modal expansions have restrictions, which we highlight
using an example with slow convergence. The particular shape originates from
particle production during inflation (common in trapped inflation) and entails
both localized and oscillatory features. We show that this shape can be
recovered efficiently using a Fourier basis and outline the prospect of future
model parameter extraction and N-body simulations based on modal techniques.
View original:
http://arxiv.org/abs/1110.1369
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