Article,
General CMB and Primordial Bispectrum Estimation I: Mode Expansion, Map-Making and Measures of f_NL
(2009)cite arxiv:0912.5516 Comment: 41 pages, 17 figures.
Abstract
We present a detailed implementation of two bispectrum estimation methods
which can be applied to general non-separable primordial and CMB bispectra. The
method exploits bispectrum mode decompositions on the domain of allowed
wavenumber or multipole values. Concrete mode examples constructed from
symmetrised tetrahedral polynomials are given, demonstrating rapid convergence
for known bispectra. We use these modes to generate simulated CMB maps of high
resolution (l > 2000) given an arbitrary primordial power spectrum and
bispectrum or an arbitrary late-time CMB angular power spectrum and bispectrum.
By extracting coefficients for the same separable basis functions from an
observational map, we are able to present an efficient and general f_NL
estimator for a given theoretical model. The estimator has two versions
comparing theoretical and observed coefficients at either primordial or late
times, thus encompassing a wider range of models, including secondary
anisotropies, lensing and cosmic strings. We provide examples and validation of
both f_NL estimation methods by direct comparison with simulations in a
WMAP-realistic context. In addition, we show how the full bispectrum can be
extracted from observational maps using these mode expansions, irrespective of
the theoretical model under study. We also propose a universal definition of
the bispectrum parameter F_NL for more consistent comparison between
theoretical models. We obtain WMAP5 estimates of f_NL for the equilateral model
from both our primordial and late-time estimators which are consistent with
each other, as well as with results already published in the literature. These
general bispectrum estimation methods should prove useful for the analysis of
nonGaussianity in the Planck satellite data, as well as in other contexts.
