Large-scale structure, the cosmic microwave background, and primordial non-gaussianity

TL;DR

This work assesses which data source—CMB or LSS—offers a tighter probe of primordial non-gaussianity by focusing on the bispectrum as the diagnostic. Through comparative analysis across several models, it concludes that the CMB generally provides a more sensitive measure of primordial non-gaussianity than galaxy surveys. It further argues that if CMB maps are Gaussian, any non-gaussian signatures in LSS can be attributed to biasing, a point demonstrated for local non-linear biasing, and it discusses how the bispectrum evolves with redshift, highlighting the relative dominance of primordial versus gravitational contributions at high redshift. The treatment integrates both skewness and bispectrum calculations to delineate the observable signatures and their interpretation for upcoming MAP/Planck and SDSS/2dF data.

Abstract

Since cosmic-microwave-background (CMB) and large-scale-structure (LSS) data will shortly improve dramatically with the Microwave Anisotropy Probe (MAP) and Planck Surveyor, and the Anglo-Australian 2-Degree Field (2dF) and Sloan Digital Sky Survey (SDSS), respectively, it is timely to ask which of the CMB or LSS will provide a better probe of primordial non-gaussianity. In this paper we consider this question, using the bispectrum as a discriminating statistic. We consider several non-gaussian models and find that in each case the CMB will provide a better probe of primordial non-gaussianity. Since the bispectrum is the lowest-order statistic expected to arise in a generic non-gaussian model, our results suggest that if CMB maps appear gaussian, then apparent deviations from gaussian initial conditions in galaxy surveys can be attributed with confidence to the effects of biasing. We demonstrate this precisely for the spatial bispectrum induced by local non-linear biasing.