Primordial non-gaussianity, statistics of collapsed objects, and the Integrated Sachs-Wolfe effect

TL;DR

This paper demonstrates how small local primordial non-gaussianity affects the statistics of collapsed objects and explores how galaxy surveys and CMB correlations can constrain non-gaussianity, providing a new analytic approach and promising observational prospects.

Contribution

It introduces a simple analytic method to approximate non-gaussian initial conditions with gaussian ones for predicting their effects on collapsed objects and analyzes the potential of galaxy surveys and CMB correlations to measure primordial non-gaussianity.

Findings

Analytic expressions relate non-gaussian effects to gaussian initial conditions.

Predicted galaxy survey and CMB cross-correlation can constrain f_{NL} to ~0.1 and 3.

Existing data shows a hint of large local primordial non-gaussianity with f_{NL} = 236 +/- 127.

Abstract

Any hint of non-gaussianity in the cosmological initial conditions will provide us with a unique window into the physics of early universe. We show that the impact of a small local primordial non-gaussianity (generated on super-horizon scales) on the statistics of collapsed objects (such as galaxies or clusters) can be approximated by using slightly modified, but gaussian, initial conditions, which we describe through simple analytic expressions. Given that numerical simulations with gaussian initial conditions are relatively well-studied, this equivalence provides us with a simple tool to predict signatures of primordial non-gaussianity in the statistics of collapsed objects. In particular, we describe the predictions for non-gaussian mass function, and also confirm the recent discovery of a non-local bias on large scales (arXiv:0710.4560, arXiv:0801.4826) as a signature of primordial non-gaussianity. We then study the potential of galaxy surveys to constrain non-gaussianity using their auto-correlation and cross-correlation with the CMB (due to the Integrated Sachs-Wolfe effect), as a function of survey characteristics, and predict that they will eventually yield an accuracy of Delta f_{NL} ~ 0.1 and 3 respectively, which will be better than or competitive with (but independent of) the best predicted constraints from the CMB. Interestingly, the cross-correlation of CMB and NVSS galaxy survey already shows a hint of a large local primordial non-gaussianity: f_{NL} = 236 +/- 127.