The Impact of Non-Gaussian Errors on Weak Lensing Surveys

TL;DR

This paper assesses how non-Gaussian errors affect weak lensing surveys, finding that while the overall signal-to-noise ratio can be significantly degraded, the impact on individual cosmological parameters is relatively small, guiding future survey designs.

Contribution

The study uses the halo model to quantify non-Gaussian covariance effects on weak lensing, including survey size effects, and compares results with previous conflicting studies.

Findings

Non-Gaussian correlations can reduce the signal-to-noise by up to a factor of 2.

Parameter errors increase by less than 10% in most cases.

Line-of-sight projection and geometric factors mitigate non-Gaussian impacts.

Abstract

The weak lensing power spectrum carries cosmological information via its dependence on the growth of structure and on geometric factors. Since much of the cosmological information comes from scales affected by nonlinear clustering, measurements of the lensing power spectrum can be degraded by non-Gaussian covariances. Recently there have been conflicting studies about the level of this degradation. We use the halo model to estimate it and include new contributions related to the finite size of lensing surveys, following Rimes and Hamilton's study of 3D simulations. We find that non-Gaussian correlations between different multipoles can degrade the cumulative signal-to-noise for the power spectrum amplitude by up to a factor of 2 (or 5 for a worst-case model that exceeds current N-body simulation predictions). However, using an eight-parameter Fisher analysis we find that the marginalized errors on individual parameters are degraded by less than 10% (or 20% for the worst-case model). The smaller degradation in parameter accuracy is primarily because: individual parameters in a high-dimensional parameter space are degraded much less than the volume of the full Fisher ellipsoid; lensing involves projections along the line of sight, which reduce the non-Gaussian effect; some of the cosmological information comes from geometric factors which are not degraded at all. We contrast our findings with those of Lee & Pen (2008) who suggested a much larger degradation in information content. Finally, our results give a useful guide for exploring survey design by giving the cosmological information returns for varying survey area, depth and the level of some systematic errors.