Hunting down horizon-scale effects with multi-wavelength surveys

TL;DR

Next-generation multi-wavelength surveys combining SKA and Euclid-like data can significantly improve constraints on primordial non-Gaussianity and detect horizon-scale general relativistic effects, surpassing cosmic variance limits.

Contribution

This paper demonstrates that combining large volume surveys via the multi-tracer method can enhance detection of horizon-scale effects beyond previous limitations.

Findings

Forecasted error on local PNG: σ(f_NL) ≈ 1.4-0.5

GR effects detectable with S/N ≈ 14

Multi-wavelength surveys improve constraints beyond cosmic variance

Abstract

Next-generation cosmological surveys will probe ever larger volumes of the Universe, including the largest scales, near and beyond the horizon. On these scales, the galaxy power spectrum carries signatures of local primordial non-Gaussianity (PNG) and horizon-scale general relativistic (GR) effects. However, cosmic variance limits the detection of horizon-scale effects. Combining different surveys via the multi-tracer method allows us to reduce the effect down cosmic variance. This method benefits from large bias differences between two tracers of the underlying dark matter distribution, which suggests a multi-wavelength combination of large volume surveys that are planned on a similar timescale. We show that the combination of two contemporaneous surveys, a large neutral hydrogen intensity mapping survey in SKA Phase1 and a Euclid-like photometric survey, will provide unprecedented constraints on PNG as well as detection of the GR effects. We forecast that the error on local PNG will break through the cosmic variance limit on cosmic microwave background surveys and achieve $\sigma(f_{NL})\simeq1.4-0.5$, depending on assumed priors, bias, and sky coverage. GR effects are more robust to changes in the assumed fiducial model, and we forecast that they can be detected with a signal-to-noise of about $14$.