Extending cosmological tests of General Relativity with the Square Kilometre Array

TL;DR

The paper forecasts the potential of the Square Kilometre Array (SKA) to test deviations from General Relativity on cosmological scales through upcoming HI surveys, promising significant improvements over current methods.

Contribution

It presents new forecasts for SKA's ability to detect deviations from GR using HI surveys, highlighting potential precision improvements in cosmic growth and expansion measurements.

Findings

SKA Phase 1 can measure fσ8 to sub-1% accuracy up to z≈1.

SKA1-MID Band 2 could outperform DESI and Euclid in constraining modified gravity.

A future SKA Phase 2 survey could detect 2% modifications to the Poisson equation up to z≈2.

Abstract

Tests of general relativity (GR) are still in their infancy on cosmological scales, but forthcoming experiments promise to greatly improve their precision over a wide range of distance scales and redshifts. One such experiment, the Square Kilometre Array (SKA), will carry out several wide and deep surveys of resolved and unresolved neutral hydrogen (HI) 21cm line-emitting galaxies, mapping a significant fraction of the sky from $0 \le z \lesssim 6$. I present forecasts for the ability of a suite of possible SKA HI surveys to detect deviations from GR by reconstructing the cosmic expansion and growth history. SKA Phase 1 intensity mapping surveys can achieve sub-1\% measurements of $f\sigma_8$ out to $z\approx 1$, with an SKA1-MID Band 2 survey out to $z \lesssim 0.6$ able to surpass contemporary spectroscopic galaxy surveys such as DESI and Euclid in terms of constraints on modified gravity parameters if challenges such as foreground contamination can be tackled effectively. A more futuristic Phase 2 HI survey of $\sim10^9$ spectroscopic galaxy redshifts would be capable of detecting a $\sim 2\%$ modification of the Poisson equation out to $z\approx 2$.