Covariance of dark energy parameters and sound speed constraints from large HI surveys

TL;DR

This paper assesses how large HI surveys with SKA can constrain dark energy's sound speed, considering parameter covariances and redshift tomography, and finds that constraints are feasible especially for low sound speeds.

Contribution

It demonstrates that redshift tomography reduces degeneracies in dark energy parameters, enabling meaningful sound speed constraints from HI surveys.

Findings

Redshift tomography lifts degeneracy between dark energy parameters.

Galaxy auto-correlation dominates the information for sound speed constraints.

Constraints are weakly affected by HI bias models.

Abstract

An interesting probe of the nature of dark energy is the measure of its sound speed, $c_s$. We review the significance for constraining sound speed models of dark energy using large neutral hydrogen (HI) surveys with the Square Kilometre Array (SKA). Our analysis considers the effect on the sound speed measurement that arises from the covariance of $c_s$ with the dark energy density, $\Omega_\LLambda$, and a time-varying equation of state, $w(a)=w_0+(1-a)w_a$. We find that the approximate degeneracy between dark energy parameters that arises in power spectrum observations is lifted through redshift tomography of the HI-galaxy angular power spectrum, resulting in sound speed constraints that are not severely degraded. The cross-correlation of the galaxy and the integrated Sachs-Wolfe (ISW) effect spectra contributes approximately 10 percent of the information that is needed to distinguish variations in the dark energy parameters, and most of the discriminating signal comes from the galaxy auto-correlation spectrum. We also find that the sound speed constraints are weakly sensitive to the HI bias model. These constraints do not improve substantially for a significantly deeper HI survey since most of the clustering sensitivity to sound speed variations arises from $z \lsim 1.5$. A detection of models with sound speeds close to zero, $c_s \lsim 0.01,$ is possible for dark energy models with $w\gsim -0.9$.