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

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 (H I) 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(de), 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 H I-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 per cent 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 H I bias model. These constraints do not improve substantially for a significantly deeper H I survey since most of the clustering sensitivity to sound speed variations arises from z less than or similar to 1.5. A detection of models with sound speeds close to zero, c(s) less than or similar to 0.01, is possible for dark energy models with w greater than or similar to -0.9.