Can a galaxy redshift survey measure dark energy clustering?

Abstract

A wide-field galaxy redshift survey allows one to probe galaxy clustering at largest spatial scales, which carries invaluable information on horizon-scale physics complementarily to the cosmic microwave background (CMB). Assuming the planned survey consisting of $z\ensuremath{\sim}1$ and $z\ensuremath{\sim}3$ surveys with areas of 2000 and $300\text{ }\text{ }{\mathrm{deg}}^{2}$, respectively, we study the prospects for probing dark energy clustering from the measured galaxy power spectrum, assuming the dynamical properties of dark energy are specified in terms of the equation of state and the effective sound speed ${c}_{\mathrm{e}}$ in the context of an adiabatic cold dark dominated matter model. The dark energy clustering adds a power to the galaxy power spectrum amplitude at spatial scales greater than the sound horizon, and the enhancement is sensitive to redshift evolution of the net dark energy density, i.e. the equation of state. We find that the galaxy survey, when combined with CMB expected from the Planck satellite mission, can distinguish dark energy clustering from a smooth dark energy model such as the quintessence model (${c}_{\mathrm{e}}=1$), when ${c}_{\mathrm{e}}\ensuremath{\lesssim}0.04$ (0.02) in the case of the constant equation of state ${w}_{0}=\ensuremath{-}0.9$ ($\ensuremath{-}0.95$). An ultimate full-sky survey of $z\ensuremath{\sim}1$ galaxies allows the detection when ${c}_{\mathrm{e}}\ensuremath{\lesssim}0.08$ (0.04) for ${w}_{0}=0.9$ ($\ensuremath{-}0.95$). These forecasts show a compatible power with an all-sky CMB and galaxy cross correlation that probes the integrated Sachs-Wolfe effect. We also investigate a degeneracy between the dark energy clustering and the nonrelativistic neutrinos implied from the neutrino oscillation experiments, because the two effects both induce a scale-dependent modification in the galaxy power spectrum shape at largest spatial scales accessible from the galaxy survey. It is shown that a wider redshift coverage can efficiently separate the two effects by utilizing the different redshift dependences, where dark energy clustering is apparent only at low redshifts $z\ensuremath{\lesssim}1$.