Designing a space-based galaxy redshift survey to probe dark energy

TL;DR

A space-based galaxy redshift survey optimized for parameters like redshift accuracy and range can significantly improve dark energy constraints, especially when including growth rate data, and is feasible with current technology.

Contribution

This paper provides a detailed analysis of optimal survey parameters for space-based galaxy redshift surveys to maximize dark energy constraints, including new fitting formulae and the importance of growth rate information.

Findings

Including growth rate information improves the figure-of-merit by ~3x.

A survey covering ~20,000 deg^2 from z=0.5 to 2 is near optimal.

Space-based surveys uniquely access low redshift ranges and enable comprehensive 3D mapping.

Abstract

A space-based galaxy redshift survey would have enormous power in constraining dark energy and testing general relativity, provided that its parameters are suitably optimized. We study viable space-based galaxy redshift surveys, exploring the dependence of the Dark Energy Task Force (DETF) figure-of-merit (FoM) on redshift accuracy, redshift range, survey area, target selection, and forecast method. Fitting formulae are provided for convenience. We also consider the dependence on the information used: the full galaxy power spectrum P(k, P(k) marginalized over its shape, or just the Baryon Acoustic Oscillations (BAO). We find that the inclusion of growth rate information (extracted using redshift space distortion and galaxy clustering amplitude measurements) leads to a factor of ~ 3 improvement in the FoM, assuming general relativity is not modified. This inclusion partially compensates for the loss of information when only the BAO are used to give geometrical constraints, rather than using the full P(k) as a standard ruler. We find that a space-based galaxy redshift survey covering ~20,000 deg^2 over 0.5< z < 2 with \sigma_z/(1+z)<= 0.001 exploits a redshift range that is only easily accessible from space, extends to sufficiently low redshifts to allow both a vast 3-D map of the universe using a single tracer population, and overlaps with ground-based surveys to enable robust modeling of systematic effects. We argue that these parameters are close to their optimal values given current instrumental and practical constraints.