Robustness to systematics for future dark energy probes

We extend the figure of merit formalism usually adopted to quantify the statistical performance of future dark energy probes to assess the robustness of a future mission to plausible systematic bias. We introduce a new robustness figure of merit which can be computed in the Fisher matrix formalism given arbitrary systematic biases in the observable quantities. We argue that robustness to systematics is an important new quantity that should be taken into account when optimizing future surveys. We illustrate our formalism with toy examples, and apply it to future Type Ia supernova (SN Ia) and baryonic acoustic oscillation (BAO) surveys. For the simplified systematic biases that we consider, we find that SNe Ia are a somewhat more robust probe of dark energy parameters than the BAO. We trace this back to a geometrical alignment of systematic bias direction with statistical degeneracy directions in the dark energy parameter space.