Is dark energy dynamical? Prospects for an answer

Recent data advances offer the exciting prospect of a first look at whether dark energy has a dynamical equation of state or not. While formally theories exist with a constant equation of state, they are nongeneric—Einstein’s cosmological constant is a notable exception. So limits on the time variation, $w^{\prime }$, directly tell us crucial physics. Two recent improvements in supernova data from the Hubble space telescope allow important steps forward in constraining the dynamics of dark energy, possessing the ability to exclude models with $w^{\prime }\gtrsim 1$, if the universe truly has a cosmological constant. These data bring us much closer to the “systematics” era, where further improvements will come predominantly from more accurate, not merely more, observations. We examine the possible gains and point out the complementary roles of space and ground based observations in the near future. To achieve the leap to precision understanding of dark energy in the next generation will require specially designed space based measurements; we estimate the confidence level of detection of dynamics (e.g., distinguishing between $w^{\prime }=0$ and $w^{\prime }=1$) will be $\sim 1.8\sigma$ after the ongoing generation, improving to more than $6.5\sigma$ in the dedicated space generation.