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.

Figure 1

Contours show Monte Carlo estimations of the dark energy equation of state today, $w_0$, and a measure of its time variation, $w^{\prime }=w_a/2$, for various types of supernova data sets and systematic uncertainties. The contours enclose 39% of the probability so $1\sigma$ estimation of the parameters is found by projection to the respective axis. Contour points near the $w_0+w_a=w(z\gg 1)=0$ boundary line are not robust but the vast majority of the contour is valid.

Figure 2

As Fig. 1, for various types of data sets that might become feasible through future experiments. The solid black curve is the same as in Fig. 1; other curves take 10 times the statistics but include two further sources of systematics: data sample heterogeneity and extinction correction scatter.