Chasing the phantom: A closer look at type Ia supernovae and the dark energy equation of state

Some recent observations provide > 2σ evidence for phantom dark energy—a value of the dark energy equation of state less than the cosmological-constant value of −1. We focus on constraining the equation of state by combining current data from the most mature geometrical probes of dark energy: type Ia supernovae (SNe Ia) from the Supernova Legacy Survey (SNLS3), the Supernova Cosmology Project (Union2.1), and the Pan-STARRS1 survey (PS1); cosmic microwave background measurements from Planck and WMAP9; and a combination of measurements of baryon acoustic oscillations. The combined data are consistent with w 1⁄4 −1 for the Union2.1 sample, though they present moderate (∼1.9σ) evidence for a phantom value when either the SNLS3 or PS1 sample is used instead. We study the dependence of the constraints on the redshift, stretch, color, and host galaxy stellar mass of SNe, but we find no unusual trends. In contrast, the constraints strongly depend on any external H0 prior: a higher adopted value for the direct measurement of the Hubble constant (H0 ≳ 71 km=s=Mpc) leads to ≳2σ evidence for phantom dark energy. Given Planck data, we can therefore make the following statement at 2σ confidence: either the SNLS3 and PS1 data have systematics that remain unaccounted for or the Hubble constant is below 71 km=s=Mpc; else the dark energy equation of state is indeed phantom.