Chasing the phantom: A closer look at Type Ia supernovae and the dark
  energy equation of state
  Some recent observations provide $> 2\sigma$ 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 $w = -1$ for the Union2.1 sample, though they present
moderate ($\sim 1.9\sigma$) 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
$H_0$ prior: a higher adopted value for the direct measurement of the Hubble
constant ($H_0 \gtrsim 71~\text{km/s/Mpc}$) leads to $\gtrsim 2\sigma$ evidence
for phantom dark energy. Given Planck data, we can therefore make the following
statement at $2\sigma$ 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.
