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 .