The state of the dark energy equation of state circa 2023

Understand

We critically examine the state of current constraints on the dark energy (DE) equation of state (EoS) $w$.

• Our study is motivated by the observation that, while broadly consistent with the cosmological constant value $w=-1$, several independent probes appear to point towards a slightly phantom EoS ($w \sim -1.03$) which, if confirmed, could have important implications for the Hubble tension.
• We pay attention to the apparent preference for phantom DE from Planck Cosmic Microwave Background (CMB) data alone, whose origin we study in detail and attribute to a wide range of (physical and geometrical) effects.
• We deem the combination of Planck CMB, Baryon Acoustic Oscillations, Type Ia Supernovae, and Cosmic Chronometers data to be particularly trustworthy, inferring from this final consensus dataset $w=-1.013^{+0.038}_{-0.043}$, in excellent agreement with the cosmological constant value.