Uplifting, Depressing, and Tilting Dark Energy

Current data in the form of baryon acoustic oscillation, supernova, and cosmic microwave background distances prefer a cosmology that accelerates more strongly than $\mathrm{\Lambda }$CDM at $z\approx 0.5-1.5$, and more weakly at $z\lesssim 0.5$. We examine dark energy physics that can accommodate this, showing that interactions (decays, coupling to matter, nonminimal coupling to gravity) fairly generically tend not to give a satisfactory solution (in terms of fitting both distances and growth) even if they enable the effective dark energy equation of state to cross $w=-1$. To fit the cosmological data it appears the dark energy by itself must cross $w=-1$, a highly unusual physical behavior.