Interpreting DESI 2024 BAO: Late-time dynamical dark energy or a local effect?

Abstract

We perform fits to DESI, CMB, and supernova data to understand the physical origin of the DESI hint for dynamical dark energy. We find that the parametrization $w={w}_{0}+(1\ensuremath{-}a){w}_{a}$ can, misleadingly, hint for a phantom Universe, although quintessence models fit the data well. Model-independently, we find that deviations from $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ are driven by low-$z$ supernova data and take place only at very low redshifts $z<0.1$. Therefore, either our Universe underwent dramatic changes very recently or we do not fully understand our local Universe in a radius of about $300\text{ }\text{ }{h}^{\ensuremath{-}1}\text{ }\mathrm{Mpc}$.