Modified gravity interpretation of the evolving dark energy in light of DESI data
Anton Chudaykin, M. Kunz
Abstract
The Dark Energy Spectroscopic Instrument (DESI) collaboration has recently released measurements of baryon acoustic oscillation (BAO) from the first year of observations. A joint analysis of DESI BAO, cosmic microwave background (CMB), and Type Ia supernovae (SNe Ia) probes indicates a preference for time-evolving dark energy. We evaluate the robustness of this preference by replacing the DESI distance measurements at $z<0.8$ with the Sloan Digital Sky Survey (SDSS) BAO measurements in a similar redshift range. Assuming the ${w}_{0}{w}_{a}\mathrm{CDM}$ model, we find an evolution of the dark energy equation of state parameters consistent with $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$. Our analysis of ${\ensuremath{\chi}}^{2}$ statistics across various BAO datasets shows that DESI's preference for evolving dark energy is primarily driven by the two luminous red galaxy (LRG) samples at ${z}_{\mathrm{eff}}=0.51$ and ${z}_{\mathrm{eff}}=0.71$, with the latter having the most significant impact. Taking this preference seriously, we study a general Horndeski scalar-tensor theory, which provides a physical mechanism to safely cross the phantom divide, $w=\ensuremath{-}1$. Utilizing the effective field theory of dark energy and adopting the ${w}_{0}{w}_{a}\mathrm{CDM}$ background cosmological model, we derive constraints on the parameters ${w}_{0}=\ensuremath{-}0.856\ifmmode\pm\else\textpm\fi{}0.062$ and ${w}_{a}=\ensuremath{-}0.5{3}_{\ensuremath{-}0.26}^{+0.28}$ at 68% CL from Planck CMB, Planck and Atacama Cosmology Telescope (ACT) CMB lensing, DESI BAO, and $\mathrm{Pantheon}+\text{datasets}$, showing good consistency with the standard ${w}_{0}{w}_{a}\mathrm{CDM}$ model. The modified gravity model gives results discrepant with $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ at the $2.4\ensuremath{\sigma}$ level, while for ${w}_{0}{w}_{a}\mathrm{CDM}$ it is at $2.5\ensuremath{\sigma}$, based on the best-fit ${\ensuremath{\chi}}^{2}$ values. We conclude that modified gravity offers a viable physical explanation for DESI's preference for evolving dark energy.