The cosmic microwave background (CMB) and baryon acoustic oscillations (BAO) are two of the most robust observations in cosmology. The recent BAO measurements from the DESI collaboration have presented, for the first time, inconsistency between BAO and CMB within the standard cosmological model $\mathrm{\Lambda }\mathrm{CDM}$, indicating a preference for dynamical dark energy over a cosmological constant. We analyze the theoretical implication of the DESI BAO observation for dark energy and gravity employing a nonparametric reconstruction approach for both the dark energy equation of state $w_{\mathrm{DE}}(a)$ and the effective field theory coefficients. We find that the DESI data can rule out quintessence dark energy by indicating a crossing of the phantom divide at $z\lesssim 1$. Furthermore, when analyzed within the broad context of Horndeski gravity which includes general relativity and many known modified gravity theories such as generalized Galileons, $f(R)$, and Brans-Dicke, our result implies that gravity should be nonminimally coupled to explain the observations, establishing the DESI result as the first hint of modified gravity. Based on these insights, we propose the thawing gravity model to explain the nonminimal coupling and phantom crossing indicated by observation, which also fits better to DESI BAO, CMB, and type Ia Supernovae data than $\mathrm{\Lambda }\mathrm{CDM}$.

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• Alternative gravity theories
• Cosmology
• Dark energy
• Evolution of the Universe