NEC violation and "beyond Horndeski'' physics in light of DESI DR2

Abstract: Recent DESI DR2 results provide compelling evidence that the dark energy equation of state evolves from $w<-1$ at high redshift to $w>-1$ today, reinforcing interest in null energy condition (NEC) violation. While NEC violation may be crucial for dark energy and solving the cosmological singularity problem, it generically leads to ghost or gradient instabilities. It is well established in the study of nonsingular cosmology that the effective field theory (EFT) operator $\delta g^{00} R^{(3)}$ enables fully stable NEC violation and represents ``beyond Horndeski'' physics. We investigate its implications in the observable Universe and confirm within the EFT framework that $\delta g^{00} R^{(3)}$ can also stabilize NEC violation in dark energy, as indicated by the latest DESI DR2 BAO observations. Furthermore, our results show that current data already impose a nontrivial constraint on the EFT coefficient $\tilde{m}^2_4$ associated with $\delta g^{00} R^{(3)}$, indicating $\tilde{m}^2_4 \neq 0$ at approximately $2\sigma$. This finding suggests an unexpected possible connection between the primordial Universe and the late observable Universe.