Perturbative unitarity and NEC violation in genesis cosmology

Abstract

Explorations of the violation of null energy condition (NEC) in cosmology could enrich our understanding of the very early universe and the related gravity theories. Although a fully stable NEC violation can be realized in the "beyond Horndeski" theory, it remains an open question whether a violation of the NEC is allowed by some fundamental properties of UV-complete theories or the consistency requirements of effective field theory (EFT). We investigate the tree-level perturbative unitarity for stable NEC violations in the contexts of both Galileon and "beyond Horndeski" genesis cosmology, in which the universe is asymptotically Minkowskian in the past. We find that the constraints of perturbative unitarity imply that we may need some unknown new physics below the cut-off scale of the EFT other than that represented by the "beyond Horndeski" operators.

Constraints on inflation with null energy condition violation from advanced LIGO and advanced Virgo's first three observing runs

“…Nevertheless, scenarios that involve NEC violation have been conjectured to account for a range of early Universe phenomena, including the inflationary epoch and the genesis of primordial black holes [25][26][27][28][29][30][31][32]; see, for example, the review by [21]. Advances in modified gravity theories, especially within the framework of "beyond Horndeski" models [33][34][35][36][37][38][39], have shown that stable NEC violations are theoretically feasible, circumventing the issue of pathological instabilities.…”