Perturbative unitarity and NEC violation in genesis cosmology

TL;DR

This paper examines whether stable null energy condition violations in genesis cosmology models are compatible with fundamental principles like perturbative unitarity, suggesting potential need for new physics beyond current effective field theories.

Contribution

It analyzes perturbative unitarity constraints on NEC violation in Galileon and beyond Horndeski genesis models, highlighting possible limitations of current EFT frameworks.

Findings

Perturbative unitarity constrains NEC violation models.

Potential necessity for new physics below EFT cutoff.

Stable NEC violation may not be fully compatible with known theories.

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.