Bounce beyond Horndeski with GR asymptotics and $\gamma$-crossing

TL;DR

This paper demonstrates that healthy bouncing cosmological solutions in beyond Horndeski theories can asymptotically match General Relativity in both past and future, resolving the apparent pathology of $ extgamma$-crossing.

Contribution

The authors confirm that $ extgamma$-crossing is healthy and construct a stable bouncing universe that reduces to GR asymptotically in both directions.

Findings

Confirmed $ extgamma$-crossing as healthy in unitary gauge

Constructed a stable bouncing universe with GR asymptotics

Demonstrated compatibility of beyond Horndeski with GR in cosmological solutions

Abstract

It is known that beyond Horndeski theory admits healthy bouncing cosmological solutions. However, the constructions proposed so far do not reduce to General Relativity (GR) in either infinite past or infinite future or both. The obstacle is so called $\gamma$-crossing, which off hand appears pathological. By working in the unitary gauge, we confirm the recent observation by Ijjas [arXiv:1710.05990] that $\gamma$-crossing is, in fact, healthy. On this basis we construct a spatially flat, stable bouncing Universe solution whose asymptotic past and future are described by GR with conventional massless scalar field.