Null Energy Condition violations in bimetric gravity

TL;DR

This paper investigates the null energy conditions in ghost-free bimetric gravity, revealing a strong anti-correlation between the foreground and background NECs, with implications for cosmological constants and energy condition violations.

Contribution

It demonstrates that in ghost-free bimetric gravity, the NECs for foreground and background sectors are strongly anti-correlated, a novel insight into energy conditions in such theories.

Findings

NECs are strongly anti-correlated between sectors

NECs can only be simultaneously fulfilled when saturated

Most cases involve NEC violation in one sector

Abstract

We consider the effective stress-energy tensors for the foreground and background sectors in ghost-free bimetric gravity. By considering the symmetries of the theory, we show that the foreground and background null energy conditions (NECs) are strongly anti-correlated. In particular, the NECs can only be simultaneously fulfilled when they saturate, corresponding to foreground and background cosmological constants. In all other situations, either the foreground or the background is subject to a NEC-violating contribution to the total stress-energy.