NEC violation in mimetic cosmology revisited

Understand

In the context of Einstein gravity, if the null energy condition (NEC) is satisfied, the energy density in expanding space-times always decreases while in contracting space-times the energy density grows and the universe eventually collapses into a singularity.

• In particular, no non-singular bounce is possible.
• It is, though, an open question if this energy condition can be violated in a controlled way, i.e., without introducing pathologies, such as unstable negative-energy states or an imaginary speed of sound.
• In this paper, we will re-examine the claim that the recently proposed mimetic scenario can violate the NEC without pathologies.