TY - JOUR

T1 - NEC violation in mimetic cosmology revisited

AU - Ijjas, Anna

AU - Ripley, Justin

AU - Steinhardt, Paul J.

N1 - Funding Information:
We thank Slava Mukhanov for suggesting that we consider NEC violation in mimetic cosmology and Lasha Berezhiani for valuable contributions during the initial stages of the project. This research was partially supported by the U.S. Department of Energy under grant number DE-FG02-91ER40671 .
Publisher Copyright:
© 2016 The Authors

PY - 2016/9/10

Y1 - 2016/9/10

N2 - 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 letter, we will re-examine the claim that the recently proposed mimetic scenario can violate the NEC without pathologies. We show that mimetic cosmology is prone to gradient instabilities even in cases when the NEC is satisfied (except for trivial examples). Most interestingly, the source of the instability is always the Einstein–Hilbert term in the action. The matter stress-energy component does not contribute spatial gradient terms but instead makes the problematic curvature modes dynamical. We also show that mimetic cosmology can be understood as a singular limit of known, well-behaved theories involving higher-derivative kinetic terms and discuss ways of removing the instability.

AB - 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 letter, we will re-examine the claim that the recently proposed mimetic scenario can violate the NEC without pathologies. We show that mimetic cosmology is prone to gradient instabilities even in cases when the NEC is satisfied (except for trivial examples). Most interestingly, the source of the instability is always the Einstein–Hilbert term in the action. The matter stress-energy component does not contribute spatial gradient terms but instead makes the problematic curvature modes dynamical. We also show that mimetic cosmology can be understood as a singular limit of known, well-behaved theories involving higher-derivative kinetic terms and discuss ways of removing the instability.

KW - Ghost

KW - Gradient instability

KW - Mimetic cosmology

KW - Non-singular bounce

KW - Null energy condition

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U2 - 10.1016/j.physletb.2016.06.052

DO - 10.1016/j.physletb.2016.06.052

M3 - Article

AN - SCOPUS:84976641787

SN - 0370-2693

VL - 760

SP - 132

EP - 138

JO - Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics

JF - Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics

ER -