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 -