NEC violation in mimetic cosmology revisited

Anna Ijjas, Justin Ripley, Paul J. Steinhardt

Research output: Contribution to journalArticlepeer-review

56 Scopus citations


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.

Original languageEnglish (US)
Pages (from-to)132-138
Number of pages7
JournalPhysics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics
StatePublished - Sep 10 2016

All Science Journal Classification (ASJC) codes

  • Nuclear and High Energy Physics


  • Ghost
  • Gradient instability
  • Mimetic cosmology
  • Non-singular bounce
  • Null energy condition


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