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NEC violation in mimetic cosmology revisited

Author(s): Ijjas, Anna; Ripley, Justin; Steinhardt, Paul J.

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Abstract: 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. (C) 2016 The Authors. Published by Elsevier B.V.
Publication Date: 10-Sep-2016
Electronic Publication Date: 28-Jun-2016
Citation: Ijjas, Anna, Ripley, Justin, Steinhardt, Paul J. (2016). NEC violation in mimetic cosmology revisited. PHYSICS LETTERS B, 760 (132 - 138). doi:10.1016/j.physletb.2016.06.052
DOI: doi:10.1016/j.physletb.2016.06.052
ISSN: 0370-2693
EISSN: 1873-2445
Pages: 132 - 138
Type of Material: Journal Article
Journal/Proceeding Title: PHYSICS LETTERS B
Version: Final published version. This is an open access article.

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