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Title: NEC violation in mimetic cosmology revisited

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. Finally, 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.

Authors:
 [1]; ORCiD logo [1];  [1]
  1. Princeton Univ., Princeton, NJ (United States)
Publication Date:
Research Org.:
The Trustees of Princeton Univ., Princeton, NJ (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1259571
Grant/Contract Number:
FG02-91ER40671
Resource Type:
Journal Article: Published Article
Journal Name:
Physics Letters. Section B
Additional Journal Information:
Journal Volume: 760; Journal Issue: C; Journal ID: ISSN 0370-2693
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; null energy condition; non-singular bounce; ghost; gradient instability; mimetic cosmology

Citation Formats

Ijjas, Anna, Ripley, Justin, and Steinhardt, Paul J. NEC violation in mimetic cosmology revisited. United States: N. p., 2016. Web. doi:10.1016/j.physletb.2016.06.052.
Ijjas, Anna, Ripley, Justin, & Steinhardt, Paul J. NEC violation in mimetic cosmology revisited. United States. doi:10.1016/j.physletb.2016.06.052.
Ijjas, Anna, Ripley, Justin, and Steinhardt, Paul J. 2016. "NEC violation in mimetic cosmology revisited". United States. doi:10.1016/j.physletb.2016.06.052.
@article{osti_1259571,
title = {NEC violation in mimetic cosmology revisited},
author = {Ijjas, Anna and Ripley, Justin and Steinhardt, Paul J.},
abstractNote = {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. Finally, 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.},
doi = {10.1016/j.physletb.2016.06.052},
journal = {Physics Letters. Section B},
number = C,
volume = 760,
place = {United States},
year = 2016,
month = 6
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record at 10.1016/j.physletb.2016.06.052

Citation Metrics:
Cited by: 19works
Citation information provided by
Web of Science

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  • 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 thatmore » 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. Finally, 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.« less
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