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A covariant Lagrangian for stable nonsingular bounce
The nonsingular bounce models usually suffer from the ghost or gradient
instabilities, as has been proved recently. In this paper, we propose a
covariant effective theory for stable nonsingular bounce, which has the
quadratic order of the second order derivative of the field but the
background set only by . With it, we explicitly construct a fully
stable nonsingular bounce model for the ekpyrotic scenario.Comment: 12 pages, 6 figures; published in JHEP; an Appendix and references
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