Journal
PHYSICAL REVIEW D
Volume 89, Issue 12, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevD.89.123520
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Funding
- U.S. Department of Energy [DE-FG02-91ER40671]
- European Research Council [256994]
- John Templeton Foundation
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We explore a new type of entropic mechanism for generating density perturbations in a contracting phase in which there are two scalar fields but only one has a steep negative potential. This first field dominates the energy density and is the source of the ekpyrotic equation of state. The second field has a negligible potential, but its kinetic energy density is coupled to the first field with a nonlinear sigma-model type interaction. We show that for any ekpyrotic equation of state it is possible to choose the potential and the kinetic coupling such that exactly scale-invariant (or nearly scale-invariant) entropy perturbations are produced. The corresponding background solutions are stable, and the bispectrum of the entropy perturbations vanishes as no non-Gaussianity is produced during the ekpyrotic phase. Hence, the only contribution to non-Gaussianity comes from the nonlinearity of the conversion process during which entropic perturbations are turned into adiabatic ones, resulting in a local non-Gaussianity parameter, f(NL) similar to 5.
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