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Journal Article

General mechanism for producing scale-invariant perturbations and small non-Gaussianity in ekpyrotic models

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Ijjas,  Anna
String Cosmology, AEI-Golm, MPI for Gravitational Physics, Max Planck Society;

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Lehners,  Jean-Luc
Theoretical Cosmology, AEI-Golm, MPI for Gravitational Physics, Max Planck Society;

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1404.1265.pdf
(Preprint), 227KB

PhysRevD.89_123520.pdf
(Any fulltext), 197KB

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Citation

Ijjas, A., Lehners, J.-L., & Steinhardt, P. J. (2014). General mechanism for producing scale-invariant perturbations and small non-Gaussianity in ekpyrotic models. Physical Review D, 89: 123520. doi:10.1103/PhysRevD.89.123520.


Cite as: https://hdl.handle.net/11858/00-001M-0000-0024-7722-D
Abstract
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 non-linear 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 non-linearity of the conversion process during which entropic perturbations are turned into adiabatic ones, resulting in a local non-Gaussianity parameter $f_{NL} \sim 5$.