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

Ijjas, Anna; Lehners, Jean-Luc; Steinhardt, Paul J.

doi:10.1103/PhysRevD.89.123520

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

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Ijjas, Anna
Lise Meitner Excellence Group: Gravitational Theory and Cosmology, AEI-Hannover, 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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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 $f_{NL} \sim 5$.