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

Anna Ijjas, Jean-Luc Lehners, Paul J. Steinhardt
2014 Physical Review D  
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
more » ... ible 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∼ 5.
doi:10.1103/physrevd.89.123520 fatcat:vfdboar675ed3kaqkxce3ws3a4