On the Spectrum of Fluctuations in an Effective Field Theory of the Ekpyrotic Universe

TL;DR

This paper analyzes the fluctuation spectrum in an effective field theory of the Ekpyrotic Universe, showing that the expected scale-invariant adiabatic fluctuations are not generated during the collapse-to-expansion transition.

Contribution

It demonstrates that the coupling of growing modes during the pre-impact phase to decaying modes post-impact prevents scale-invariant fluctuation spectra in this model.

Findings

No scale-invariant spectrum of adiabatic fluctuations is produced.

Growing modes couple only to decaying modes after the impact.

Results may not apply to improved Ekpyrotic models.

Abstract

We consider the four-dimensional effective field theory which has been used in previous studies of perturbations in the Ekpyrotic Universe, and discuss the spectrum of cosmological fluctuations induced on large scales by quantum fluctuations of the bulk brane. By matching cosmological fluctuations on a constant energy density hypersurface we show that the growing mode during the very slow collapsing pre-impact phase couples only to the decaying mode in the expanding post-impact phase, and that hence no scale-invariant spectrum of adiabatic fluctuations is generated. Note that our conclusions may not apply to improved toy models for the Ekpyrotic scenario.