Comment on "Density perturbations in the ekpyrotic scenario"

TL;DR

This paper critiques the ekpyrotic scenario's claim of a scale-invariant spectrum, showing that the predicted spectrum depends on arbitrary choices, thus questioning its predictive power.

Contribution

It demonstrates that the spectrum of primordial perturbations in the ekpyrotic scenario is dependent on arbitrary matching variables, challenging its predictive validity.

Findings

Spectrum depends on arbitrary matching variable

No microphysical principle to fix the matching

Ekpyrotic scenario lacks predictive power

Abstract

In the paper ``Density perturbations in the ekpyrotic scenario'', it is argued that the expected spectrum of primordial perturbations should be scale invariant in this scenario. Here we show that, contrary to what is claimed in that paper, the expected spectrum depends on an arbitrary choice of matching variable. As no underlying (microphysical) principle exists at the present time that could lift the arbitrariness, we conclude that the ekpyrotic scenario is not yet a predictive model.