Reply to "Bouncing Universes and their perturbations: remarks on a toy model"

TL;DR

This paper defends the validity of a cosmological bouncing model's impact on primordial perturbations against recent criticisms, clarifying misconceptions about the scale factor expansion and scalar field mass assumptions.

Contribution

It refutes recent objections to the bouncing cosmology model, demonstrating the physical plausibility of the scalar field mass and the scale factor assumptions.

Findings

The minimum size of the Universe at the bounce is not necessarily the Planck scale.

The scalar field mass at the bounce is smaller than the Planck mass for reasonable scale factors.

Criticisms based on polynomial scale factor assumptions are misleading or incorrect.

Abstract

In this web note, we reply to a recent paper, gr-qc/0404126, confirming a previous work of ours in which a cosmological bouncing phase was shown to have the ability of modifying the spectrum of primordial perturbations (PRD 68, 103517 2003), but challenging its physical conditions of validity. Explicitly, Ref. gr-qc/0404126, besides pretending our Taylor series expansion of the scale factor close to the bounce amounts to choosing a family of polynomial scale factors, also claims that the bounce affects the spectrum only if the mass scale of the scalar field driving the dynamics is of the order of the Planck mass. We show that these objections are either misleading or incorrect since the minimum size of the Universe a_0 (value of the scale factor at the bounce) is either not physically specified, as required in a closed Universe, or implicitly assumed to be the Planck mass. We calculate this mass and obtain that, unsurprisingly, for a reasonable value of a_0, i.e. much larger than the Planck length, the scalar field mass is smaller than the Planck mass.