Gauge-Invariant Perturbations at a Quantum Gravity Bounce

Round 1

Reviewer 1 Report

In this paper titled “Gauge-Invariant Perturbations at a Quantum Gravity Bounce” the authors explore the evolution of scalar perturbations in cosmological scenarios with modified Friedmann equations. As a paradigmatic example, they consider modifications arising from quantum gravity-like scenarios. They focus on the gauge-invariant perturbation variables ζ and R which are physically related to the power spectrum of the CMB. 

The work is well-written and organized. The focus is very sharp and well-posed. I checked some equations and they seem to be consistent. For the above reasons, the manuscript can be recommended for publication in Universe. I only have one minor suggestions that may presumably give broader perspectives to the work: modifications to Friedmann equations induced by quantum gravity-like corrections also appear, for instance, within the framework of generalized Barrow entropy, see

Phys. Rev. D 102 (2020) no.12, 123525

Eur. Phys. J. C 82 (2022) no.6, 558

It would be interesting, if possible, to look for some connection between the two frameworks and try to reformulate the obtained results in terms of Barrow language. This point could be discussed at least qualitatively.  

Author Response

We included a paragraph with the example mentioned by the reviewer at the end of section 4.2 (lines 237-244) : We comment that the modified Friedmann equation arising from Barrow entropy is an example of the F(\rho) case and the conservation laws of general relativity therefore continue to hold on super-horizon scales.

Reviewer 2 Report

In this paper the authors consider the evolution of scala perturbations in a cosmological scenarios starting from a modified Friedmann  equation. Perturbations which arise in quantum gravity are considered, in particular introducing gauge-invariant perturbations variables.

The authors show in particular that for a modified Friedmann equation, some relations between the perturbation variables are not true, in general, even if the relations still hold for suitable modifications. 

The paper is very interestin, is well written and certainly it gives an excellent contribute to the topic.

For this reaso I strongly recommend for publication on Universe

Author Response

We thank the reviewer for the report. We did not make any changes, as none were suggested.