Gravitational Particle Production and the Hubble Tension

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this proposal the author suggests that gravitational particle production results in an effective increase of the energy density in an FLRW universe, which results in a change in the Hubble parameter. This is offered as a possible resolution of the Hubble tension.

Let me say up front that although I am skeptical, I find the proposal intriguing and deserving of publication. However, I'd like to offer two important points of possible criticism that, I think, the author should seriously consider addressing before publication.

First, in the case Hawking radiation, we take it for granted that it is a form of evaporation: That is to say, the energy source of the radiation is the infalling matter, and thus the Hawking black hole loses mass over time. In other words, the black hole converts infalling matter into outgoing radiation, it does not create mass-energy. Why is this not the case in the situation considered by the author? Why does gravitational particle production increase \rho_{eff} as opposed to simply taking, as its energy source, \rho_{bg}, so that it's \rho_{eff} that remains governed by the Friedmann equations?

Second, assuming that \rho_{eff} changes as the author suggests, doesn't it imply that the effective stress-energy tensor of matter is no longer conserved, T^{\mu\nu}_{;\mu}\ne 0? In other words, Einstein's field equations are no longer valid, which of course means that the Friedmann equations are not valid anymore either. Does this not reveal a fundamental disconnect between treating matter semiclassically when it comes to particle production, yet classically when it comes to the background and the gravitational field equations? I sense an important contradiction here with the potential to undermine the author's argument.

Having said the above, I feel strongly that the author's proposal, even if wrong, deserves to see the light of day as opposed to just being dismissed by a lone skeptical (and possibly misguided) reviewer.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Your work is done at a high theoretical level. There are, however, some inaccuracies that are eliminated by re-working the manuscript. These inaccuracies are collected in a separate file universe-3156457-reviewer-v1.pdf sent to you. After corrections and the elimination of inaccuracies, the work can be accepted for publication.

 

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Moderate editing of English language required.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf