Dark Energy from Virtual Gravitons (GCDM Model vs. ΛCDM Model)

Round 1

Reviewer 1 Report

Referee’s report to the paper “Graviton Dark Energy (GCDM model vs LCDM model)” by Leonid Marochnik and Daniel Usikov.

The authors start from a theory, proposed by one of them, where Einstein's equations are endowed with an extra interaction of the graviton. In some previous articles they derive field equations that can be used to explain the accelerating expansion of the universe. In this new work, using these equations, they compare the SnIa observations with the fit made using the LCDM model. They claim that their new theory fits these observations better than LCDM.

The paper is difficult to follow, I tried to see the previous papers and they are also very difficult to understand. The paper is very confusing in several parts. For example, the authors make several times reference to equation (7), but this equation does not exist. Instead of this, equation (9) is only trivial nomenclature.

Or, for example, equation (6) doesn't make sense in the pdf version I received, as well as an equation written on line 188, etc. I also found several sentences difficult to understand. The article also has relations that are false, for example, in line 249 the authors state that z_0=a_{today}-1. If z_0 is the redshift and a is the scale factor, this relationship is incorrect. Or on line 310 they write a relationship between a and H that is wrong. If these relationships are used in the calculations, we cannot trust the numerical results that the authors show in the paper. Also, sometimes authors write explicit kappa and sometimes not, even in the same equation, for example, see equations (12) and (13), and this is very confusing.

I find also some points that must be clarify.

1.- In equation (21) they neglect the contribution of radiation. At present, the error bars are small enough to detect this contribution, I believe that for the data we currently have, this contribution cannot be neglected.

2.- My main objection to this paper is that the condition that the graviton self-interaction starts at the same time as the EDE is completely arbitrary and the justification is very weak. It seems to me a pure ansatz, a proposal like so many others, it has no physical support.

There is a similar theory of the graviton as dark energy that should be cited in the paper, see Rev. Mex. Fis. 68 (2022), 020705. arXiv:2203.07876.
doi.org/10.31349/RevMexFis.68.020705.

On the other hand, the SnIa observations are not the best way to compare a new theory with LCDM, indeed, these observations still contain too big errors. The best way to compare theories now is using the CMB observations.

I cannot recommend this paper to be published in the Universe journal.

Author Response

Please find authors' response in the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

This model starts from assuming a graviton dominated  universe. The idea is actually a speculation and I don’t see how this can solve the cosmological constant problem that still remains an open issue in this framework.
Consequently the authors must explain to readers why, in view of the cosmological constant problem, one should prefer their model to LCDM. In my view the Occam razor suggests the second and so I’m not so confident about this approach.

moreover the model is not actually well compared as the authors claimed with data. The analysis is very trivial and needs a severe improvement.

last but not least presentation and style are very poor. Typos very frequent. The work needs a great restyle.

Author Response

please find authors'  reply in the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Second referee’s report to the paper “Graviton Dark Energy (GCDM model vs LCDM model)” by Leonid Marochnik and Daniel Usikov.

I thank the authors for their answers to my questions and comments, the paper is still confusing, but in fact the document has improved. However, the main objection remains, this theory is much more complicated than the LCDM and we get the same level of confidence with it, at least with the supernovae observations. It is pending what happens with the other observations. This theory remains as one of the hundred proposals to solve the problem of dark energy, but possible.

Author Response

Please check the response to reviewer's comments in the attached file.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors of this paper have to justify why the gravitons:

1) do not interact to each other, giving interference,

2) they are not detectable besides "indirect" effects, namely dark energy (universe speed up)

Moreover, the authors have to prompt comments about the possibility of unifying dark matter and dark energy in a unified model under the idea of gravitons. 

Finally the authors are invited to explain how it is possible that gravitons, exhibiting gravity by construction, can instead provide repulsive effects (negative pressure) from a physical viewpoint.

Minor points: a) the manuscript is very badly writte, a complete revision is needful. The authors might do that in bold in the text; b) formulae are not well displayed. Why don't the authors use Latex? Or if not Latex they could restyle the overall presentation.

After these changes I need to see the manuscript again.

Author Response

Please check the response to reviewer's comments in the attached file.

Author Response File: Author Response.docx