The Unsettled Number: Hubble’s Tension

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This contribution, aimed to improve our  understanding of the present situation of the so-called "Hubble tension", is largely historical rather than technique. It abounds in fine details likely unknown to many young researchers, both in physical as well in theoretical cosmology.  The physics behind each method of measuring distances is succintly but clearly explained and appropiate references are included for those who choose to go deeper in the subject.

It is obvious to me that this work should be published, though I recommend the authors a further revision of the English.

Comments on the Quality of English Language

As  I said above, it would be helpful that the  English were further revised.

Author Response

Thanks for your comments. We rephrased several small sentences, particularly in the last sections, to make the reading easier. One of us is a native speaker and has not found further idiomatic problems.

Additionally, we made several changes to answer other referees’ comments, as well as comments from other colleagues, as we recently posted the manuscript in the arXiv. See below:

We made the following changes to the manuscript. All changes are in bold letters and marked in yellow:

1. We slightly changed wording in the abstract.
2. Typos: We changed the word “universe” for “Universe” everywhere in the text, and other minor typos.
3. We have written two new sections, 32 and 33, and the corresponding explanations are in the Introduction (lines 131-134). These sections were requested by the referees and colleagues.
4. In section 27, lines 1246-1248 and 1270-1275, we added some remarks made by other colleagues, as we recently posted the manuscript in the arXiv.
5. In section 31: Lines 1422-1423, we added a new comment.

In line 1428,  we changed the phase “…neutron coalescence and therefore “standard siren” was picked...” to “…neutron coalescence. The name “standard siren” was picked…”.

In the same section, lines 1436-1437, we changed “The associated uncertainties will much be reduced in the next years when more such events are measured.” to “The associated uncertainties will be greatly reduced in the coming years as more events of this type are measured.”

6. As above mentioned, we added two new sections, 32 and 33.
7. In new section 34, we added sentences in lines: 1532-1534, 1538-1540, 1557-1558, and 1562-1563. Some of these were due to referee’s comments and requests by other colleagues.

Reviewer 2 Report

Comments and Suggestions for Authors

This is a very nice manuscript providing a historical overview of the Hubble constant up to present days when the Hubble tension is one of the major problems in cosmology. The manuscript is written very well and I have no problems accepting it. I have only three small recommendations:

1) among all the methods for measuring H0 it would be interesting to mention BH shadows: see https://arxiv.org/abs/1905.10509 , https://arxiv.org/abs/1906.10825 , https://arxiv.org/abs/2001.02986 , https://arxiv.org/abs/2205.03396 , https://arxiv.org/abs/2301.00490
2) the last sentence says "In fact, there is already a plenty of possible explanations to the Hubble tension, see for instance a review of solutions". It would be good if the authors can briefly mention some of these, e.g. phantom dark energy, early dark energy, extra neutrinos, etc.
3) it could be good to mention the possibility recently discussed in https://arxiv.org/abs/2308.16628 (https://www.mdpi.com/2218-1997/9/9/393) that probably the Hubble tension requires modifications both in the early Universe (before the CMB) and in the late Universe (after the CMB), which is currently being studied.

With these minor modifications the paper can be published.

Author Response

Thanks for your comments. Related to your three points:

1. We wrote a new section on Black Holes Shadows (section 32), where all of your cited references were incorporated. Thanks for pointing us this subject!
2. We preferred not to mention alternative models for dark energy since there are simply too many and the scope of this historical manuscript is other. However, …
3. We have cited your suggested reference (2308.16628) in line 1560.

Additionally, we made several changes to answer other referees’ comments, as well as comments from other colleagues, as we recently posted the manuscript in the arXiv. See below:

1. We slightly changed wording in the abstract.
2. Typos: We changed the word “universe” for “Universe” everywhere in the text, and other minor typos.
3. We have written two new sections, 32 and 33, and the corresponding explanations are in the Introduction (lines 131-134). These sections were requested by the referees and colleagues.
4. In section 27, lines 1246-1248 and 1270-1275, we added some remarks made by other colleagues, as we recently posted the manuscript in the arXiv.
5. In section 31:

Lines 1422-1423, we added a new comment.

In line 1428,  we changed the phase “…neutron coalescence and therefore “standard siren” was picked...” to “…neutron coalescence. The name “standard siren” was picked…”.

In the same section, lines 1436-1437, we changed “The associated uncertainties will much be reduced in the next years when more such events are measured.” to “The associated uncertainties will be greatly reduced in the coming years as more events of this type are measured.”

6. As above mentioned, we added two new sections, 32 and 33.
7. In new section 34, we added sentences in lines: 1532-1534, 1538-1540, 1557-1558, and 1562-1563. Some of these were due to referee’s comments and requests by other colleagues.
8. We rephrased several small sentences, particularly in the last sections, to make the reading easier.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors provide an interesting overview of the history of cosmology through the years and the exciting ability nowadays to measure precisely the rate of acceleration and the value of H0. The focuse on the Hubble tension and discuss various aspects of it. I think the article is publishable, I have a minor comment on the new physics perspective of the Hubble tension around the era that corresponds to the redshift of the Cepheids. For example in refs. 2208.07972, 2201.07647 such a perspective of physics change at the redshift of the Cepheids was discussed. I think a discussion on this topic can add an interesting perspective in the article. 

Author Response

Thanks for your comments. Related to your point,  we preferred not to mention alternative models for dark energy or related subjects because there are simply too many and the scope of this historical manuscript is other. However, we wrote a section (section 32) on black hole shadows to measure H0 and we saw convenient to cite your referred works, see lines 1463 and 1464.   

Additionally, we made several changes to answer other referees’ comments, as well as comments from other colleagues, as we recently posted the manuscript in the arXiv. See below:

1. We slightly changed wording in the abstract.
2. Typos: We changed the word “universe” for “Universe” everywhere in the text, and other minor typos.
3. We have written two new sections, 32 and 33, and the corresponding explanations are in the Introduction (lines 131-134). These sections were requested by the referees and colleagues.
4. In section 27, lines 1246-1248 and 1270-1275, we added some remarks made by other colleagues, as we recently posted the manuscript in the arXiv.
5. In section 31:

Lines 1422-1423, we added a new comment.

In line 1428,  we changed the phase “…neutron coalescence and therefore “standard siren” was picked...” to “…neutron coalescence. The name “standard siren” was picked…”.

In the same section, lines 1436-1437, we changed “The associated uncertainties will much be reduced in the next years when more such events are measured.” to “The associated uncertainties will be greatly reduced in the coming years as more events of this type are measured.”

6. As above mentioned, we added two new sections, 32 and 33.
7. In new section 34, we added sentences in lines: 1532-1534, 1538-1540, 1557-1558, and 1562-1563. Some of these were due to referee’s comments and requests by other colleagues.
8. We rephrased several small sentences, particularly in the last sections, to make the reading easier.