Comments on P. Jordan’s Cosmological Model

Round 1

Reviewer 1 Report

This paper provides a useful overview of the cosmology of P. Jordan, and a useful first English translation of a key paper on this topic by Jordan. It could be published subject to some major revisions. In my view, there are five such revisions should be carried out:

(i) In the section describing the basics of Jordan’s cosmology, the physics of his cosmic model needs to be made clearer. While the authors discuss Jordan’s ideas of a varying gravitational constant and the creation of matter, it is not made clear whether the resulting cosmic model is static or dynamic model. In particular, it needs to be spelled out whether the model is a steady-state model of the expanding universe in the style of Hoyle et al, or a steady-state model of a static universe in the style of Sambursky.

(ii)For the same reason, the model is therefore not placed in correct historical context as regards the history of cosmology. While the paper correctly places Jordan’s model in the historical context of the large numbers hypothesis of Eddington and Dirac, it would benefit by placing the model in the historical context of other attempts at steady-state cosmologies, by Einstein, Dirac, Hoyle and others.  

(iii) The paper would benefit greatly by a discussion of the reaction of Jordan’s peers to his cosmology, and today’s view (see for example McCrea’s review in Nature 172, 3–4 (1953)).

(iv)The authors provide an English translation of Jordan’s paper in an Appendix. For reasons of clarity, this appendix would be better as a self-contained appendix at the very end, i.e., after the authors’ own list of references.

(v)The English of the paper is rather poor and would benefit greatly by advice from a native speaker. I have supplied detailed language corrections to the abstract; similar corrections should be applied throughout the paper. In addition, I would suggest that the title of Jordan’s paper is better translated as ‘Remarks on Cosmology’ (or possibly ‘Comments’, but not ‘Thoughts’).

Comments for author File: Comments.pdf

Author Response

Following the first two suggestions, the authors revised the structure of the paper, and added a section on static and steady universes (section2).
Indeed, if the large numbers study context was explicit, that was not the case for this second aspect on stationarity.
Therefore, the authors recalled what is a static universe not to be confused with a steady one; they exposed the attempts preceding Jordan's work; and explained why Jordan's universe could be considered as a steady cosmological model.
Following the third remark, the authors wrote a section on Jordan's work and its reception (section3), quoting especially the gently suggested McCrea's review.
The appendix has been moved at the very end of the paper, as requested.
Finally, the English has been reviewed to the most possible extent given such short notice.

Reviewer 2 Report

The article is an introduction to the English translation of a not very popular work by P. Jordan. The work itself is not so well known, because it's mostly, what the authors call, numerology. I understand the main idea of the article is to tell the story of that field of physics which many big names like Eddington, Dirac and Jordan have explored at some point of their careers. I think it's interesting and educative to see the translated Jordan's article published. My problem is with the comments by the authors.

First, there are errors in the English, mostly typos. But also there are some places which I had to reread to understand. So for me they need to work on the text to improve its clarity and consistence. For example, line 14-19 and 46-52 repeat the same thing.

Second, I believe the comments are too long. They should be about 3 pages at most and they should be something like an introduction to the translation. I think they should focus mostly on the historical context of the article. Meaning - very little formulas or numbers, just telling us why this was interesting at the time and how the ideas evolved. Instead, they are partially retelling the translated article and other results.

Third, and related to the second, the scientific value of this article is very low in terms of modern cosmology. It has a historical value. But from the article, we didn't read why and when these ideas have been abandoned. Which is clearly important to put the whole thing in some context.

Considering all this, I would not recommend publishing the article in its current form. It needs serious revisions to improve its quality and to give the reader enough context of otherwise very controversial part of the history of science.

Author Response

First, the English has been elaborated, as much as we could in such short notice.
To avoid redundancy, the introduction has been rewritten as an explication of the structure of the paper. The structure has also been revised, to provide a context (sections 1 and 2) of the translated paper and not a paraphrasing of it.
Secondly, the comment on the paper itself has been reduced to put special emphasis on three important aspects of Jordan's article (section 4). However, it was impossible to reach the three pages of comment recommended as the first reviewer asked for further developments of the historical context. To meet both remarks, we have chosen to improve the historical context but to save only the comments on Jordan's translation that seemed to us the most relevant for modern cosmologists : his system of characteristic units that do not invoke any constants from microphysics but solely on cosmological constants, the variation of Newton's constant G which has been widely explored since Jordan's epoch and Jordan's spontaneous creation of black holes.
Thirdly, the authors extended the contextual approach, adding a section about static and steady universes (section 2) and, in a new 'Jordan's work' section (section 3), contextualising the translated paper in Jordan's career and explaining the cold reception his cosmological model received.
The present paper is not focused on cosmological development, but on the establishment of an historical analysis. Its principal aim is to link Jordan's model to the work of other contemporary authors, and also to highlight the three above-mentioned important aspects of Jordan's article.

Round 2

Reviewer 1 Report

Much improved MS. A few minor changes to English required, flagged in file uploaded

Comments for author File: Comments.pdf

Author Response

The authors would like to thank the reviewer for his relevant remarks.
As requested, corrections have been made:
l.6 the sentence has been rephrased.
l.17 the word static has been changed for stationary.
l.99 (previously 74) the word develop has been replaced by consider.
l.104 (previously 80), 108 (84), and 113 (89) the expression steady state has been preferred to the only word steady.

Reviewer 2 Report

The article has significantly improved its quality by including much more historical context. I consider it can be published as a historical review, but I have some suggestions for minor revisions which I leave to the authors to decide whether to implement them:

1. The English is much better but still it could use a reading by a native speaker
2. Dirac's cosmology model and the steady state model haven't been explained even though they are referred to in the text. It could help to include a sentence or two on both Dirac's cosmology model and the steady-state models.
3. alpha - denotes both the fine-structure constant in 4.2 and the Hubble constant in 4.1. I'd suggest either changing notations or making it explicit in the text that notations change between the two sub-sections.
4. I find the sentence on line 162 confusing: "So, G is not a constant anymore, it is proportional to e^2 h which decreases as h^2." . Since I don't think the text states the dependence h(e), I guess, it could be made clearer. 
5. Notation-wise, it's a bit confusing that "e" is used both as an exponent and as a Coulombian charge. Also "h" in cosmological context usually means the dimensionless Hubble parameter, while here it's the Planck constant. So maybe it's a good idea somewhere to introduce the notations used after line 144.
6. Finally, with regards to the statement about the compactness of the predicted by Jordan stars, the compactness of the black holes is considered to be 1. So I'm not sure what this super compact objects would be. Maybe that deserves a word, or not, but in any case, it would be nice to include for reference that for neutron stars the compactness is about 0.2 and for black holes it's considered to be 1 for completeness.

Author Response

The authors would like to thank the reviewer for his relevant remarks.
1. Given the particular circumstances, it has not been possible to strikingly improve the English level of the piece.
2. A development of Dirac's cosmological model have been added in section 1 and of 1948 Steady State Theory in section 5.
3. To explicit the use of the notation alpha, a footnote have been added in section 4.1.
4. To explain the possible confusion in l.186 (previously 162), a short reference to l.170 (146) have been written.
5. To avoid any confusion between e (= 2.71...) and e (= Coulombian charge), the first one have been substituted by exp.
6. In the footnote dedicated at the definition of Xsi, a comment on the compactness of black holes and neutron stars have been written to enlight the specific value of 5/3.

Round 3

Reviewer 1 Report

The paper still contains many minor errors in language. In particular, the abstract contains a number of grammatical errors that should be corrected.

Author Response

We apologize for the formulation that seem to be sometimes confusing or difficult to read. However,  with all due respect, the remarks are currently too vague to allow us performing precise and pertinent modifications. It would be more efficient if you could please point us the sentences that should be rewritten.

Reviewer 2 Report

The authors have implemented my suggestions and I think the article is already quite informative for this part of the history of cosmology. I recommend the article for publishing as it is. One thing that the authors could double-check is whether Jordan's model is static or stationary, because I see this has been modified between revisions (line 17).