This is a very good and clear article.
My only suggestion would be to get rid of the most speculative part, which I does not stand on the same footing as the rest of the text, and that will not be obvious to the common reader. One could easily delete the part starting with "While the main arena ..... up to .... astrophysical obstacles" At least the second of those paragraphs should be omitted in this sort of test, I believe.
Thanks. I have removed that paragraph. - EL
This is a good article with an excellent balance of detail and clarity.
I am afraid I don't share the other two reviewers' enthusiasm for this article. While it begins with a fine outline, the execution leaves much to be desired. In some places it is just plain sloppy (e.g., "dark energy is the name given to unknown physics" -- no, it is the name given to the mysterious energy form with repulsive gravity that is causing the Universe to speed up); almost everywhere the level is inconsistent from very technical jargon (e.g., k-essence or higher derivative theories) to very accessible language; the best example of jargon and incomprehensibility is Fig. 2: conformal horizon, BW and VM both undefined. I am confident that Fig 2 is eqully useless to expert or pedestrian. Contrast Fig 2 with Fig 1, which is at the other extreme; at least it might be useful to the pedestrian. The reference list is very author-centric and probably not useful to either experts or pedestrians. A good example of this is Kowalski et al; no where is the reader told where he/she might find the article (which I suspect does not exist yet). The coverage of topics within the review is uneven (e.g., Dark Universe does not even mention dark matter). I would suggest the author sit down and think carefully about what needs to be said about dark energy and who the reader is; then make a detailed outline with space allocations and figure ideas; and then take some time to write the article (rather than cutting and pasting from a variety of material). This review does not do justice to one of the most exciting topics in all of science, nor to the author who is an important worker in this field. I cannot recommend it.
Unhappy third reviewer
[Reply to third reviewer - Regarding your comment on the definition, the wording was carefully chosen so that it would not refer solely to a form of energy, since we do not know that acceleration arises from a physical component. I did not understand the comment about the reference list being very author-centric, as my name appears only once and that is for a community bibliographic resource. As to dark matter, it would be treated in another article; this one was specifically restricted to dark energy. Please also see comments after the fourth review. -EL]
I will lie somewhere in between previous reviewers and our unhappy (anonymous) third reviewer. I think the review is acceptable as a start, but I share many of the concerns of Unhappy above, in that it could be improved for better reader accessibility, more completeness, and better referenced.
So how would I improve...
The article would be improved if it better targetted a single audience. I would say that right now it is written for someone who is comfortable with GR, quantum field theory, and cosmology, but has somehow missed out on the details of Dark Energy. I think it needs to aim a lot lower. I would target a 1st year PhD Astronomy student who has not had GR, not had Quantum field theory, does not know what a Kaluza-Klein theory, but has had the basic suite of undergraduate classes.
Dark Energy Section: and will determine the fate of the universe. [I am always told this is not strictly true..If it persists, it will]
equation of state description will only inform those who already know what equation of state in a GR context means
Physical Origin Section:
I find this whole section aimed well above my target reader. A 1st year Astronomy graduate student will not understand what is being talked about here. I think the different theories need to be talked about from a more physical and intuitive perspective. E.g. Cosmological constant represents an energy tied to space - it does not vary either in time, or across the Universe. It is a fundamental property of the Universe, which can be expressed as a geometric constant in Einstein's GR equations, but also as a form of Energy in his equations. Maybe talk about Zeldovich and Cosmological Constant Problem here?
Then take on Scalar Field theories... Multi dimensions.. etc.
Detecting Dark Energy Section:
Might lower the level again on how acceleration is detected, but compared to above it, is more accessible. (Do we need to even say a(t)?)
Second paragraph.. I think it is fair to say that the SN Ia results were supported early on when MAXIMA and BOOMERANG saw first peak (e.g Balbi et al), indicating a flat Universe - at complete odds with the SN results unless there was dark energy, and then confirmed independently when large scale structure experiments (2dF) measured Omega_M, and could be combined with CMB measurements. e.g. Peacock et al. 2001, to see dark energy without resorting to the SN.
I find the current description of observational scenario a little funny. I would add a section of where the SN observations are going, then do the same thing for BAOs.
Then talk about a third way - the growth of structure - right now it has far more info on it then the other two methods, which are far more advanced. I think collapsing down the growth of structure to the salient points would make it more clear, and better balance the text. I believe that weak Gravitational Lensing was first detected in 1988 Fort, et al. or if you want just random cosmological detection without reference to a cluster then Mould et al 1994 well ,sort of saw something...not sure what you are referencing here in 2000.
Reference of combined constraints on Dark Energy is unknown to me, and I think you are better referencing primary sources here.
Figure 1: This is a good figure, but it is not strictly an observers view of the universe. We cannot measure the x-axis ... we measure Distance, not time. So I am afraid this too is a theorists view of the Universe - it just has data on it that you have moved into this space, e.g. points positions on x-axis assumes the cosmology?
Figure 2: This figure doesn't do much for me - I can sort of figure out what is going on, given what I know. But, your reading audience will be left in the dark.
- - -
[Thanks for your comments. As you have guessed, many of the characteristics of the article were dictated by the encyclopedia format, the space and reference restrictions, and the specific charge given to me for the article. In this format, much vocabulary is not defined within the article as the reader is assumed to come to the article for the answer to a specific question, or to be led on to other articles for explanation. For example, you are right that I would normally explain the cosmological constant in much more detail, but the format here is instead to hyperlink the word to a separate article. A similar response holds for your point on SN observations. Regarding dark energy and the fate of the universe, the properties of dark energy - including whether it fades away, and hence no longer dominates - determines the fate. I agree that Maxima and Boomerang played key roles in developing our view of the accelerating universe, but I did not have space to go into the history; here I say "by 2003". A similar point holds for weak lensing and the statistically significant detection of the cosmological (rather than individual cluster) signal. I spend a couple of more sentences on growth probes simply because they are less familiar, and less often explained how they see dark energy. You are right about Fig. 1 not being truly observed quantities - I decided against complicating the figure by having faintness (of SN) along the top axis and redshift along the right axis - now I explain this further in the caption. Fig. 2 is a compact way of showing the unity between the expansion history (the value of the curve), the distance (area under the curve), and the acceleration (slope of the curve) - now I explain this further in the caption, along with what BW and VM are. This sort of plot is analogous to similar diagrams for inflation, hinting at the similarity between early universe and current acceleration. I share your hesitation about the article used as a standalone, but that is not the purpose of this format: the reader is supposed to progress from article to article. And of course space constraints for the encyclopedia format do not allow as much detail as a full monograph. Finally, regarding Kowalski et al., this article has been submitted to ApJ and should be publicly accessible soon. -EL]