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Diffusion damping

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This peer review discussion has been closed.

I haven't worked on this article in a few months, so I've gotten a bit lost in developing it. I'd like eventually to get it to FA status. My main worries are explanatory power and completeness. What do you think?

Thanks -- Rmrfstar (talk) 14:11, 5 February 2009 (UTC)[reply]

Comments by Doncram

edit

I think it is great that you are here and trying to explain this cool stuff to the rest of us. I have a few suggestions.

  • It is very dense reading for the somewhat educated lay person that you should be aiming to inform.
I continue to work on this.
  • It strikes me that "diffusion damping" is a term which could be used to describe the reduction of diffusion in many other settings. In marketing, for example, there are diffusion models regarding the spread of consumer interest in buying a new product, and perhaps there could be "damping" effects there. Diffusion is a general process in various engineering areas. So, I question whether the article title should not be the more specific alternative name given, "Photon diffusion damping". You seem to be claiming it here for a very specific phenomenon. It may be that this is the short term used in a narrow field for this, but maybe that is local jargon, which is to be avoided, and a longer more specific term is appropriate for the main name. If you switched to a longer name, it would certainly be okay to give the short, jargon name as an alternative, as used in the field.
This is a point well taken. In fact, the article used to be entitled "Photon diffusion damping". I forget the exact reason why I moved it, but I'm pretty sure it was good. The edit summary of the redirect created by the move says that the reason is printed on page 228 of Modern Cosmology, which I read a while back.
  • The first sentence essentially describes damping as the effect of being damped, and you have to go off elsewhere to discover what being damped is. Avoid that, make the lead as readable as possible. Perhaps reorder, and use reduced rather than damped in the first sentence, and use the damped word with its link soon thereafter but not in the first sentence. Perhaps: "Photon diffusion damping, also called diffusion damping or Silk damping, is a process (effect?) which is believed to have reduced density inequalities (anisotropies) in the early universe, making the cosmic microwave background radiation (CMB) more uniform." I think it is very important for the first sentences to be simple, to stand alone, not requiring readers to go elsewhere to understand what they mean.
I've tried to address this by incorporating some of your suggestions.
  • The second sentence is also a little hard for me. It is stated, with no room for question, that "Around 300,000 years after the Big Bang, during recombination, diffusing photons travelled from hot regions of space to cold ones, equalising their temperatures." How on earth would you know that? Since when have you known that? It is not obvious or known to most people. I think it is appropriate to qualify that in some way, perhaps along the lines of (but corrected to be factually accurate, i am making this up): "Theories which have become accepted in the field of physics during the 1990's assert that during the formation of the universe, about 300,000 years after the Big Bang, diffusing photons travelled from hot regions of space to cold ones, equalising their temperatures. This was during the theorized phase of recombination."
The lede is not the place for such qualifications. Your style says the same thing but in a very roundabout fashion. Everything is a theory, and photon diffusion is well-accepted theory.
  • Also, please provide a better link for recombination than to point to a disambiguation page which covers genetic recombination and other types of recombination. You need to link to an article, or part of a more general article about the formation of the universe, specifically about this. I have not checked many other of your links, but you should check them all to ensure they don't point to disambiguation pages.
Done.
  • Also, within that 2nd sentence, does the theory really suggest that temperatures were literally equalized, that is made to be identical? I simply do not believe that is likely. I would believe there could be a process which reduced differences in temperatures (or moderated temperature differences?) across the universe but it seems obvious to me that there exist remaining temperature differences, i.e. it was not permanently equalized, and I do not believe it ever was uniformly equalized at any point in the past. Of course, you may observe that i also don't know much about this, but i think you mean something less than achieving actual equalization, and if so you should avoid implying actual equalization.
I changed "equalising" to "averaging", which, I think, makes it sound more like an ongoing process towards equalisation.
I changed this back because of Cryptic's comment below. I can't think of another way of rewording it.
  • In the second paragraph, you refer to what Silk damping is, before you define it. I suggest reordering. Your order: "Though general diffusion damping can damp perturbations in collisionless dark matter simply due to photon dispersion, the term "Silk damping" applies only to damping of adiabatic models of baryonic matter, which is coupled to the diffusing photons, not dark matter[3], and diffuses with them.[4][5] Sometimes the terms are used interchangeably.[1] Photon diffusion was first described in Joseph Silk's 1968 paper entitled "Cosmic Black-Body Radiation and Galaxy Formation"[6], which was published in The Astrophysical Journal. Silk damping was thus named after its discoverer." Perhaps better, mention Jonas Silk first, and drop a sentence: "Photon diffusion was first described in Joseph Silk's 1968 paper entitled "Cosmic Black-Body Radiation and Galaxy Formation"[6], which was published in The Astrophysical Journal. Though general diffusion damping can damp perturbations in collisionless dark matter simply due to photon dispersion, the term "Silk damping", which Silk described, applies only to damping of adiabatic models of baryonic matter, which is coupled to the diffusing photons, not dark matter[3], and diffuses with them.[4][5] Sometimes the terms are used interchangeably.[1]
Fixed, I think.
  • In the second paragraph, though, I am confused again. Is the article about photon diffusion damping, also known simply as diffusion damping or as Silk damping, after the discoverer. Or, is the article about photon diffusion damping and also about Silk damping, which is something else? I have to go back to the first sentence of the first paragraph and try to unravel it. Up front, perhaps you had cleverly avoided exactly equating Silk damping to photon diffusion damping, because you used the word "and" rather than the more straightforward word "or", in the sentence: Diffusion damping, also called photon diffusion damping, and Silk damping, ...." If you had used or it would have been more clear that Silk damping is the same thing. However, i and most readers would read this as equating the two, just as if you had used or. And further, the sentence continues, basically saying that "Photon diffusion sampling and Silk damping is an effect..." What do you mean, are they two processes that have one effect? Or are they two names for one effect? It is confusing for me to see, in the second paragraph, that you may be using Silk damping to refer to a special case of photon diffusion damping or to something else. If it is a special case, state up front that the article is about Photon diffusion damping, and a special case termed Silk damping. If it is something different, then perhaps Silk damping should be covered in a different article, or the intro sentences need to clarify the fact that this article is about two effects or processes, not one.
I've reworded this enough, I think.
  • In the section "Explanation", again I react that this is a theory, not long known, and i wish you would say that. I am not sure Explanation is the proper title to the section. Do you mean for the lead section to be a dense statement and this to be a more expanded one? I am not familiar with any journal articles in field areas that i know, which have an intro then an Explanation section. Is this used in science articles in wikipedia (not my forte)? Anyhow, I suggest looking at the tables of contents of some other science articles which are good in your view, or perhaps which are featured articles, and reconsidering whether this organization and this section title could be improved. The current organization is:
1 Explanation
2 Mechanism
3 Parameters
4 Effects
Actually it makes me wonder what the section on parameters is going to be, given that you include formulae before you get there, wouldn't parameters be some of the terms in the formulae? I don't see how a paper can be organized which plans to wait to define parameters later. Actually i think you need expanded section titles that are more meaningful to readers.
This has changed a lot.
  • In the "Mechanism" section, the first equation given should be given a bit more preamble, and perhaps some terms could be defined before the equation is stated. Anyhow, before or immediately after the equation is stated, every term should be defined. As written, the equation is given, then there is something else, then you have a "where" clause which defines terms. You have to drop the something else, in my view, and "Where" should be the next word after the equation.
Partially fixed. More prose is forthcoming.
  • Before you get to the equation, you state "This term, when factored into the Boltzmann equation for the CMB, reduces the amplitude of perturbations..." What is the CMB? It has not been defined. Oh, no, i see the acronym was defined in the first sentence of the article, where it was too dense for me to absorb. I suggest not using the very first sentences to define all acronyms, but rather use the longer phrase once up front, then use it again later and define the acronym then.
Fixed.
  • Are there any relevant wikipedia articles which could be suggested in a "See also" section? There is no such section, often useful.
I created one.
  • I note there is no "External links" section either, are there no professors or university or government websites that have anything to do with this? Some links to alternative explanations of this stuff would help some percentage of readers.
Thanks for the reminder! I added the only one I could think of.
  • About the footnotes and references, I am no expert. It basically looks good, although the abbreviations "astro-ph/9406071" and a couple others should probably be expanded, to comply with wikipedia's wp:MOS Manual of Style section on referencing (perhaps wp:ref?).
Those abbreviations refer to the designation of the article at the linked-to website.

I know this must be hard to try to explain, for lay audience, and appreciate that you are trying. Hopefully my comments and reactions may be of some use to you. If you find this helpful, i hope also that you might consider reviewing another article. Visit wp:pr or its backlog at Wikipedia:Peer review/backlog/items, where i came across your request. doncram (talk) 05:22, 12 February 2009 (UTC)[reply]

Followup: I get the impression that the requestor of the peer review finds my comments not very helpful or useable. I guess that because User:Cryptic C62 provided a separate review carefully avoiding comparison to what i had offered, and the requestor carefully credits Cryptic C62 in edit summaries in the article, and the requestor responds point by point to Cryptic C62, while not responding to my earlier review. That's fine. There's no requirement that a PR requestor to acknowledge a peer reviewer's comments in any way, although I do hope that any later FA reviewers will consider whether or not my comments have been addressed. If the requestor or anyone else could give me feedback at my Talk page or elsewhere on what might have made my comments more helpful, however, I would appreciate receiving that. Anyhow, no issue here by me, and good luck with your article. doncram (talk) 17:33, 28 February 2009 (UTC)[reply]
Not so! I just read and addressed his comments first, because I know him in real life and he was bugging me about it. I was in the middle of addressing your criticisms directly when you wrote your followup!! You're comments are very helpful. Please don't think they aren't. I'm very grateful for them. -- Rmrfstar (talk) 17:46, 28 February 2009 (UTC)[reply]
Thanks for your followups. As i indicated, i am not experienced in science article reviewing and I do appreciate the feedback, though it is not required or necessary, thanks. doncram (talk) 15:47, 1 March 2009 (UTC)[reply]

Comments by Cryptic C62

edit

Some of my comments may repeat Doncram's sentiments. I didn't read his comments so as to approach the article with fresh eyes.

Lead
  • "Diffusion damping, also called photon diffusion damping, and Silk damping" As this is written, it implies that the latter two terms can be used interchangeably with the first, but the second paragraph implies that Silk damping refers to something more specific.
Changed.
Better, but now I've noticed this: "Diffusion damping ... is a process which reduced density inequalities" Is there any adjective you can put before "process" to make it more descriptive?
"Physical process" is now used.
  • "diffusing photons travelled from hot regions of space to cold ones, equalising their temperatures" Whose temperatures? The photons, or the regions?
Fixed.
"averaging the temperatures of these regions" When using "average" as a transitive verb, it implies (at least to me) that the photons went around with calculators finding the average temperature. Equalizing worked better, I think.
I switched this back.
  • "This effect, called photon diffusion damping, is responsible ... for the formation of galaxies and galaxy clusters later on" Later on isn't particularly scientific... Are we talking in terms of thousands of years? Millions of years?
Do I really need to specify this? It's not really within the scope of the article. I replaced "later on" with "eventual" to make the sentence less informal.
Solid.
  • "...these being the dominant large scale structures observed in the universe" Why is it relevant that galaxies are the most observed large structures? Or am I misreading?
You are misreading. I've made the sentence less ambiguous, I think.
You have not. It still reads the same way. --Cryptic C62 · Talk 18:12, 2 March 2009 (UTC)[reply]
I should explain: it's not that the galaxies are the most often observed; rather they are observed to be the largest and most common. You said you were reading it like my second example, I think. This cannot be done with the current version.
  • ""Silk damping" applies only to damping of adiabatic models of baryonic matter, which is coupled to the diffusing photons, not dark matter, and diffuses with them" Wow. This is a mess. Adiabatic? Baryonic? No links, no explanation. The phrase "diffuses with them": What is diffusing? What is "them"?
I moved this to a later section.
  • "Sometimes the terms are used interchangeably." Ah.
Yes. -- Rmrfstar (talk) 14:50, 28 February 2009 (UTC)[reply]
  • "different sorts of plasma may experience different sorts of diffusion damping" Is "different sorts" really the most scientific way to describe this?
It's scientifically correct, and the kind of statement that a physicist might make.
  • "Diffusion damping occurs on a small scale, the Silk scale. This scale corresponds to the size of galaxies in the present day" I realize you'd like to incorporate this into the lead without getting overly technical, but without more information to flesh this bit out, it's essentially useless. --Cryptic C62 · Talk 17:47, 28 March 2009 (UTC)[reply]
See the present version. -- Rmrfstar (talk) 16:56, 29 March 2009 (UTC)[reply]
Introduction
  • Ick. What a terrible name for a section. How about "History" or "<insert anything besides introduction>" ?
I want a really simple introduction. In naming this section, I was inspired by "Introduction to General Relativity".
  • The subsections are confusing as all hell, too. The current layout implies that "diffusion damping" refers to two separate non-simultaneous processes, but nothing in the lede gives any indication of this.
You're right. I fixed deleted those two stupid headers.
  • Does the available literature actually refer to recombination happening in a "soup" ?
Actually yes.
  • "this soup, a plasma, was largely opaque to the electromagnetic radiation of photons: permanently excited photons were scattered by the protons and electrons very often, too often to travel very far" Is the traveling bit after the colon supposed to be a result of the opacity? Or an reason for the opacity? Being unfamiliar with the processes at play here, I really don't understand how these two phrases are connected. Also, how/why does the frequency of the scattering of excited photons dictate how far they travel? Are you saying that each individual photon gets scattered multiple times, and is thus unable to travel far? --Cryptic C62 · Talk 03:46, 6 March 2009 (UTC)[reply]
Reworded. -- Rmrfstar (talk) 14:33, 15 March 2009 (UTC)[reply]
Mechanism
  • "which in turn depends on a crucial value, the diffusion length, λD" What is a "crucial value"?
It is a value with great significance. Does this really need to be explained better?
  • "The diffusion length relates how far photons travel during diffusion" Relates them to what?
I am using the word "relates" here in the sense of "says": "... diffusion length says how far photons travel...".
  • What exactly distinguishes those terms that are defined on the list and those that are explained in the paragraph?
They are defined in the list; they are explained and related in the prose.
  • "η is the conformal time" vs. "conformal time at decoupling (η*)" Why is there sometimes an asterisk and sometimes not?
The asterisk denotes a specific value, as x0 may denote a specific value of x.
  • The lead should summarize all of the main points of the article, but I don't see anything from Mechanism appearing there.
I added a third paragraph.

--Cryptic C62 · Talk 20:29, 16 March 2009 (UTC)[reply]

Parameters
  • "  is approximately equal to  " In the second expression, it isn't entirely clear that the 1/2 is a power. Can it be placed in parentheses? Or can it simply be written as a radical?
    Done: written as a radical.
  • Why is it that there's a subsection on Ionisation fraction and baryon density, but there are also several sentences about that subsubject immediately before the relevant subsection?
    Done: merged sections.
  • "early isocurvature fluctuations, or fluctuations which do not require a constant ratio of baryons and photons" As this is written, it implies that these two fluctuations are two different processes. If that's the case, some kind of explanation or linking needs to be given for "early isocurvature fluctuations". If they are the same process and the second clause is meant to explain the first, it needs to be reworded to make this clearer.
    Done: reworded the whole paragraph.
  • "thereby masking the effects of other, model-dependent phenomena." Why is dependent italicized? --Cryptic C62 · Talk 17:16, 3 May 2009 (UTC)[reply]
  • "This means that without an accurate model of diffusion damping, scientists cannot judge the relative merits of cosmological models, whose theoretical predictions cannot be compared with observational data, this data being obscured by damping effects." This snake gets a bit awkward at the end. I'm not sure that the second comma is necessary. Consider adding "as" after the final comma and rewriting.
  • Model dependence has no wikilinks. Given the technical nature of what you're explaining, surely you can find something to link to, yes?
  • "In this case, increases in baryon density do not require a corresponding increases in photon density" Yipe. "a corresponding increases" Singular or plural mate, you can't have both!

--Cryptic C62 · Talk 15:24, 3 April 2009 (UTC)[reply]

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