Talk:Dark matter

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Contents 1 Earlier reference to dark matter 2 Zwicky quote 3 Sidis 4 Contradiction 5 Intro Paragraph 6 Significant news? 7 Is dark matter pseudoscience? 8 Dark Matter is a new current event! 9 Dark matter important? 10 Dark Matter Observed - August 2006, Kavli Institute 11 Critics comment in Observation section 12 More explanation

[edit] Earlier reference to dark matter

The earliest reference to "dark matter" appears to be by Arthur Eddington in 1917: "Some of the regions are in the galactic plane, where obscuring matter is generally suspected -- and, indeed, is known to exist in certain parts. It would seem that the dark matter must be very local."; Eddington, A. S., "Researches on globular clusters" The Observatory, Vol. 40, p. 394-401 (1917) And by Henry Norris Russell in 1921: "In view of the great numbers of dwarf stars, which give out far less light in proportion to their masses, and the existence of unknown quantities of dark matter, it is probable that the radiation per unit mass for the Galaxy as a whole is much less than this"; Russell, H. N., "Radiation pressure and celestial motion" Astrophys. J., 53, 1-12 (1921)

So what does Zwicky write, which differentiates his "dark matter" from those of Eddington and Russell? I see that Eddington talks about an "obscuring" matter which is not necessarily the same as a "dark mass", but both Eddington and Russell would have been aware that dark obscuring matter must have mass. --Iantresman 08:32, 20 June 2006 (UTC)

It seems that Zwicky's contribution was not in identifying dark matter, but suggesting that in Nebulae, "the gravitational interactions of its component masses is so high as to cause it to rotate like a solid body".[1]. But earlier commentators suggest "dark matter" in relation to galactic rotation, eg.:

"Photos of the region in Sagittarius where the nucleus is supposed to exist, reveal sufficient dark matter to obscure a mass sufficient to account for the observed rotation." Harper, W. E., "Astronomical Research during 1929" (Retiring President's Annual Address), (1930) Journal of the Royal Astronomical Society of Canada, Vol. 24, p.97[2]

Isn't the development of "dark matter" something along these lines:

• Obsuring dark matter exists (Eddington/Russell)
• Dark matter affects nebular/galactic rotation (W. E. Harper, 1930)
• Dark matter affects rotation like a solid disk (Zwicky, 1933, 1937)
• Dark matter is non-Baryonic (attribution, date?)
• Differentiation of Cold Dark Matter and Hot Dark Matter (attribution, date?)

--Iantresman 09:23, 20 June 2006 (UTC)

Eddington, Russell, and Harper are talking about the dynamics of light obscuring matter which is really just gas and dust of the interstellar medium. What's more, Zwicky didn't measure the rotational velocities, but only velocity dispersions of clusters. The attribution to Zwicky for a "discovery" of dark matter is really something of an honorary title because no one would have remembered Zwicky's work if it hadn't been for Rubin and others finding other velocities that didn't match the mass-to-light ratios. There is sometimes some confusion about whether "dark matter" could just be normal matter that's "difficult" to see (for example MACHOs), but the determination of the non-baryonic nature of dark matter really happens when detailed CMB studies come out predicting that such material should be in the proportions that dark matter is supposed to be in. I hope this helps explain the sequence. --ScienceApologist 14:02, 20 June 2006 (UTC)

• So what's the difference between Baryonic dark matter and that of Eddington, Russell, and Harper?

Mostly context and the way in which it is observed. The "dark matter" of Eddington is observed to exist because of dark nebulae. Baryonic dark matter (if it exists as such) is observed to exist because of gravitational effects. --ScienceApologist 14:56, 20 June 2006 (UTC)

• What distinguishes Zwicky's dark matter from that of Eddington, Russell, and Harper? --Iantresman 14:46, 20 June 2006 (UTC)

The dark matter of Zwicky's is a gravitational mass while the dark matter of Eddington is a material that is obscuring light. --14:56, 20 June 2006 (UTC)

• Presumably, Zwicky's dark matter is not non-Baryonic matter, which I though is the most commonly thought of, type of dark matter? --Iantresman 14:46, 20 June 2006 (UTC)

Zwicky didn't inquire into the nature of the phenomena causing the observations, he just reported unusually high velocity dispersions for galaxy clusters. --14:56, 20 June 2006 (UTC)

In which case "dark matter" is all quite similar; it's method of observation is what distinguishes Zwicky from the others. Baryonic dark matter could be Eddington's... and Zwicky just proposed another way to try and detect it? --Iantresman 15:18, 20 June 2006 (UTC)

I'm not sure your characterization is exactly correct -- Zwicky didn't really make any claims on the kinds of matter doing the gravitating other than it was required by Newton's Law of Universal Gravitation, but it is true that to a certain extent baryonic matter could be the same type as that seen in dark nebula, except with improved techniques for observing these nebula in other wavebands, the matter in them is far from dark. CO emission, in fact, is rather bright in Eddington's "dark nebula". So it's not exactly baryonic "dark matter" in the usual sense of dark matter. --ScienceApologist 16:15, 20 June 2006 (UTC)

Didn't William James Sidis predict dark matter before Zwicky? In his book "The Animate and the Inanimate" and according to the wikipedia article on William James Sidis, he proposed the existance of dark matter. This book was published in 1925, and the article says Zwicky proposed its existence in 1933. Shouldn't this receive mention in this article? --Anonymous 17:09, 08 July 2006

From the Wikipedia article in question, I can only gather that The Sidis book where the "prediction" takes place is a)about cosmology, b) often (apparently erroneously) taken as a prediction of black holes, and c) according to the editor, a prediction of dark matter. I don't have the book here, so I cannot verify this, but if I understand from Sidis' biography correctly he was a theoretician and not an astronomer. Without reading the book itself, I suspect that he was probably referring to a dust solution of GR and applying it to the FRW metric which is often seen as the cosmological equivalent of a dark matter prediction, especially considering that the observable luminous matter density of the universe is lower than what is needed for closure. However, this is not a prediction of dark matter per se, it's actually a prediction of a closed universe and dark matter was the stuff that would allow for such a thing. Interestingly enough, we don't live in a closed universe so it is somewhat misleading to state that Sidis "predicted" the stuff, and even so, there were people before him who applied dust models to FRW. I'd also point out that this is probably based on the grander ideas of cosmological reversability, an idea that Sidis seems to have taken to heart, appears to be a major impetus for Sidis cosmology which does lead naturally to an insistence on universe closure, approach of the Big Crunch and an oscillating universe. While a viable solution to the Friedman equations, it just so happens that our universe is not such a place nor is cosmological reversability given much thought these days. I think the major issue is that Sidis' "prediction" was a fluke coincidence and although it does correspond in theory to dark matter's function in a matter dominated universe, it's entirely by accident that it worked out that this particular model has relevance to our universe. While there are differences, this is somewhat akin to using a atomistic alchemist's theories on how iron atoms can be "fused" to form gold as a basis for claiming that said alchemist "predicted" nuclear fusion. The superficial prediction is correct but the details of the science supporting it is only coincidental. --ScienceApologist 10:04, 9 July 2006 (UTC)

It seems to me that others did predict "dark matter" before Zwicky, whose only claim to fame was trying to quantify it. Zwicky's dark matter also bears little resemblance what dark matter is considered to be today. --Iantresman 12:55, 9 July 2006 (UTC)

Zwicky, of course, didn't "predict" dark matter -- he observed it. That's why he's famous. The measurements Zwicky made of the phenomena are actually very close to the accepted measurements today, so actually it does bear a remarkable resemblance. --ScienceApologist 15:34, 9 July 2006 (UTC)

• No, Zwicky observered the rotation of the galaxy, and infered viscosity caused by "dark matter". It seems to me he is well-known for applying "dark matter" to the galaxy rotation problem, which he thought could be due to "dust particles and gases". But he was not the first to suggest "dark matter".

This is a misinterpretation. Viscocity was what prevented the dynamics of the system from keeping the galaxy from flying apart. Zwicky is suggesting that dust and gases could serve as a glue of sorts, but his dark matter observations were much clearer with regards to the dynamics of clusters. --ScienceApologist 17:28, 9 July 2006 (UTC)

• While some of the "Earlier reference to dark matter"(above) are, as you correctly point out, in refernce to its obscuring properties, rather than mass, it is also clear that Birkeland preceeded them all when he wrote:

"It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in 'empty' space. [..]

This might be considered a prediction for the interstellar medium which is a very different thing than dark matter. However, Birkeland wasn't the first to make a prediction of this sort. There were others before him who opined that the ISM might have gaseous components. --ScienceApologist 17:28, 9 July 2006 (UTC)

"There is also another question which naturally presents itself for investigation : Will the assumed density of flying corpuscles in space bring about any appreciable resistance to the motion of the heavenly bodies?". Polar Magnetic Phenomena and Terrella Experiments, in The Norwegian Aurora Polaris Expedition 1902-1903 (publ. 1913, p.720)

Seems like a prediction of solar wind type of phenomena. Again, very different from dark matter. --ScienceApologist 17:28, 9 July 2006 (UTC)

It was pointed out to me by a colleague that Oort likely beat Zwicky by a year in proposing dark matter. He was looking at the orbits of stars at large scale heights out of the milky way disk and determined they needed much more matter than was present in the stars and gas clouds, and couldn't be dust because of the low extinction of the "extragalactic nebulae" (galaxies) in the area. The reference is Oort 1932, Bull. Astr. Inst. Neth., 6, 249 (http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1932BAN.....6..249O&data_type=PDF_HIGH&type=PRINTER&filetype=.pdf for the article in ADS). It wasn't clear reading it if he was proposing that there had to be exotic dark matter, so I'll leave it up to the regulars to decide if it's worth including.

You should tell you colleague that Oort's suggestion for dark matter in the disk has been disputed by more recent papers. Oort's analysis has not really stood the test of time and shouldn't be considered an earlier observation than Zwicky's because of this. --ScienceApologist 15:02, 9 September 2006 (UTC)

[edit] Zwicky quote

"We must know how much dark matter is incorporated in nebulae in the form of cool and cold stars, macroscopic and microscopic solid bodies, and gases. [..] we infer that the cores of nebulae possess considerable internal viscosity. The problem of deducing theoretical values for this viscosity therefore arises. It will be interesting to see whether the gravitational interactions in dense stellar systems are sufficient to account for the fact that such systems rotate like solid bodies, or if it is necessary to introduce matter in the form of dust particles and gases."[1].

The quote Ian inserted is interesting, but turns out to be not directly related to the subject of dark matter. Zwicky was making some interesting observations regarding the dynamics of the "nebula" including that there wasn't a reliable measurement for the luminous mass of the objects. Zwicky was commenting that it would be important to measure the material that was difficult to detect. He wasn't really discussing dark matter per se. This might be a good quote to include on the Fritz Zwicky page because it illustrates his care in distinguishing between what was and wasn't observed. However, it doesn't really fit with the dark matter article. --ScienceApologist 17:28, 9 July 2006 (UTC)

In which case, why don't you provide the appropriate quote, because I can't find it. Or maybe Zwicky didn't didn't propose dark matter, as it is generally thought of. --Iantresman 17:32, 9 July 2006 (UTC)

Not everything can be boiled down to "appropriate quotes". Zwicky's writing is notoriously convoluted even while in hindsight his work is remarkably predictive. I am not opposed to including a quote that is succinct and helps the reader understand what he was doing, but Zwicky sort of prided himself in being an obscure read and difficult to comprehend (not exactly in line with Wikipedia:Style guide). There was a saying the gist of which went that everything that is discovered today in astronomy was already discovered by Zwicky, but wasn't paid attention to because no one could understand it. --ScienceApologist 17:38, 9 July 2006 (UTC)

These two quote suggest that Zwicky is credited with the first EVIDENCE or DISCOVERY of dark matter:

• "The first evidence of dark matter was found by the Swiss astronomer Fritz Zwicky" (my emphasis), Galactic Dynamics by James Binney, Scott Tremaine
• "The Swiss American astronomer Fritz Zwicky was the first who discovered dark matter in galaxy clusters" (Zwicky, 1933) Proceedings of an Issi Workshop, 19-23 March 2001, Bern, Switzerland by Philippe Jetzer, Rudolf Von Steiger, K Pretzl

These quotes suggest that dark matter is IMPLIED or INFERRED:

• "We use the term dark matter to denote any form of matter whose existance is inferred solely from gravitational effects" Galactic Dynamics, op cit.
• "This is the path that led Zwicky (1933) to infer the existence of dark matter" Merging Processes in Galaxy Clusters By L Feretti, I M Gioia, G Giovannini

We can see that those people who accept dark matter will see Zwicky as the discoverer, and those astronomers who are a little more cautious, that he provided evidence that infers dark matter.

One view is subjective, the other is accurate. There is NO DOUBT that Zwicky came up with the first evidence that infers dark matter.

Consquently, other people predicted dark matter before Zwicky. --Iantresman 18:14, 9 July 2006 (UTC)

I follow and agree with your analysis all the way up to the very last sentence. In science, inference and evidence are really all we ever go on. There are "strong" inferences and there are "weak" inferences. However, writing from this "accurate" perspective is cumbersome in the English language which deals exclusively with definitional forms that are nuanced mostly by means of adjective modification. It's just not reasonable to go through all science articles and change the statements that are absolutes into statements that accurately reflect the scientific method. For example, take a random science article like Geomagnetic reversal and look at the language that is used. Very first sentence: "A geomagnetic reversal is a change in the orientation of Earth's magnetic field such that the positions of magnetic north and magnetic south become interchanged." It's actually inaccurate to say this. A more accurate statement would be: "An inferred geomagnetic reversal is an inferred change in the inferred orientation of the Earth's inferred magnetic field such that the inferred positions of inferred magnetic north and inferred magnetic south are inferred to become interchanged". Some of these inferences are strong, some of them are weak, but they are all inferences. Similarly Zwicky was making inferences which at the time to many astronomers appeared weak but have over the course of the years shown to be much stronger than one may have guessed in the 1930s. So we've come to a bit of a quandry here, in science writing it is not always best to have utter accuracy because relying on accuracy alone is too linguistically cumbersome.

Where does this leave us? Well, "dark matter" is a phrase that can be interpreted in a very broad sense ("matter that is dark"), and inasmuch as one wants to rely on this interpretation it is perfectly reasonable to state that various people made a "prediction" of dark matter in history. What we really are trying to decide is what the research origins of the phenomenological "dark matter" concept in physical cosmology are. I haven't seen any evidence that before Zwicky there were people who predicted that there was vast amounts of matter that would be so biased away from the properties of local astronomical objects (e.g. the Sun, the planets, the gas and debris of the solar system) that it would cause the observed dynamics of large objects to deviate substantially from Keplerian. If someone finds a source to such a claim, it would definitely be a prediction of "dark matter" in the strict sense of the term the article uses, but simply predicting that some of the matter in the universe isn't as bright as stellar objects as Birkeland pointed out, or proposing a matter-dominated cosmological model like Sidis did, is not really a prediction of "dark matter".

--ScienceApologist 18:57, 9 July 2006 (UTC)

• ScienceApologist wrote: I haven't seen any evidence that before Zwicky there were people who predicted that there was vast amounts of matter that would be so biased
• Reply: Birkeland wrote: "... it is not impossible that future investigations will show that without coming into conflict with experience in any way here mentioned, we may reckon that there are more than ten thousand times greater masses gathered as flying corpuscles in "empty" space than the masses in the stars and nebulae" --Iantresman 21:01, 9 July 2006 (UTC)

But this isn't light-biased mass, Ian. This is normal matter. It exhibits thermal radiation. It is detectable in ways other than dynamical measurements. Birkeland is predicting a massive component of the visible universe might be in the ISM, he's not predicting dark matter. --ScienceApologist 21:05, 9 July 2006 (UTC)

And Zwicky (quote above) was summising that it might be "necessary to introduce matter in the form of dust particles and gases." I don't see the difference. --Iantresman 21:52, 9 July 2006 (UTC)

The difference is that when Zwicky was proposing (as all good conservative scientists do) that given enough ISM material the discrepancies in dynamics could be avoided. Birkeland was discussing the components of the ISM, but made no reference to the dark matter dynamics. This all boils down to dark matter being different from the ISM. --ScienceApologist 22:28, 9 July 2006 (UTC)

• Birkeland says: "Will the assumed density of flying corpuscles in space bring about any appreciable resistance to the motion of the heavenly bodies?"
• He is indicating that the extra mass may cause an appreciable change in the motion of heavenly bodies. This is the prediction. Zwicky found it. --Iantresman 00:09, 10 July 2006 (UTC)

No. Birkeland's "resistance to the motion of heavenly bodies" is not the same thing as dynamical velocities exceeding Keplerian predictions based on mass-to-light assumptions. In fact, they're entirely different arguments. You can use this as a prediction of solar wind or similar phenomena, but it is not a prediction of dark matter. --ScienceApologist 17:51, 10 July 2006 (UTC)

And when Zwicky says, above, "..if it is necessary to introduce matter in the form of dust particles and gases", that surely isn't dark matter either? --Iantresman 20:43, 10 July 2006 (UTC)

As we currently understand dark matter, dust and gas is not dark matter, no. --ScienceApologist 20:45, 10 July 2006 (UTC)

[edit] Sidis

I've been looking through the web version of The Animate and the Inanimate at sidis.net, and I can't find anything corresponding to dark matter. He talks about dark sections of the universe where the second law of thermodynamics runs in reverse and stars absorb light (or emit darkons?). This idea, however, has no obvious connection to dark matter beyond the shared attribute "dark." If somebody can point me to a specific chapter that really does talk about dark matter, that would be much appreciated (and the chapter # should be added to the citated reference). Otherwise, I'm inclined to say that the claim should be removed. --Reuben 22:00, 5 August 2006 (UTC)

I have found several sections on the web that say that sidis was the first to predict dark matter. Anyone can look them up.

Anyway: Definition from dictionary.com, Dark matter - Physical objects or particles that emit little or no detectable radiation of their own and are postulated to exist because of unexplained gravitational forces observed on other astronomical objects. Dark matter is believed to be part of the missing mass.

He describes matter in his book that absorbs light (no not darkons!?!?!?) and other matter simple too dim to see. He predicted that 10 times more matter exists than we can see. One of the reasons apart from his prediction of the reversal of the 2nd law of thermodynamics was that the universe seemed to be gravitationally much larger than is visible through light. That seems to qualify as dark matter. Matter that is dark. Also since noone even knows what dark matter is made of today, I can't see why the claim is invalid. --Shane --t_a_y_l_o_s

• Fan web sites are not a very authoritative reference. One of the ones that was recently cited as a reference on the page goes on to claim that William Sidis "probably" coined the term libertarianism (link), which is patent nonsense (see Online Etymology Dictionary).
• No, being dark and made of matter doesn't make it dark matter. Dark matter is a technical term with a specific meeting that's not related to what Sidis was proposing. In fact, absorbing light is one of the properties that dark matter specifically must not have. The crux of Sidis's argument is the part about the second law of thermodynamics running backwards, and this has absolutely no analog in the idea of dark matter as we know it. It appears to me to be quite a stretch and a weak argument to draw such a connection.
• (Please feel free to ignore the darkons; that's just an old physics joke that came to mind.)
• Please provide a specific citation for the prediction of 10x more dark matter than visible matter. From what I can see, Sidis proposes approximately equal amounts of negative space and positive space. --Reuben 01:03, 6 August 2006 (UTC)

The Animate and the Inanimate chapter 13, last paragraph http://www.sidis.net/ANIM13.htm "If we take 18,000,000 as the approximate number of visible stars, and allow about 9 times as many that are dark or too faint to be seen..."

Yes he proposes equal ammount of positive and negetive space but that doesn't mean that every positive object(operating under the 2nd law) emits enough light that we can see it.

Where does it say that dark matter cannot absorb light? Wiki quote: "MACHOs- Massive Compact Halo Objects assumes that the dark matter consists of condensed objects such as black holes, neutron stars, white dwarfs, very faint stars, or non-luminous objects like planets. The search for these consists of using gravitational lensing to see the effect of these objects on background galaxies." It seems to me that black holes absorb light, and black holes are under the "potential explanation for part of dark matter" list. http://en.wikipedia.org/wiki/Cold_dark_matter

I have defined dark matter according to dictionary.com above, if you have another defintion could you please post it so that I can understand your arguement better. Thanks. --t_a_y_l_o_s 02:55, 6 August 2006 (UTC)

From Wikipedia, the Free Encyclopedia: "In cosmology, dark matter refers to matter particles, of unknown composition, that do not emit or reflect enough electromagnetic radiation (light) to be detected directly, but whose presence may be inferred from gravitational effects on visible matter such as stars and galaxies." [3] In all seriousness, that should help you understand the argument; Sidis is not proposing modern dark matter at all. -- SCZenz 04:09, 6 August 2006 (UTC)

• Here's another one that may be useful. From Ned Wright at UCLA: Glossary of Astronomical and Cosmological Terms. One thing to understand is that when he says dark matter doesn't emit, doesn't absorb, and doesn't scatter light, that's an approximate, shorthand way of speaking. What's really meant is "very, very little." Although MACHOs should indeed absorb some light, it will be a very small amount; you would have a very hard time detecting them by their absorption, unlike some other famous objects such as the Horsehead Nebula. Sidis's negative space regions are supposed to absorb essentially all the light that enters them, so that they would hide whatever's behind them.
• Dark matter does indeed lens the light from background objects. That's been experimentally observed. So dark matter does lens light, but it doesn't significantly absorb, emit, or scatter it.
• Thanks for the chapter 13 citation. I'll check it out. It sounds like that refers to faint but ordinary stars, so that the 90% number is separate from his thesis about the negative space regions. --Reuben 04:16, 6 August 2006 (UTC)

SCZenz- yes it doesn't sound like the modern dark matter, but so what, when Fritz Zwicky proposed dark matter he didn't predict the modern version of dark matter either but we call what he predicted dark matter anyway.

Reuben- I see what you mean and I agree that sidis's negetive absorbing stars are not modern dark matter.

I don't think that every photon going into the negetive regions of space's of sidis would be absorbed because that would mean that all lines of sight had matter directly in the way, but you are right it would absorb a lot of light. Plus his dim stars could be dark matter.

Also the only thing i can find wrong in Sidis's logic in his book is time-symmetry, he assumes time-symmetry but in fact time is only very slightly time-asymmetrical, creating an arrow of time. --t_a_y_l_o_s 04:46, 6 August 2006

Actually, what Zwicky found is exactly dark matter in the modern sense. When people say that Zwicky was the first to discover dark matter, it's because his observations pointed to the phenomenon we still know today as "dark matter," and similar observations today continue to provide strong evidence for dark matter. --Reuben 18:37, 8 August 2006 (UTC)

I see. Well.. do you think that Sidis deserves credit for being the first to predict dark matter? I think he does. --t_a_y_l_o_s 20:41, 8 August 2006

Check the discussion in the section above. Dark matter != matter that is dark. --ScienceApologist 21:06, 8 August 2006 (UTC)

From looking at The Animate and the Inananimate, I would say no. What Sidis was talking about isn't related to what we now call "dark amtter." And if you take dark matter to mean just "matter that is dark," then the proposal of dark stars by John Michell in 1783 would be an earlier prediction of dark matter (among others). So either way, I don't think it's right to say that Sidis was the first to predict dark matter. --Reuben 21:35, 8 August 2006 (UTC)

[edit] Contradiction

From the article:

• Hot dark matter - nonbaryonic particles that move ultra-relativistically
• Warm dark matter - nonbaryonic particles that move relativistically
• Cold dark matter - nonbaryonic particles that move non-relativistically

But then in the next paragraph:

If the dark matter is composed of abundant light particles which remain relativistic until shortly before recombination, then it may be termed "hot". The best candidate for hot dark matter is a neutrino....

And Hot dark matter says:

Hot dark matter is a form of dark matter, which consists of particles that travel with relativistic velocities....

So, relativistic or ultra-relativistic? Which is it? Rangek 16:56, 15 August 2006 (UTC)

This is not a contradiction, although it could be made more clear. Ultra-relativistic is "really, really relativistic" not "beyond relativistic" so ultra-relativistic particles are a sub-set the relatavistic ones. Vondo 16 August 2006 (UTC)

The definition of "ultra" is beyond. Therefore, ultra-relativistic IS "beyond relativistic". I get what you are trying to say, but "ultra-relativistic" is not how you say it.

How about "at nearly the speed of light"? Rangek 02:33, 16 August 2006 (UTC)

According to Wiktionary:ultra- it can also mean excessively, to an extreme. So ultrarelativistic can also mean extremely relativistic. Besides, the word seems to be in common use (Google)Lars T. 16:20, 16 August 2006 (UTC)

Okay. Then I guess the issue is really with Hot dark matter and Warm dark matter Rangek 17:24, 16 August 2006 (UTC)

[edit] Intro Paragraph

I added the word "theoretical" in the opening sentence, and it was cited as 'weasly' and removed. I'm assuming the editor was referring to 'weasel words' as defined by Wikipedia. This article is about an unproven theory. This is unclear to the casual reader until later in the article. Theory/theoretical are only considered weasel words when used as synonyms for guess or speculation, which is clearly not how I am using it. I've reverted the article so that others can see how it reads for discussion purposes. Please review wikipedia's 'words to avoid' and provide input here so we can come to a consensus before modifying any further. CPitt76 02:02, 21 August 2006 (UTC)

All theories in science are "unproven" since science is inductive and doesn't rely on proof but rather observations. The problem with placing theoretical as an adjective here is that dark matter is an observationally driven phenomenological theory. It isn't "theoretical" in the same sense that general relativity is and was when it was proposed. Of course, dark matter "theory" exists, but it's not quite correct to consign the entire subject to this side of the theoretical/observational divide. --ScienceApologist 02:05, 21 August 2006 (UTC)

My point was that your argument that theory is a weasel word is not valid in this article since we are talking about a theory. I'm not discounting the theory, just making it clear to casual readers not familiar with the topic that we are discussing a theory. As it reads now, the layman will think this is a universal truth, which is not the case. CPitt76 02:36, 21 August 2006 (UTC)

"Universal truth"? What kind of philosophy of science are you spouting? On a more serious note, the problem with presenting dark matter as a "theory" for the causal reader is that, more than many other theories, it is based primarily on astronomical observations. Dark matter was driven by observational astrophysics as opposed to theoretical astrophysics. See the problem. In this particular subject, there are a few people who are adamantly, for one reason or another, opposed to the existence of dark matter. What they are arguing against is not just "theory" but the implications of a number of different observations. See the distinction? --ScienceApologist 02:43, 21 August 2006 (UTC)

There's substantial room to improve the intro, and if it's done well, it should be able to satisfy both of your concerns. As ScienceApologist said, the idea of dark matter is primarily driven by observation, not theory, so "theoretical" seems out of place in the definition. However, observation doesn't demand that dark matter be made up of some yet-undiscovered fundamental particle. That's just one type of (theoretical) dark matter candidate. I suggest that the definition be reworked to focus on the ways that we (indirectly) see the dark matter, instead of ideas about what it might be. Then, various dark matter candidates such as WIMPs, axions, etc. can be correctly described as theoretical later in the text. --Reuben 02:45, 21 August 2006 (UTC)

I agree with eliminating the speculation, and highlighting the observations. It's much more objective and will allow for a more unbiased interpretation by the reader. ScienceApologist, care to take a stab? You seem more passionate and knowledgeable about the subject than I am. Hopefully it won't interfere with your ability to come up with such zingers as "What kind of philosophy of science are you spouting?"

To respond (seriously) to your last post, SA, based on your last post, it seems that you assume I lack an understanding of the role observation played in recognizing dark matter, which isn't the case. I wasn't trying to get bogged down in a debate about the theory itself. Your initial comments on the edit page took issue with "theoretical" as "weasly". I took this to mean you thought it was considered a Wikipedia weasel word. The use of theoretical here did not meet that definition. If you weren't using weasly in that sense, I misunderstood and apologize. CPitt76 03:29, 21 August 2006 (UTC)

I do consider theoretical and theory to be words that are often misunderstood by the general public and so we must take extra care to use them only where they are 100% appropriate. The scientific community can play fast-and-loose with their terminology and jargon when they're h'yucking it up around the water cooler, but when writing an article that any random fop will find interesting, one misuse of the word "theory" will lead to 10,000 bulletin board posts saying "See? According to Wikipedia dark matter is only a theory." Yikes. --ScienceApologist 06:40, 21 August 2006 (UTC)

I have made a first stab at reworking the intro paragraph. Your comments are welcome. --Reuben 15:35, 21 August 2006 (UTC)

A vast improvement over the previous write, in my opinion. A more objective and well written presentation. Thanks for taking the time. CPitt76 01:45, 22 August 2006 (UTC)

[edit] Significant news?

"Cosmic smash-up provides proof of dark matter"; 21:50 21 August 2006; NewScientist.com news service; David Shiga; http://www.newscientistspace.com/article/dn9809-cosmic-smashup-provides-proof-of-dark-matter.html Mitchconquer 09:45, 22 August 2006 (UTC)

I know next ot nothing about dark matter, but Space.com has an article about it today, a big discovery about dark matter, and it seems like it's a really big deal, possibly throwing current ideas about gravity into question. So I thought I'd just post it so others can decide what to make of it. There's a picture as well. --Age234 20:22, 21 August 2006 (UTC)

The Bullet cluster is significant in that it may be impossible for MOND et al. to explain. However, it has been known for a while now. The Chandra results announced today appear to be just an incremental improvement in the characterization of the bullet cluster. Although it is being reported as "the first direct detection of dark matter," the dark matter is still detected indirectly through its gravitational effects, using weak lensing, which has been done before. The bullet cluster actually reinforces our current understanding of gravity, possibly ruling out some (very very clever and interesting) alternative ideas. --Reuben 01:18, 22 August 2006 (UTC)

Yes, and I was expecting to see new results on the DM-DM cross-section, but as you can read here they're still doing the simulations. The old result from 2004 is sigma\lesssim 1 cm^2 (m/gram). Count Iblis 01:27, 22 August 2006 (UTC)

Could you please add a sourced statement to Bullet cluster saying "... from 2004 is sigma\lesssim 1 cm^2 (m/gram)" and why this is important? (rules out most MOND?) (with wikilinks to the dimensions if you could) WAS 4.250 01:36, 22 August 2006 (UTC)

This particular result constrains self-interacting dark matter, not MOND. However, the limit on the self interaction cross section is not as strong as constraints from other observations. Count Iblis 12:34, 22 August 2006 (UTC)

B.t.w. what happened to the "research due to be fully published in spring 2006, researchers from the University of Cambridge Institute...". ???? Count Iblis 01:32, 22 August 2006 (UTC)

talk from Dark Matter 2006, paper in the arxiv --Reuben 01:40, 22 August 2006 (UTC)

Thanks! Count Iblis 12:34, 22 August 2006 (UTC)

"In research due to be fully published in spring 2006" <-- I hope this can be removed before it is 2007. --JWSchmidt 01:06, 10 December 2006 (UTC)

[edit] Is dark matter pseudoscience?

"NASA Finds Direct Proof of Dark Matter" (Aug 21, 2006). This seems to fail at least two important criteria as defined in the Pseudoscience article:

• "Evasion of peer review prior to publicizing them (called "science by press conference")". Sure, the material will be printed in The Astrophysical Journal Letters, but this does seem like science by press release.
• "Assertion of claims that a theory predicts something that it has not been shown to predict"; at best the observations is evidence that is consistent with the theory, which is FAR from being proof?

I also wonder whether (a) the observations were predicted, (b) the conclusions are testable. --Iantresman 08:54, 22 August 2006 (UTC)

Please see WP:Point, Ian. Jefffire 09:59, 22 August 2006 (UTC)

It says that "discussion .. is prefered", not "illustrative edits". My comment falls into the former, and you side-stepped the issue. --Iantresman 10:42, 22 August 2006 (UTC)

Can you find a reliable source calling dark matter a pseudoscience? Otherwise spare us the OR. If any of the page regulars would are curious as to what Ian's motives may be on this, take a scroll through the Pseudoscience talk page. Jefffire 10:47, 22 August 2006 (UTC)

I'm pleased to hear that you require a reliable source, and assume that a press release claiming "proof" is not one. Indeed, Wikipedia reliable sources tells us "In science, avoid citing the popular press"[4], and even "... arXiv (or similar) preprints and conference abstracts should be considered to be self-published"[5]. So let's wait to read the final article in The Astrophysical Journal Letters --Iantresman 11:04, 22 August 2006 (UTC)

Nice to see you sidesteping the question and forming strawmen already. I'll take this to mean that you can't find a reliable source to support your POV. Jefffire 11:13, 22 August 2006 (UTC)

I asked a legitimate question, not to be confused with whether we describe dark matter as pseudoscience, which indeed requires a reliable source. --Iantresman 13:57, 22 August 2006 (UTC)

The press conference was called only after two papers were accepted for publication in ApJ:

http://xxx.lanl.gov/abs/astro-ph/0608407

Comments: Accepted for publication in ApJL

http://xxx.lanl.gov/abs/astro-ph/0608408

Comments: Accepted for publication in ApJ

Count Iblis 12:40, 22 August 2006 (UTC)

Thanks for that, that covers the science by press release, though I still think that "proof" is preposterous. --Iantresman 13:57, 22 August 2006 (UTC)

No-one here is calling it "proof", it's evidence. Write a letter to NASA chiding them for exaggeration, but leave the good people here to get on with their business. Jefffire 14:11, 22 August 2006 (UTC)

The paper above, "A direct empirical proof of the existence of dark matter", as accepted by ApJL, says just that. Shall we chide ApJL too? --Iantresman 14:44, 22 August 2006 (UTC)

Yes, empirical proof which always comes with some small print. This is also true for well established theories. You can't be 100% sure that tomorrow the laws of physics will be completely different than what they are today. E.g. gravity could stop to exist and the Earth would float away from the Sun. There is no way to rule this out with 100% certainty. Count Iblis 15:02, 22 August 2006 (UTC)

I think it is a very powerful statement that the observation is consistent with dark matter, and not with any proposed alternative theory—especially since the folks with the alternative theories have been trying pretty hard. But yes, we can't ever say any theory is capitol-T True in any sense; we can only say that our models describe reality. When the proof stacks up, of course, it's easier just to say "we found dark matter," and in pratice that's what we do. The same thing has happened with many physics discoveries—you could argue we have no proof that electrons exist either. -- SCZenz 15:50, 22 August 2006 (UTC)

Iantresman, the recent observation of dark matter (see the SLAC Today reference in the new addition to the observation section in the main article) takes care of any arguments that dark matter is pseudoscience, and the article has been appropriately updated to reflect this. Your question was valid regarding prior observations in that they were not submitted at the time to peer reviewed journals. This new observation will be submitted to no less than two peer-reviewed astrophysical journals, and a variety of physics phenomena were cross-referenced to obtain a more complete understanding of the data. Since I work closely with this field, I will stay on top of this article. Cheers, Astrobayes 18:43, 23 August 2006 (UTC)

Thanks for that. But to suggest "proof" is a bit strong, and does seem to falled into the pseudoscience description above. --Iantresman 20:50, 23 August 2006 (UTC)

The recent observation does not fall under the category of pseudoscience. The dark matter was observed, not inferred (please see ref [6] in the article).

Sorry for the split paragraph. This is incorrect. If you could observe it it would not be dark matter. The articles demonstrate that the dark matter halo was massively inferred. Some articles (but not all) state 7000 distinct measurements. What kind of measurements you might ask?

http://climatesci.atmos.colostate.edu/

"In addition to the Chandra observation, the Hubble Space Telescope, the European Southern Observatory's Very Large Telescope and the Magellan optical telescopes were used to determine the location of the mass in the clusters. This was done by measuring the effect of gravitational lensing, where gravity from the clusters distorts light from background galaxies as predicted by Einstein's theory of general relativity."

So the dark matter halo was not observed, it's presence and shape was derived from gravitational lensing observations. For a simple system this might well be fairly precise, but I suggest that for a galaxy cluster, there is too much information and I don't expect results would be spot on.

One final note, looks to me like the blue halo is not mass (dark matter or otherwise) but rather it outlines where the cluster's gravitational field is strong enough to cause gravitational lensing. And gas that hot (emits x-rays) does not come from simple compression, but usually from hot new stars forming, some of which go supernova and heat the surrounding gas. At this distance their remnants would not be visible.

64.172.115.2Rich

The hot plasma in clusters of galaxies has very little to do with star formation. In fact, most of the hydrogen and helium in the plasma have never been processed through a star, but are primordial gas. This gas is heated up by falling into the potential well of the cluster, thermalizing to several million degrees Kelvin, which almost totally ionizes it and causes it to emit mostly in X-rays (thus why it's observed with X-ray observatories). Any atoms heavier than helium in the gas would be from supernovae, planetary nebluae, etc, but the metalicity of the plasmas is small compared to that of most stars.

What was overlayed in blue in the bullet cluster image on the main page is a weak lensing observable called the convergence (or really one step from a direct observable - you directly measure the shear then use a mathematical transform to obtain it). If gravity has a 1/r^2 force law, then the convergence is simply the surface density of matter scaled by a cosmological scaling constant, so the blue overlay really does show where the mass is. If gravity does not have a 1/r^2 force law then the convergence is a non-local function, and you have to take a given mass distribution and calculate the resulting shear and run it through the same transform to get what the kappa distribution of it would be (see the Angus paper in the refernces for an example). For all of the gravity laws, however, the convergence is a set of second derivatives of the surface potential, and as such the blue overlay does show where the surface gravity is located (and for anyone confused by the term "surface," it means an integrated 2-dimeinsional distribution).

So, is the bullet cluster observations a direct measurement of dark matter? The answer depends on what definition you're using for direct. To a particle physicist, the answer would be no, because it didn't measure an individual particle of dark matter. To an astronomer, the answer would be yes, because it was a measurement of a direct observable, instead of a result which depends on modeling of the data to determine (for instance, the relative height of the third peak to the second peak in the cosmic microwave background power spectrum shows good evidence that dark matter must exist, but the evidence relies on models fitting to the data).65.24.145.231 01:52, 19 November 2006 (UTC)

The hypothesis of the existence of dark matter is testable, was tested, was confirmed, and is now going to be published in two astrophysical peer-reviewed journals. According to we scientists, the word proof is appropriate here since we predicted something, then saw it - so the article now reflects that fact. Thanks to the hard work of astrophysicists and engineers, the issue of the existence of dark matter has been put to rest. The article will be updated regularly as new news comes in. Cheers, Astrobayes 21:30, 23 August 2006 (UTC)

Ian seems to be a troll, in my opinion. He has taken up similar issues on other pages, including on the pseudoscience page concerning evolution! Raistlin7000 19:01, 20 September 2006 (UTC)

[edit] Dark Matter is a new current event!

http://www.newscientistspace.com/article/dn9809-cosmic-smashup-provides-proof-of-dark-matter.html

[edit] Dark matter important?

Why is dark matter such a revelation? Why do people attempt to describe it with new particles such as neutrinos, and other as yet undiscovered matter? Why don't people just assume the obviously simple answer: dark matter is matter that doesn't emit light, and is therefore almost impossible to see.

It seems so obvious to me that dark matter is spacerock that aren't suns or hot nebulae. I'm not perporting to know a great deal about this, nor do I want to change the article in any way to reflect my view. What I'm looking for is a rational explanation as to why I'm wrong - because obviously plenty of physicists think I am. Am I alone in this confusion? Fresheneesz 18:35, 22 August 2006 (UTC)

This is covered in the article. MACHO's. Jefffire 18:39, 22 August 2006 (UTC)

In brief, thermal radiation ensures that all "normal matter" will emit electromagnetic radiation. --ScienceApologist 19:28, 22 August 2006 (UTC)

There's also measurements of the amount of mass in the universe that don't correspond to the amount of everyday (Cosmologists call it Baryonic) matter we predict. I think it'll be explained under big bang model or something similar. There is baryonic dark matter which you describe and the predicted non-baryonic dark matter (as well as dark energy!). I'll try and find the best article to link you too. Jameskeates 11:51, 25 August 2006 (UTC)

Try Lambda-CDM model and links contained on that page.Jameskeates 11:53, 25 August 2006 (UTC)

As for "normal matter" that will emit EM thermal radiation, thats simply not true. Matter needs be hot to emit thermal radiation, and even hot matter might not emit enough to be "directly detected" thus making it dark matter. But I'll look into the rest, thanks for the pointers. Fresheneesz 09:00, 3 September 2006 (UTC)

If an object has 0K temperature then it will not emit thermal radiation. Otherwise it will emit thermal radiation. Indeed, there is some question as to whether matter at very cold temperatures of space can be observed, but we have observed the infrared glow from cold gas clouds down to the temperature of the CMB (~3K). So the only far-reaching possibility is that there are some very compact objects holding the baryonic mass at low tempertures. These are the so-called MACHOs and there are a number of reasons that astrophysicists believe there cannot possibly be enough of them to solve the Dark Matter problem. First of all, there would have to be an enormous number of them and they should be observed through microlensing events such as those looked for by OGLE. Secondly, compact objects need time to cool down to low temperatures since a collapsin mass naturally heats up as the particles lose potential energy. To get cold compact objects in the right amount and distributed through the universe in this way just doesn't correspond to our observations. --ScienceApologist 18:20, 5 September 2006 (UTC)

Alright, some of that does make sense. However, I'd be interested to know how far away the 3K gas clouds are from us, and how they know the temperature solely based on its radiation. Fresheneesz 21:44, 5 September 2006 (UTC)

The details of infrared astrophysics isn't exactly germane to this discussion. If you are interested in looking at the components of the interstellar medium, I'd suggest starting at that article and doing research in this subject. The majority of objects in our galaxy are warmer than 30 K due to the fact that cooling processes are so difficult to come by. The higher the temperature, the easier to see. The background at 3 K is a subject of the foreground papers of WMAP and was very well-researched by astronomers on that observing team. --ScienceApologist 23:25, 5 September 2006 (UTC)

But see, I think this article should have that sorta explanation so people don't have to research it to understand dark matter. Fresheneesz 07:38, 6 September 2006 (UTC)

Fresheneesz, you commented that you didn't know "...how they know the temperature solely based on its radiation." The answer is that for any radiation wavelength there is a corresponding peak temperature with a predictable functional form off-peak. This is called a black-body curve. As far as any object needing to be "hot" to emit radiation, the word "hot" is a misnomer because if an object has a temperature above 0 Kelvin then it will emit radiation because the molecules making up that matter will either have rotational transitions or vibrational transitions or other atomic transitions. Although this information will not apply directly to this article, I hope it helps you. Cheers, Astrobayes 15:18, 6 September 2006 (UTC)

When I said "hot", I meant it needs to be hot to be able to be *detected*, not to emit. If you're looking into an abyss, and expect to see some rocks at 10K 100 light-years away, I wouldn't be surprised if you get disappointed. Also, when you're looking at far off objects, nothing you can see is really going to be close to a black-body - of course i'm just speculating.

When you say the information doesn't apply directly to this article, do you mean that it isn't an appropriate subject, or is shouldn't be linked to? Fresheneesz 19:47, 7 September 2006 (UTC)

The fact is, we know exactly how 10K rocks radiate; they actually do look more or less like a blackbody. We can thus estimate how many there could be 100 light-years away without noticing... The answer is: not enough. Astrophysicists have thought through these questions in great detail.

As for what's relevant for the article, Wikipedia is an encyclopedia rather than a FAQ. That is, we present the state of current research, not answers to every question anyone might ask about it. As discussed below, counting stars and gas clouds is an extraordinarily complex job done by very skilled professionals, and it is not at all easy to explain in detail—although perhaps we should have a Wikipedia article about it and link there from here when we mention the estimates of "ordinary"/"visible' matter. -- SCZenz 20:59, 7 September 2006 (UTC)

[edit] Dark Matter Observed - August 2006, Kavli Institute

Yesterday, I put in a reference to the SLAC Today article describing the observation of dark matter in the Bullet Cluster. I didn't have much time to make that addition very attractive other than to put in a sentence or two, and I really appreciate the individual(s) who made the last paragraph in the Observation section more descriptive after I logged off. The section looks great now, and I just have to say that it is exciting that such a new discovery could have such implications for current astrophysical models. Now that dark matter has been observed, any arguments that dark matter is pseudoscience in the article should be removed now as they have lost their steam. Thanks again for helping with this contribution. Cheers, Astrobayes 18:39, 23 August 2006 (UTC)

I disagree. If in fact the existance of dark matter is confirmed, wikipedia should still cover allegations, yet mention that those allegation may, or are, no longer valid. Fresheneesz 09:13, 3 September 2006 (UTC)

Also note that dark matter has not been "observed", despite anyone that might say so. The event is simply more evidence. Yet, this new evidence does almost nothing to tell us what makes up this dark matter - and its not at all unreasonable to cover legitimate critics when scientists are proposing any and every possible explanation for dark matter. Fresheneesz 09:16, 3 September 2006 (UTC)

I agree here - the ref 11 just says that the presence of dark matter is confirmed by grav lensing studies, it has not been "observed" or seen in any light - that is part of the DEFINITION of dark matter. Please get rid of that "has been observed" part, especially "optically" - the data was from xrays etc. varunbhalerao 18 October 2006

Go to this link. See the material highlighted in blue? That's dark matter - as has been confirmed by the hundreds of physicists working on the project using x-ray detectors of the matter in the region. I collaborate with the institute that was responsible for this discovery, and I can tell you that this direct observation of Dark Matter is indeed a milestone. And this observation, which the article now properly reflects, outdates the objections made in that arXiv paper which supplies only computer models as refutation of the observation of Dark Matter seperate from ordinary matter. This is exciting news and I'm glad that Wikipedia is able to bring this knowledge to the public by accurately reflecting it in the article. Cheers, Astrobayes 18:12, 5 September 2006 (UTC)

Well, I guess my point is that there are criticisms of *explanations* for dark matter. Also, when i said it hasn't been "observed" i meant in the physical sense. If we observed it, it wouldn't be dark - we merely infered its existance. So yea, my point is that much of dark matter theory is still legitimately under fire - for example, theorys about MACHOs and WIMPs (and now even super WIMPS). Fresheneesz 21:54, 5 September 2006 (UTC)

There is a difference between theories about what dark matter is and whether dark matter exists. There is (basically) no controversy that dark matter exists and that it is mostly non-baryonic. There is still controversy over the detailed composition of the stuff. When you say it hasn't been "observed" what you mean is in the "particle physics" sense, not the physical sense. Astrophysical observations are still physical. --ScienceApologist 23:28, 5 September 2006 (UTC)

I'd say there's no controversy about what it is, because we have no clue. ;) It's sure as heck been observed, about as directly as it can be. If people are really concerned about semantics, we could say "detected directly" instead or some such. -- SCZenz 01:50, 19 October 2006 (UTC)

That's not actually better: in the dark matter field, "direct detection" means DAMA/NaI, CDMS, etc., i.e. dark matter particles scattering in your detector. --Reuben 02:30, 19 October 2006 (UTC)

[edit] Critics comment in Observation section

I removed the phrase that "critics point to..." because the statement was sourced from a blog, not a scientist studying dark matter. And the source paper for the comments in that blog (found here) is on the LANL arXiv pre-print database, which is not a peer-reviewed source of scientific research (see also WP:OR), but is rather a database containing original research by numerous non-scientists in addition to scientists publishing research prior to submission to peer-reviewed journals. And the paper in question uses computer models as the basis for their criticism of dark matter theories, in contrast to and published prior to the recent observational discovery of dark matter seperate from matter by physicists in association with the Kavli institute at SLAC. While there are problems with dark matter, in the interest of the highest quality article standards at Wiki, the criticisms of dark matter ideas should come from much more credible sources than blogs and non peer-reviewed pre-prints, especially in light of the observational discovery of dark matter. This is why I made the edit. Astrobayes 19:25, 28 August 2006 (UTC)

I see my comments have since been reverted. I don't want to get into an edit-and-revert debaucle over something so simple so I'm calling for some perspective by individuals who regularly watch this article. Here's the situation: in August 2006, astrophysicists observed dark matter seperately from ordinary matter. This direct observation was confirmed by collaborators at institutions all over the USA and across the world. These results have by consensus been so convincing that the results are going to make it into no less than two peer-reviewed astrophysics journals. Thus far, any claims that this direct observation of dark matter doesn't prove its existence have not and according to their authors' comments thus far will not be published in scientific peer-reviewed journals. The article on the arXiv (which is a non-peer reviewed database that anyone, scientist or not, can upload to) that is used to refute the dark matter observations was actually published before the August 2006 observation of dark matter. It is therefore historically inaccurate to refute a more recent peer-reviewed observation with a historical non-peer reviewed paper using computer models. The language in the article as it currently stands does not therefore reflect the current status of observations of dark matter. How should we note this in the article so that we avoid endless reverts? Thanks for your perspective on this. Cheers, Astrobayes 22:28, 1 September 2006 (UTC)

The paper said that they had proved the existance of dark matter, no dispute there. However, one would argue that proofs are never demonstrated, merely evidence that is consistent with the theory... which is far from a proof. Unfortunately most popular magazines and newspapers jumped on the soundbite, and reported the same. There is also no argument that others will accept the finding as proof, when what they really mean, is belief in the findings. Fortunately one paper does not make a scientific consensus, but we can report what one set of researchers has found --Iantresman 23:27, 5 September 2006 (UTC)

Thanks for your feedback. Language is a delicate art, and the art of semantics can often be a point of contention which in turn leads to troubled article improvements here at Wiki. I think that ScienceApologist made an appropriate and subtle correction to the wording in that section so that it reports rather than adopts a POV. Cheers, Astrobayes 05:47, 7 September 2006 (UTC)

[edit] More explanation

There are a couple things that would be very interesting to include in this article. One is how scientists estimate the expected mass of the universe, another is how scientists detect mass at a distance. Some of this may be implicit in the article, but since those are subjects that are so integral to the mystery of dark matter, I think it should be more explicit. Fresheneesz 09:11, 3 September 2006 (UTC)

The second question is answered in detail in the article, in the sections on rotation curves and gravitational lensing. For the first part, they basically count stars and gas and galaxies they can see; however, the details of this count are obscure to most people (including me), in large part I suspect because it is very tedious work. -- SCZenz 15:24, 6 September 2006 (UTC)

Fresheneesz, you ask some good questions and I sympathize with your point of view but the difficulty with answering explicitly the broad question (2nd) you've asked here is the same one that college physics and chemistry professors face when teaching a difficult subject. Higher subjects (say Quantum Field Theory or Cosmology) require an aptitude at their sub-disciplines (Electromagnetism, Celestial Mechanics, Optics, Relativity, etc.) that is not achievable in a short span of time, just as creating Wiki articles that answer in detail all of the possible questions that a complicated subject may inspire is likewise not possible in a short span of space on the article page. As SCZenz points out, it is true that the answer to your question lies in the years of tedious data acquisition and computer interpolation of the data, in synthesis with constantly updated models that must conform to the basic sub-disciplines of physics. We cannot possibly cover such things in this article past a mention in the appropriate section or appropriate links (which thus far the article does a fair job at, but you're welcome to put in new links if it would help). To inspire your curiosity further, here are some references which you may enjoy: NASA-GSFC Topics, The Planetary Mass Problem, The Universal Web, Stellar Mass Paper, The Initial Mass Function. I hope they help! Cheers, Astrobayes 05:44, 7 September 2006 (UTC)

"estimate the expected mass of the universe?" ... pointing to experiments measuring the cosmic microwave background would probably be what you are looking for (such as WMAP, COBE, VSA, etc). Also experiments such as Supernovae type Ia measure the Hubble constant which indirectly infer the mass density. By mass i mean non-relativistic mass. Baryon mass density can be found from the measuremnt of the light element abundance.

observing the dynamics of cluster and galaxies will give you an estimate (a bound more like it) on the total matter too. Theres been many observations and the current estimates of the matter density (based on particular models of course) is usually a best-fit analysis of combined data.

"how scientists detect mass at a distance" .... the real tricky part is measuring how far away the objects are from us. In a sense theres no real direct way to do this. It has quite an interesting history. Basically we look at specific objects which we know the mechanisms in which they radiate, and then measure the radiation that reaches us. Thats the jist of it although which objects we pick depends on what distances we are looking at: redshifts below or above one. Theres plenty of info on it that doesnt recquire much knowledge of physics except the basics.

I guess im trying to say is that the information is out there; not necessarily on wiki but its definatly out there. you just have to link to it. i dont think its necessary to have to put it in the article per se. that seems a little redundant. im sorry i dont have the references off the top of my head but you can find them easily these days like arxiv.org, prola.aps.org, etc ... or just try searching for it =) --Blckavnger 22:40, 15 November 2006 (UTC)

It seems to me that there are quite a number of misunderstandings of which I'll mention just two: 1) While some of the discussors treat dark matter as ordinary matter of which the light cannot be detected from Earth, others treat it as devoid of visible light, and still others treat it as devoid of any electromagnetic emision, absorption, deflection, diffusion, or diffraction (i.e., perfectly transparent). Of course, all of them may (and probably do) exist. It would be helpful to specify every time which dark matter is being discussed. 2) It is unfortunate that various types of dark matter are being DEFINED by the properties of the particles BELIEVED to constitute those dark matters! (It would be appropriate to ASSUME ...) Perry Mason mighht say "the assertor assumes facts not in evidence." The down side is that it precludes the consideration of a continuum constitution (as distinct from a particulate constitution) of dark matter. Fundamental skeptic 01:06, 11 October 2006 (UTC)

Dark matter is often misunderstood, in part because of the historical references discussed in the first section above where the term was originally used to mean dark clouds which absorbed light. In modern astronomy dark matter is anything which satisfies 5 criteria: 1) It does not interact with electromagnetic radiation (light) in any observed passband at a detectable level in any way (emission, absorption, deflection, etc) except through gravity. 2) It exerts a gravitational pull on its surroundings. 3) It reacts to the gravitational pull of its surroundings (i.e. overdense dark matter distributions pull additional dark matter into them). 4) It has a very low or non-existent collisional cross-section to both other dark matter particles (if it is particle in nature) and baryons (as compared to baryon-baryon cross-sections). 5) At the time of the last scattering surface of the CMB, the largest overdensities must be diffuse enough to only cause the observed fluctuations, and must be able to collapse fast enough to form the observed galaxies, clusters, and large-scale structure in later epochs. Because these are the 5 basic observations, any dark matter candidate must satisfy all of them to be considered a viable candidate. The reason why all of the discussed candidates are particles is because nobody (afaik) has come up with a way of creating a field based candidate that satisfies all 5 observations. 65.24.145.231 23:12, 21 October 2006 (UTC)

I agree, except with point 4. The cross section only has to be an order of magnitude lower than the baryon-baryon cross section, see here and here. Count Iblis 23:34, 21 October 2006 (UTC)

Here I mean the nuclear cross section not the electromagnetic. The electromagnetic cross section is actually not so relevant for the interstellar gas which is almost a collisionless plasma. Count Iblis 21:50, 24 October 2006 (UTC)

The reason it appears that "... nobody has come up with a way of creating a field based candidate that satisfies all five observations.", is because all such (fields-based) candidates are excluded from the literature. The gates to publication are held by scientists religiously devoted to the proposition that in the last analysis, anything ponderable IS CONSTITUTED OF PARTICLES, not necessarily baryonic but particles. Anything different is excluded at thosee gates. It does not allow the larger community of scientists to discuss such candidates. Such attitude seems very much like that of the Church against the Copernican system (for some centuries). On the other hands, errors that some Sophomores would not make are published (and remain unchallenged in print) when offered by "authorities." For example, about the consideration of dark matter (DM)as a possible explanation of the Pioneer Anomaly, one of the arguments against that explanation is that the presence of the needed amount of DM would disturb too much the Ephemeredes in the Solar system, ..." especially the Earth Ephemeredes, the most accurate ...". However, a good Sophomore student would realize that the argument is based on the error of inadvertently counting twice DM mass within the Earth trajectory: once in using those Ephemeredes to determine the total mass within the Earth trajectory (including DM),and then adding the DM mass a second time to make their argument. If instead they correct for the mass of the Sun (-140 ppb) to allow for the DM contribution to the total mass, then adding the DM (to recover the total mass) does not affect a bit the Earth Ephemeredes ("the most accurate!"). The other arguments against DM are also erroneous, but those errors are more subtle. My main point is that the gate keepers will allow only BIG NAMES in physics (whether right or wrong) to stray off the mainstream path, while others (again, right or wrong) are excluded. Therefore, while it would be correct to state that "a way of creating a field based candidate that satisfies all five observations has not been published," it is quite unjustified to state that "... nobody has come up with a way of creating a field based candidate that satisfies all five observations."Fundamental skeptic 21:40, 24 October 2006 (UTC)

All this doesn't matter. Wikipedia is not the place to publish original research, nor is it the place to advertise for your favorite fringe theory. If you want to be taken seriously, publish in a journal or argue at a conference. Wikipedia is not the place to spam for your favorite ideas and explanations. --ScienceApologist 22:33, 24 October 2006 (UTC)

Let me try to understand what you are writing: Are you accusing me of advertizing here for a favorite (fringe)theory just because I am pointing out the exclusion from discusion of a whole range of ideas (giving the impression that they do not exist)? Is it OK to discuss trends blessed by the establishment, but not OK to point out the proscription of other ideas? Are you saying that if the establishment proscribes a range of ideas, we are all to abide by it (because sending someone back to the locked gates achieves nothing)? How is this different from the attitude of the Church, for several centuries, towards the Copernican ideas? Fundamental skeptic 18:17, 25 October 2006 (UTC)

No, he's writing about Wikipedia policy: Wikipedia:No original research and Wikipedia:Fringe theories, and he might have added Wikipedia:Verifiability. If Wikipedia existed at the time of Copernicus, it would indeed say that the sun goes around the earth, citing religious authorities, and Copernicus might not have been notable enough to mention as a minority opinion. That may seem to be a strange choice, but it's policy because for every Copernicus there are a million snake oil promoters, and scientists should tell the difference, not us. If you want to argue that your theory is under-represented rather than arguing that it's true, we would then ask for Wikipedia:Reliable sources. Art LaPella 23:57, 25 October 2006 (UTC)

I had to go back and read carefully Wikipedia policy to realize that you are both right: pointing to a bias (suppression of a range of view points) in the official scientific literature is itself a point of view, which naturally is not voiced there and, therefore, has no place in Wikipedia. Mea culpa. Fundamental skeptic 22:24, 26 October 2006 (UTC)