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October 10, 2005 1:48 PM   Subscribe

Does dark matter exist? Dark matter has been suggested as a solution to the galaxy rotation problem where individual stars don't seem to rotate the way Newton's laws would predict. Now, some scientists are saying that observations fit with Einstein's general relativity, without any dark matter needed. I just find it amazing that no one has tried this yet.
posted by delmoi (45 comments total) 1 user marked this as a favorite

 
oh, via slashdot.
posted by delmoi at 1:51 PM on October 10, 2005


Dark matter is what the universe came packed in.
posted by Faint of Butt at 1:53 PM on October 10, 2005


Yeah, this explanation has always troubled me a bit, to be honest. Some astrophysicists estimate that up to 90% of the universe is made up of this mysterious substance, yet no one's seen it yet.

I'm hardly an expert, but it sounds to me like when the ancients claimed that Mars' and Juiper's moons sometimes revolved backwards, so that their observations did not conflict with the geocentric (earth-centered) universe which the church and others believed in. Ptolmy (sp?) and others actually worked out this highly convlouted equation which would allow for these - for each moon in conflict no less.

Sounds to me like we have to take "dark matter" on faith, as fact because we don't understand the right question to ask yet.

Just my opinion, of course. I'm not a cosmologist, but I play one on MetaFilter.
posted by TheStorm at 2:04 PM on October 10, 2005


Sure, any honest astrophysicist will tell you that "dark matter" is just a placeholder for what we dont know. Also, there is a sizeable group (MOND - Modified O? Newtonian Dynamics) who dont believe we know enough about how gravity works at large distances to make such judgements.

This paper is still fairly recent so a lot of people havent weighed in yet. Still, there is at least one other paper out already disputing their methodology.
posted by vacapinta at 2:12 PM on October 10, 2005


Yeah, well, WTF is gravity then?
posted by The Jesse Helms at 2:15 PM on October 10, 2005


It's totally heretical, but I like Reg Cahill's idea that the attraction between massive objects is a process of "space" being continually consumed by those objects.

Basically, you have some random events wayyy down there on the (only just) sub-quantum level, and these events can form statistically persistent structures in aggregate. To maintain their structure, these aggregates must be constantly replenished by unstructured, random events, and the ongoing absorption of structurally unstable space brings the structurally stable regions topologically closer.

My description probably doesn't really describe the model all that well. It's a very Fuller-ish, New Kind of Science sort of idea.
posted by sonofsamiam at 2:24 PM on October 10, 2005


> Yeah, well, WTF is gravity then?

Gravity is just a theory
posted by spazzm at 2:24 PM on October 10, 2005


Dark matter is one of the greatest unsolved mysteries in science today, and it's been a bit of an embarrassment to the scientific community. If this guy solved it, there's most definitely a nobel prize with his name on it.

Cooperstock and Tieu specifically refute the practice of some scientists making up equations in order to fit the data. What is needed is a greater understanding of what is going on. We have already seen that Newton's laws don't scale well. (Quantum physics, relativity)

Basically, there are two solutions to the dark matter problem: either 98% of the universe is made up of some sort of matter which is impossible to detect, or newton's laws need modification wrt galaxies. I usually find myself in the latter camp, but many respected scientists lie on both sides. I really hope they figure this one out, because I am enough of a nerd that it actually has kept me up nights!
posted by anomie at 2:27 PM on October 10, 2005


Gravity is just a theory

Double irony because the actual mechanisms of evolution are indeed better understood than gravity, which is really only well-developed in a predictive, quantitative sense. The mechanism by which we all gravitate is still pretty hotly debated.
posted by sonofsamiam at 2:30 PM on October 10, 2005


One of my mentors, and old physics curmudgeon that retired recently, would never speak of dark matter except in a comical tone. Same goes for string theory.

But Doc. O. would make a point of letting the young kids know that such things were just fancies. Maybe they would pan out, maybe not. But the young 'uns tend to take the ideas of great theoretical physicists without the necessary grain of salt. Until you get the experimental verification, it's still possible for them to be wrong. Sometimes spectacularly so.

It's alright. One quibble I would make with anomie is the part about this being embarrassing. I don't really see it that way. We don't understand some things. That's OK. It's a chance to figure it out.

Alas, this stuff actually does keep me up at nights, too.
posted by teece at 2:37 PM on October 10, 2005


Very interesting work. How is it being received by their peers?
posted by caddis at 2:43 PM on October 10, 2005


From the paper:
One might be inclined to question how this large departure from the Newtonian picture regarding galactic rotation curves could have arisen since the planetary motion problem is also a gravitationally bound system and the deviations there using general relativity are so small. The reason is that the two problems are very different: in the planetary problem, the source of gravity is the sun and the planets are treated as test particles in this field (apart from contributing minor perturbations when necessary). They respond to the field of the sun but they do not contribute to the field. By contrast, in the galaxy problem, the source of the field is the combined rotating mass of all of the freely-gravitating elements themselves that compose the galaxy.

I agree with you delmoi, it seems amazing that nobody looked at this before.
posted by caddis at 2:53 PM on October 10, 2005


Yeah, this explanation has always troubled me a bit, to be honest. Some astrophysicists estimate that up to 90% of the universe is made up of this mysterious substance, yet no one's seen it yet.

Maybe this would be a good question for AskMe, but... why is this troubling? It seems to me that there must be all sorts of entities in our physical theories that we can't "see" -- we posit them because they do a good job of explaning effects in the world. The distinction between physical entity and theoretical construct doesn't seem to have a very crisp line, so I'm wondering what specifically bugs physicists about dark matter. Why is dark matter more problematic than things like gravitational forces or light waves or quantum states or cell membrane pumps?

Do scientists tend to have some sort of standard of "observability" that dark matter isn't meeting? Or is it because dark mater hasn't led us to make any novel hypotheses that have later been confirmed? (Like string theory, I guess....)
posted by painquale at 2:54 PM on October 10, 2005


I thought gravity was a fold on in space.

(Hence large gravity needed for theoretical 'Warp')
posted by countzen at 3:03 PM on October 10, 2005


I will then shed a tear for the graviton.
posted by The Jesse Helms at 3:06 PM on October 10, 2005


It was a dark and stormy matter, this mysterious wormhole between threads.
posted by bashos_frog at 3:13 PM on October 10, 2005


Damn! That wormhole sucked me right into the pop music thread...
posted by InfidelZombie at 3:22 PM on October 10, 2005


Why is dark matter more problematic than things like gravitational forces or light waves or quantum states or cell membrane pumps?

It hasn't been observed. All the other things you mention (gravity, light waves, quantum states) has been observed. They can be manipulated in repeatable experiments.

Another problem with dark matter is that there's a great deal of difficulty explaining it if it actually exists.
How come it's not detectable? How come it doesn't congregate in galactic cores, but seem to be spread evenly throughout a galaxy?
If it's so common, how come there are no pure dark-matter galaxies (which would be detectable by their gravitatonal effect on other celestial objects)?

Is there an astrophysicist in the house?
posted by spazzm at 3:24 PM on October 10, 2005


How did I get here? I was just in the pop music thread, then I felt like I was being drunk, and now you're all spouting greek.
posted by The Monkey at 3:29 PM on October 10, 2005


Do scientists tend to have some sort of standard of "observability" that dark matter isn't meeting?
You wish. In the 19th and early 20th century, scientists believed in a substance called aether. When they couldn't observe it through testing, they simply assumed that it was the fault of their instruments, rather than the non-existence of the substance.
Science, as rigorous and regimented as it may be, is only as objective as its practitioners.
posted by boo_radley at 3:40 PM on October 10, 2005


I agree with you delmoi, it seems amazing that nobody looked at this before.

Ah, delmoi and caddis, if you look at the mathematics of doing large-scale physics like galaxy simulation with general relativity, you would not be in the least bit surpised that no one has looked at this.

OK, I jest, but I once was in a GR class with a brilliant astrophysicist who did his PhD on the accretion disks of black holes (computer simulation of their shape). He wrote down an equation once (I forget which one, but it was very key), and actually turned to the class and said: "I've spent a couple of summers trying to wrap my mind around the derivation of this, and I must admit it's still a little murky. So if you don't want to just take this on faith, your going to have to figure it out on your own, probably." (A reasonable paraphrase).

The mathematics of GR is very intimidating. Few things can be solved analytically. It's hard stuff.
posted by teece at 3:50 PM on October 10, 2005


If it's so common, how come there are no pure dark-matter galaxies?

From the Wikipedia article on Dark matter: "Recently, astronomers from Cardiff University claim to have discovered a galaxy made almost entirely of dark matter, 50 million light years away in the Virgo Cluster, which was named VIRGOHI21. Unusually, VIRGOHI21 does not appear to contain any visible stars: it was seen with radio frequency observations of hydrogen."

It's also worth noting that this paper, if correct, provides an explanation for galactic rotation speeds without invoking dark matter, but galactic rotation speeds are only one of two lines of evidence for dark matter, the other being the large-scale structure of the universe. Again, see the Wikipedia article, or for a good explanation at a not-too-technical level, I recommend Alpha and Omega: The Search for the Beginning and End of the Universe.
posted by DevilsAdvocate at 4:12 PM on October 10, 2005


It's not so much that we can't observe it. The problem is that there is quite a bit of strong evidence that there is "something" there, along with a lack of corresponding evidence that "something" consists of MACHOS such as planets, dead stars, or just floating gas.

For a while, there was some money put on the WIMPS, and in fact, the quest for "dark matter" did turn up at least one nice result with the discovery of nutrino mass.

But I'm leaning a bit more towards the possibility that our understanding of general relativity on large scales needs to be adjusted.
posted by KirkJobSluder at 4:25 PM on October 10, 2005


caddis writes "How is it being received by their peers?"

Well, it's not published yet, but there's already a refutation out there....
posted by mr_roboto at 4:38 PM on October 10, 2005


Hello!
People!
I'm here!
I exist!
Duh!
posted by darkmatter at 4:43 PM on October 10, 2005


I agree with you delmoi, it seems amazing that nobody looked at this before.

Of course people have looked at this before. The problem is that nobody can figure it out exactly. Neither can any computer figure out all the interactions either within galaxies or in galactic clusters. (I shared a desk with a guy working for Frank Shu (spiral density waves) who told me all about the difficulties in just modeling how a galaxy spins) So, you have to rely on approximations - either Newtonian or estimating contributions to the metric in General Relativity.

What these guys are saying is that the assumptions others have been making are wrong by improperly calculating the outlying contributions (those that deviate from the model of a Newtonian point-source) They may be right. They may be wrong. I'll wait for a more experienced GR person to weigh in.

Dark Matter is a "wait and see" There may be something more fundamental involved that we dont understand yet or has yet to be discovered. A lot of smart people have tackled this problem - the theories - WIMPS, MACHOS etc. all have their drawbacks...

(I'm no astrophysicist. I just got an undergraduate degree in it and did some independent work in both Galactic Dynamics and Radioastronomy)
posted by vacapinta at 4:51 PM on October 10, 2005


Galactic rotation curves are not the only evidence for dark matter. The WMAP experiment puts dark matter as constituting about 26% of the energy content of the Universe (baryonic matter is 4%, and 70% is dark energy). Of course, this is also just gravitational evidence of dark matter, but solving the galactic rotation problem isn't the whole story.

Likewise, once the LHC comes online, we might get a look at the lightest supersymmetric particles (if they exist). There are some supersymmetric dark matter models, so it might be best to wait for experiment (vs. observation) before trying to say exactly what's going on.

In any case, there's still dark energy, which is far more elusive than dark matter.

Also, physics is made up anyway, so it doesn't really matter.
posted by dsword at 4:58 PM on October 10, 2005


This paper seems pretty dubious. There is a technical problem with it according to this paper, but aside from that, how can general relativity possibly lead to a significant effect? In the case of galaxy rotation we are dealing with very weak fields, and Newtonian dynamics is an excellent approximation (assuming something ad hoc like MOND is not operating). The general relativistic galaxy treatment must be at most a small perturbation to this Newtonian picture (assuming standard general relativity, as laid out by Einstein, is correct), so really their paper is wrongheaded and must contain some mathematical goof.

Dark matter is not going to go away. There is too much strong and independent evidence for it now.
posted by snoktruix at 4:59 PM on October 10, 2005


I'm fascinated by this and will definitely be following developments. Back when I was studying astrophysics I always used to say that dark matter sounded a bit like God to me: i.e. a blatant fudge factor. It always seemed more likely to me that general relativity wasn't quite general enough yet.
posted by Decani at 5:19 PM on October 10, 2005


In the case of galaxy rotation we are dealing with very weak fields, and Newtonian dynamics is an excellent approximation (assuming something ad hoc like MOND is not operating).

Newtonian mechanics is only an excellent approximations in certain realms. It sucks in others. The math of GR is tough enough that it is by no means trivial to decide if very large scale structures can be modeled with simple Newtonian gravity or not.

We actually have a very hard time simulating things with GR -- so we don't know for sure whether Newtonian works at the scale of galaxies.
posted by teece at 5:45 PM on October 10, 2005


If you're interested in experimental particle physics, some of which deals with the questions around what particles make up dark matter, you might subscribe to Fermilab/SLAC's excellent and free Symmetry print magazine.
posted by Rothko at 6:00 PM on October 10, 2005


GR reduces to Newtonian gravity in the weak field limit, and the gravitational fields generated by a galaxy's self-gravity are quite small and well inside this weak field regime. A single galaxy is not so large scale to a cosmologist, the physics is quite well understood. The authors try to claim that non-linearities somehow conspire to produce a large effect, which I find very unlikely - throwing a small stone into an ocean is not going to produce a tidal wave, no matter how tough the equations of GR are (and gravitational waves do not seem to amplify themselves until the earth is shaking every time an apple drops. In other words, the non-linearities don't matter when you have weak fields).

I read the paper. It wasn't very good, they pull a metric out of their ass which looks quite odd. It isn't at all clear that the model is even mathematically consistent, and if they had done a rigorous analysis of this thing you would think they would explain how these weak fields can produce this massive effect in normal english somewhere in the paper. I think it's just a standard wrong paper on the arxiv basically.
posted by snoktruix at 6:11 PM on October 10, 2005


anyone else notice the domain?

http://xxx.lanl.gov ?!?

having been shackled behind a government firewall before, I can't help but think that this website is unavailable to 95% of government employees.
posted by reflection at 6:11 PM on October 10, 2005


Um, dark matter has been observed. We can observe it's mass... that's why it's dark matter. I'm not sure where all this "placeholder" talk is coming from... dark matter has a lot of evidence going for it. Without the dark matter contributing that extra 90% a lot of stuff falls down. The debate is not so much whether dark matter exists but where/how/when it came from.
posted by nixerman at 6:29 PM on October 10, 2005


Wow, thanks for the Symmetry link, Rothko.
posted by spazzm at 8:01 PM on October 10, 2005


Um, dark matter has been observed. We can observe it's mass...

No, it has not... which is sort of the point of this paper. Large masses of rotating matter gravitate according to general relativity, not Newtownian mechanics. The authors are suggesting that by carefully accounting for all of the relativistic effects one can explain the 'phantom extra mass' that usually is called dark matter.

That said, it is highly doubtful that so many other independent computational studies of galaxies were so incorrect. Also, neutrino physics has some unsolved problems in it that many are starting to believe are tied to the dark sector.
posted by fatllama at 8:21 PM on October 10, 2005


Wow, thanks for the Symmetry link, Rothko.
posted by spazzm at 11:01 PM EST on October 10 [!]


You're quite welcome, spazzm.
posted by Rothko at 8:38 PM on October 10, 2005


Why is dark matter more problematic than things like gravitational forces or light waves or quantum states or cell membrane pumps?

It hasn't been observed. All the other things you mention (gravity, light waves, quantum states) has been observed. They can be manipulated in repeatable experiments.


I guess my question, then, is what it takes for something to count as observed or manipulated.
posted by painquale at 10:10 PM on October 10, 2005


"it seems amazing that nobody looked at this before."

Although others have answered this specifically with regard to the FPP line of investigation, I just wanted to say that that statement is pretty common in the history of science. :)

Remember, light travels so fast that it took us a half-million years to notice that it travels at all - not to mention about the same amount of time to notice that the Earth is roughly spherical, even with some pretty obvious evidence right in front of us (like the curved shadow of the Earth on the Moon during an eclipse).

"Gee, I never noticed that before..." can probably be said to be the beginning of scientific exploration.

This paper is very interesting, I'm looking forward to seeing what comes of it. I'm going to forward the link to a friend or two who are astrophysicists.
posted by zoogleplex at 10:54 PM on October 10, 2005


painquale: I guess my question, then, is what it takes for something to count as observed or manipulated.

Well, I won't comment on what "it takes" in general--at least not at first--but for dark matter there are two somewhat broad classes of experiments that seek to observe dark matter. As a precursor, it is important to note that though you often hear people say that "dark matter doesn't interact with ordinary matter," this isn't really true. It rarely interacts. In physics language, dark matter has a small scattering cross section--as it travels through a sea of ordinary matter, it sees the particles making up that sea as being very widely spaced, so it doesn't run into them very often. Back to the two types of experiments...

The first are known as direct detection experiments, whereby the experimenters hope to detect the interactions of dark matter with ordinary matter by using some sort of advanced detector (namely, a large block of metal).

The second are indirect detection experiments. These experiments hope to detect the products of dark matter interactions in large celestial dark matter detectors, such as the Sun, Earth, or galactic center. For example, as dark matter travels through these large bodies, it will occasionally interact and lose energy, sometimes becoming bound by the object's gravitational field. As time goes on, the concentration increases, causing dark matter particles to run into one another. Such an interaction occasionally leads to neutrinos that may fly towards Earth, which we can detect. These neutrinos enjoy a unique energy spectrum, unlike what we would expect from fairly well understood processes going on within the Sun or Earth or what have you, and so detection of such neutrinos is interesting in that it possibly evidences the existence of dark matter.

The point is that physicists have not simply said, "Dark matter is the answer," and called it quits. Dark matter theories have been proposed to provide answers to unresolved problems, and methods to test the potential validity of said theories have been devised. We generally hope that we can come up with a theoretical framework to explain phenomena, but the usefulness of such a framework often rests on the outcomes of experiments specifically designed to falsify it.
posted by dsword at 11:13 PM on October 10, 2005


Thar be dragons in th' aether!
posted by moonbiter at 5:32 AM on October 11, 2005


Um, dark matter has been observed. We can observe it's mass...

No. We have made observations which could be explained by invisible mass. Not quite the same thing as "observing dark matter".
posted by Decani at 5:37 AM on October 11, 2005


This paper is very interesting, I'm looking forward to seeing what comes of it.

I'm afraid nothing will come of it and it will be ignored (as it seems to have been so far), because it is obviously wrong for the reasons I said above. If it makes you any more confident, I do have a PhD in astrophysics from a well known institution where a certain nobel prize winning schizophrenic math professor once worked.
posted by snoktruix at 8:12 AM on October 11, 2005


It should be pointed out that the neutrino was not observed until 25 years after it was postulated.
posted by dirigibleman at 8:38 PM on October 11, 2005


Epicycles.
posted by bshort at 9:20 PM on October 11, 2005


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