Dama

[zosima]

I just read a neat article in the New York Times about an experiment that claims to be detecting dark matter that the earth is traveling through. According to the times, the experiment is much in the minority, but nonetheless legit, as well as quite interesting.

Here's the times article:
http://www.nytimes.com/2008/04/17/science/space/17dark.html
Here's the web site for the group doing the experiment:
http://people.roma2.infn.it/~dama/web/home.html
If you want primary sources you can find them there.

So what do people think? I don't really think I'm qualified to evaluate it. Does anyone else have any opinions? It'd be neat if they were right.

Oh ya, for all you cranks out there, the NYT article mentions your favorite cosmologist, Zwicky!

 

[Cuddles]

It doesn't look especially crackpot, but it does look like they're rather overstating their case. Basically, they may not be detecting dark matter at all, and they may not be detecting any variation. Their detector is essentially the same as the various neutrino detectors around the world. Since the speed of light is slower in a medium than in a vacuum, it is possible for high energy particles to travel faster than light in the medium, and this results in a flash of light called Cherenkov radiation, similar to getting a sonic boom when you travel faster than sound. If you bury a big pot of liquid deep underground, all the regular particles will be absorbed before they reach it and only very weakly interacting ones will be detected. Neutrinos are detected in this way, but if dark matter is made up of WIMPs (Weakly Interacting Massive Particles) then it would be seen as well. (Technically neutrinos are WIMPs, but the term was coined to refer to dark matter before neutrinos were known to have mass, and since they can only make up a tiny proportion of dark matter it isn't usually used to refer to them.)

Their claim is fairly simple, and is similar to the arguments used to disprove the existence of the aether. If dark matter exists and the Earth is passing through clouds of it, more particles will be detected when you are moving against the flow of dark matter than when you are moving with it. There are two main problems with their claims. Firstly, they need to prove that there actually is an annual variation. This appears to be the main thing under question at the moment. These sort of experiments are very hard to run since they deal with very few interactions and are difficult to filter out noise. Secondly, they need to prove that they are actually detecting dark matter and not just neutrinos. While a flow of dark matter is one possible cause of such a variation, it is by no means the only possible cause.

In all fairness to the scientists, I suspect this may be another case of the media trying to sensationalise perfectly legitimate research, rather than them actually overstating the case themselves. Certainly their publications are in perfectly respectable journals, and most reactions seem to be saying that it's interesting but very preliminary rather than just dismissing it.

 

[sol invictus]

It's certainly not crackpot, but most people in the field think it's likely that the result is wrong. It conflicts with the data from several other searches (although none of them are designed in exactly the same way, so there's always room for possibility).

As Cuddles says, it's a very hard experiment, and there are lots of potential sources for seasonal variations other than DM which must be eliminated. After seeing enough exciting but wrong experimental results come and go over the years one starts to develop a nose for which are likely to be correct, and this one doesn't smell very good.

 

[zosima]

Secondly, they need to prove that they are actually detecting dark matter and not just neutrinos. While a flow of dark matter is one possible cause of such a variation, it is by no means the only possible cause.

Any thoughts on how they might be able to exclude neutrinos? I was wondering if they might have some theory that predicts that WIMPs will interact more frequently with Sodium Iodide than Water. Aren't most neutrino detectors water detectors?

If not to make a distinction between WIMPs and neutrinos, why use a different material in your detector?

Slightly tangentially, how do scientists know that neutrinos aren't responsible for the missing mass? Are they pretty sure about the rate of neutrino interaction? Because it seems like one explanation could be many more neutrinos that are much less likely to interact with normal matter.

 

[ben m]

DAMA's claimed signal is seen in flashes of scintillation light (not Cerenkov, sorry Cuddles) in 100kg of ultrapure NaI. The flashes seen are in the range of 2-6 keV, far below the range where Cerenkov light occurs. They see about 1 flash per kg-day-keV in their detector constantly, but they claim a ~0.01 flash/kg/d/kev higher rate in the summer and ~0.01 flash/kg/d/kev lower rate in the winter. It is certain that most of these flashes are backgrounds from radioactive decays in the detectors, the shielding, etc.., but such decays should not vary from season to season.

The *expected* dark matter signal is when a Galactic dark matter particle crashes into a nucleus. The incoming dark matter velocity is the sum of several components: (a) some random isotropic velocity left over from the formation of the Galaxy, (b) 220 km/s because the Sun is moving through the galaxy, and (c) 10 km/s because the Earth is moving around the Sun. You'd expect the dark matter signal to be higher in the summer when the Earth's motion *adds* to the Sun's (with respect to the Galactic rest frame) and lower in the winter when the motions partially cancel. The size of this effect is expected to be in the ballpark of a few percent.

The problem is, if DAMA is seeing 0.01 c/d/kg/kev of *variation* between 4 and 6 keV ... why did XENON10, a much cleaner experiment, see fewer than 0.003 c/d/kev/kg *total events* between 4 and 6 keV? (XENON10 was not sensitive to the 2-4 keV events which DAMA saw.)

If DAMA's signal to be dark matter, it is very very difficult to imagine (a) why no other experiments have seen it and (b) why the annual modulation would be so large a fraction of the signal. If this is the case, the dark matter involved must be something very weird---axions, with weird galactic dynamics, coupling to electrons? MeV-scale WIMPs coupling to nuclear spin? Something else?

DAMA is expected to see some neutrino events, yes, but (I think) nowhere near enough to explain their large signals---remember that most neutrino detectors are kilotonnes, not 100 kilograms. It's hard to think of a neutrino source that makes such low-energy events, and it's even harder to think of a neutrino source which is 10% stronger in summer than in winter!

The thing to remember is that DAMA's signal has nothing to do with simply seeing events. Everyone sees events: XENON, CDMS, COUPP, etc.. What's new about DAMA is this claim of seasonal variation.

The other thing to remember is that seasonal variation is *really hard* to get right. Cosmic-ray fluxes vary over the year. Temperatures vary, which might couple to radon infiltration rates, noise, etc. If could be something like, "The incompetent students from University X do the calibrations during their summer vacation, while expert postdocs from University Y are available all winter." It's a very difficult measurement, and an easy one to get subtly wrong. That said, everything I could think of has been given a careful treatment in their paper.

 

[zosima]

DAMA's claimed signal is seen in flashes of scintillation light (not Cerenkov, sorry Cuddles) in 100kg of ultrapure NaI. The flashes seen are in the range of 2-6 keV, far below the range where Cerenkov light occurs. They see about 1 flash per kg-day-keV in their detector constantly, but they claim a ~0.01 flash/kg/d/kev higher rate in the summer and ~0.01 flash/kg/d/kev lower rate in the winter. It is certain that most of these flashes are backgrounds from radioactive decays in the detectors, the shielding, etc.., but such decays should not vary from season to season.

The *expected* dark matter signal is when a Galactic dark matter particle crashes into a nucleus. The incoming dark matter velocity is the sum of several components: (a) some random isotropic velocity left over from the formation of the Galaxy, (b) 220 km/s because the Sun is moving through the galaxy, and (c) 10 km/s because the Earth is moving around the Sun. You'd expect the dark matter signal to be higher in the summer when the Earth's motion *adds* to the Sun's (with respect to the Galactic rest frame) and lower in the winter when the motions partially cancel. The size of this effect is expected to be in the ballpark of a few percent.

The problem is, if DAMA is seeing 0.01 c/d/kg/kev of *variation* between 4 and 6 keV ... why did XENON10, a much cleaner experiment, see fewer than 0.003 c/d/kev/kg *total events* between 4 and 6 keV? (XENON10 was not sensitive to the 2-4 keV events which DAMA saw.)

If DAMA's signal to be dark matter, it is very very difficult to imagine (a) why no other experiments have seen it and (b) why the annual modulation would be so large a fraction of the signal. If this is the case, the dark matter involved must be something very weird---axions, with weird galactic dynamics, coupling to electrons? MeV-scale WIMPs coupling to nuclear spin? Something else?

DAMA is expected to see some neutrino events, yes, but (I think) nowhere near enough to explain their large signals---remember that most neutrino detectors are kilotonnes, not 100 kilograms. It's hard to think of a neutrino source that makes such low-energy events, and it's even harder to think of a neutrino source which is 10% stronger in summer than in winter!

The thing to remember is that DAMA's signal has nothing to do with simply seeing events. Everyone sees events: XENON, CDMS, COUPP, etc.. What's new about DAMA is this claim of seasonal variation.

The other thing to remember is that seasonal variation is *really hard* to get right. Cosmic-ray fluxes vary over the year. Temperatures vary, which might couple to radon infiltration rates, noise, etc. If could be something like, "The incompetent students from University X do the calibrations during their summer vacation, while expert postdocs from University Y are available all winter." It's a very difficult measurement, and an easy one to get subtly wrong. That said, everything I could think of has been given a careful treatment in their paper.

Thanks for the feedback, Ben. That was a really great explanation.

 

[DeiRenDopa]

I have a question about this, from the OP:

Oh ya, for all you cranks out there, the NYT article mentions your favorite cosmologist, Zwicky!

(I added bolding).

I wasn't aware the Zwicky was a fave among crackpots, nor that Zwicky was particularly well-known for his contributions to (theoretical) cosmology.

Would you mind elaborating a bit please zosima?

That Zwicky was a larger-than-life character, with a (shall we say) strong personality is not in any doubt; however, the work he did in astronomy and astrophysics was absolutely first rate ... and of course, for someone with such a long career and such a high output, he made his share of mistakes etc ...

 

[zosima]

I have a question about this, from the OPI added bolding).

I wasn't aware the Zwicky was a fave among crackpots, nor that Zwicky was particularly well-known for his contributions to (theoretical) cosmology.

Would you mind elaborating a bit please zosima?

That Zwicky was a larger-than-life character, with a (shall we say) strong personality is not in any doubt; however, the work he did in astronomy and astrophysics was absolutely first rate ... and of course, for someone with such a long career and such a high output, he made his share of mistakes etc ...

I don't have such a long history on this forum, so I'll admit I'm working from 2 data points. Also, I'm not trying to say that Zwicky didn't have a quality scientific career and make excellent contributions. But I have seen his stuff used by Woo's in the Witt "Null Physics" thread and one other one, I think it was the thread about magnetic reconnection=fake thread, but I won't swear by that.

So I'm saying that Woos like him, not that he made Woo. From what I've read, he had many innovative ideas that were well ahead of his time. Some of them were vindicated and some not. I think the woos like to use the ones that history frowned upon and Zwicky's reputation to give people the false impression that there is actually some legitimate dispute about whether or not their wooey claims are false.

Do you think this was an unfair characterization? If so, I must admit I don't know that much about Zwicky so I'll defer to someone more knowledgeable at a heartbeat.

 

[DeiRenDopa]

I don't have such a long history on this forum, so I'll admit I'm working from 2 data points. Also, I'm not trying to say that Zwicky didn't have a quality scientific career and make excellent contributions. But I have seen his stuff used by Woo's in the Witt "Null Physics" thread and one other one, I think it was the thread about magnetic reconnection=fake thread, but I won't swear by that.

So I'm saying that Woos like him, not that he made Woo. From what I've read, he had many innovative ideas that were well ahead of his time. Some of them were vindicated and some not. I think the woos like to use the ones that history frowned upon and Zwicky's reputation to give people the false impression that there is actually some legitimate dispute about whether or not their wooey claims are false.

Do you think this was an unfair characterization? If so, I must admit I don't know that much about Zwicky so I'll defer to someone more knowledgeable at a heartbeat.

.
Thanks; I'll go look into those two data points ...

It would surely be all too easy to quote mine Zwicky and come up with ID-like support for any number of woo ideas ... he had a fiery temper, and suffered fools not at all gladly (he also suffered many first rate scientists in much the same way, sadly), and was way ahead of his time in quite a few areas.

A quick session with Google turned up this rather amusing thread: Just call it the 'Zwicky whatever' and be done with it (there's plenty more on him, he really was quite a character, by all accounts).

 

[DeiRenDopa]

A quick check turned up nothing about Zwicky in the Null Physics thread, and several references to him in some of the 'plasma cosmology' ones (his name is also invoked in at least two other threads in the last six months, but not in support of any woo ideas).

I found one post by ben_m particularly apt (excerpt, bold added):

This does NOT happen:
Q) "Bob, I understand that MOND fits rotation curves, but there are similarly slow accelerations in disk oscillations; why do those look so Newtonian? Doesn't that disprove your theory?"
A) "That's a great question, Professor Zwicky. In fact, you could say that the answer to that question would allow the theory to be conclusively disproven. The answer is presumably in my peer reviewed paper, but I'm not doing your work for you."
Q) "Oh, OK."

This happens---well, assuming that the speaker isn't full of baloney.
Q) "Bob, I understand that MOND fits rotation curves, but there are similarly slow accelerations in disk oscillations; why do those look so Newtonian?"
A) "That's a great question, Professor Zwicky. We looked into that, and the data actually disagree with the old version of the MOND, that's why we're presenting the non-Lorentz-invariant version. For this version, the fit is actually really good."
Q) "I still find it hard to believe; do you have a slide of that?"
A) "It's in the paper; let me pull up a PDF and show you."

.
Of course ben_m is right about the general tenor of any such exchange, but far too gentle (shall we say) about the words the real Zwicky would have used ... if he'd thought 'Bob' had a case less than completely iron-clad, there'd have been no question at all! And, sadly, if this was an area he (Zwicky) felt he knew something about, or if he his opinion of Bob was not high (shall we say), even if Bob did have really good grounds and could present them clearly and concisely, Zwicky would very likely have let fly anyway ...

There's also a classic post, by BeAChooser, implying that Zwicky was less than a good scientist because he did not consider electro-magnetic explanations for his observations on the Coma cluster (that lead him to postulate 'dark matter')! But this is the opposite of woos looking to Zwicky to support their ideas; in this case, Zwicky is cast as an unrepentant, craven, narrow-minded, lackey of 'the mainstream' with exceedingly limited imagination and creativity!

 

[zosima]

A quick check turned up nothing about Zwicky in the Null Physics thread, and several references to him in some of the 'plasma cosmology' ones (his name is also invoked in at least two other threads in the last six months, but not in support of any woo ideas).

I found one post by ben_m particularly apt (excerpt, bold added):.
Of course ben_m is right about the general tenor of any such exchange, but far too gentle (shall we say) about the words the real Zwicky would have used ... if he'd thought 'Bob' had a case less than completely iron-clad, there'd have been no question at all! And, sadly, if this was an area he (Zwicky) felt he knew something about, or if he his opinion of Bob was not high (shall we say), even if Bob did have really good grounds and could present them clearly and concisely, Zwicky would very likely have let fly anyway ...

There's also a classic post, by BeAChooser, implying that Zwicky was less than a good scientist because he did not consider electro-magnetic explanations for his observations on the Coma cluster (that lead him to postulate 'dark matter')! But this is the opposite of woos looking to Zwicky to support their ideas; in this case, Zwicky is cast as an unrepentant, craven, narrow-minded, lackey of 'the mainstream' with exceedingly limited imagination and creativity!

Ya, those threads are classic.

I know that Witt said he wanted to have like a million of Tired Light's babies. Tired Light happens to be Zwicky's theory. I think I assumed Zwicky was mentioned directly. Witt does mention him directly in his second white paper on his website.

 

[DeiRenDopa]

Today we see, I think, but a pale shadow of the context within which Zwicky came up with 'tired light' ... at the time what we today call H₀ (the Hubble constant) was estimated to be much greater than its currently accepted value of ~70 (km/s/Mpc, ±~10+%), and Zwicky thought the 'expanding universe/space' explanation could not be right, with such a high value of H₀. Hence his 'tired light' idea. In this particular case, he turned out to be both right and wrong - right that the contemporary value of H₀ was leading astronomers astray, wrong that the solution was 'tired light' (the history of the determination of H₀ is rich indeed; one irony is that a key breakthrough was made by Baade, with whom Zwicky had a 'complex relationship', shall we say).

Of course, the logic of woo-dom leads inexorably to attempting to hitch one's wagon to Zwicky's tired light coattails, and to glossing over the rich history (or remaining totally ignorant of it), much like Einstein's writings have been quote mined (and more) in support of all manner of the most absurd woo ...

 

[DeiRenDopa]

In the second white paper Witt does mention Zwicky ... but curiously doesn't cite the 1929 Zwicky paper as the source! (link is to a PDF).

I don't know about you, but I'd say there's no comparison between Witt and Zwicky, on just those points - Zwicky looks at six different possible explanations for the Hubble relationship, as far as it was known from astronomical observations of the time, and puts forward 'F' (now know as 'tired light') as one possible explanation, along with suggestions for where to take the idea next; Witt, on the other hand, well, JREF forum members have already commented on the quality of his work.

It looks like Witt is, primarily, trying to make his idea seem more acceptable by invoking Zwicky's name.

 

[Acleron]

{snip}
The *expected* dark matter signal is when a Galactic dark matter particle crashes into a nucleus. The incoming dark matter velocity is the sum of several components: (a) some random isotropic velocity left over from the formation of the Galaxy, (b) 220 km/s because the Sun is moving through the galaxy, and (c) 10 km/s because the Earth is moving around the Sun. You'd expect the dark matter signal to be higher in the summer when the Earth's motion *adds* to the Sun's (with respect to the Galactic rest frame) and lower in the winter when the motions partially cancel. The size of this effect is expected to be in the ballpark of a few percent.
{snip}

Thank you for an excellent explanation.

Just one question, why doesn't the dark matter also orbit around the galactic centre?

 

[ben m]

Thank you for an excellent explanation.

Just one question, why doesn't the dark matter also orbit around the galactic centre?

Dark matter does orbit the Galactic center, but at our radius there should be equal amounts orbiting clockwise, counterclockwise, around the poles, radially, etc.. In the Galactic rest frame, this collection of orbits will look like an isotropic cloud.

 

[zosima]

In the second white paper Witt does mention Zwicky ... but curiously doesn't cite the 1929 Zwicky paper as the source! (link is to a PDF).

I don't know about you, but I'd say there's no comparison between Witt and Zwicky, on just those points - Zwicky looks at six different possible explanations for the Hubble relationship, as far as it was known from astronomical observations of the time, and puts forward 'F' (now know as 'tired light') as one possible explanation, along with suggestions for where to take the idea next; Witt, on the other hand, well, JREF forum members have already commented on the quality of his work.

It looks like Witt is, primarily, trying to make his idea seem more acceptable by invoking Zwicky's name.

Agreed, Witt is no Zwicky...Witt is no wit (lol bad bad bad bad joke)

I doubt that Witt has ever actually bothered to go to the primary sources, then he'd realize he was stretching the truth. He's just making the standard woo false appeal to authority to justify his crazy theory of everything.

 

[zosima]

Dark matter does orbit the Galactic center, but at our radius there should be equal amounts orbiting clockwise, counterclockwise, around the poles, radially, etc.. In the Galactic rest frame, this collection of orbits will look like an isotropic cloud.

I just want to verify my understanding. Dark matter is supposed to be moving at the same rate as the galaxy is rotating, ya?

 

[sol invictus]

Dark matter does orbit the Galactic center, but at our radius there should be equal amounts orbiting clockwise, counterclockwise, around the poles, radially, etc.. In the Galactic rest frame, this collection of orbits will look like an isotropic cloud.

Actually there is some debate about this.

Consider beginning with a Hubble law. Plot the position versus velocity (imagine doing this in one dimension for now) - you get a straight line with slope H (the Hubble parameter). The thickness of the line is the velocity dispersion of the dark matter.

Now suppose gravitational collapse occurs around some point (call it the origin). Furthermore, suppose the line on that phase space plot we just made cannot break. Then the only thing that can happen is the line gets "wound up" (like a ball of yarn) around the origin. In that case, at any given position there will be a discrete set of velocities for dark matter, some going one way and some going the other. There will also be caustics (places where the line is vertical) where the density is much larger than the typical density.

Extending to three dimensions, you have to imagine a 3d surface winding up in a 6d space. Hard to picture, but the results are qualitatively the same.

I don't know how reasonable this idea is, but I find it very interesting.

 

[Acleron]

Dark matter does orbit the Galactic center, but at our radius there should be equal amounts orbiting clockwise, counterclockwise, around the poles, radially, etc.. In the Galactic rest frame, this collection of orbits will look like an isotropic cloud.

Please accept I'm straining for an understanding.

Dark matter only usually interacts with normal matter gravitationally. Why does dark matter contra-rotate with respect to the rotation of every thing else? Alternatively, why does the matter that we see only rotate in one direction?

 

[Dancing David]

My low brow explanation:

Most stuff bumps into other stuff as a result of the EM force. You don't fall through your chair, bits of goo stick to each other, mainly because of EM forces.

Now DM is wierd it is drawn to stuff but then can't bump or stick because it doesn't respond very well to EM forces.

I stand to be corrected by those who do know the answer.