Here in the Era of (Attempted) Dark Matter Detection, new results just keep coming in. Some are tantalizing, some simply deflating. Count this one in the latter camp.
The XENON100 experiment is a detector underneath the Gran Sasso mountain in Italy (NYT article). It’s a very promising experiment, and they’ve just released results from their most recent run. Unlike some other recent announcement, this one is pretty straightforward: they don’t see anything.
Here we see the usual 2-dimensional dark matter parameter space: mass of the particle is along the horizontal axis, while its cross-section with ordinary matter is along the vertical axis. Anything above the blue lines is now excluded. This improves upon previous experimental limits, and calls into question the possible claimed detections from DAMA and CoGeNT. (You can try to invent models that fit these experiments while not giving any signal at XENON, but only at the cost of invoking theoretical imagination.) See Résonaances or Tommaso Dorigo for more details.
No need to hit the panic button yet — there’s plenty of parameter space yet to be explored. That grey blob in the bottom right is a set of predictions from a restricted class of supersymmetric models (taking into account recent LHC limits). So it’s not like we’re finished yet. But it is too bad. This run of XENON had a realistic shot of actually finding the dark matter. It could be harder to detect than we had hoped, or it could very well be something with an extremely small cross-section, like an axion. The universe decides what’s out there, we just have to dig in and look for it.
Sean,
You mentioned axions as a possible dark matter candidate; however, I was under the impression that the parameter space for axions has been even more tightly constrained than it has for WIMPs.
This time period seems to be a crisis in cosmology in the Thomas Kuhn framework of scientific revolutions, no? None of the ideas currently on the table seem to be working: MOND has its problems, so does LCDM, so does LWDM, so do theories that posit extra dimensions, etc. It makes me wonder when there are so many people with 140+ IQs and billions of dollars of funding, why there seems to be no resolution in sight when it comes to establishing an accurate description of most of the Universe!
Has any observational evidence been found yet for when all this supposed dark matter first appeared?
In particular, do the most distant galaxies whose peripheral rotational red shifts can be meaured show the same anomalouslty fast rotation as nearby galaxies?
I gather at one time it was thought that clumps of dark matter promoted the initial formation of galaxies. But hasn’t that idea now been supplanted by the notion of this being achieved by polar jets of primordial black holes concentrating vast clouds of gas? In that case dark matter could have turned up later than the Big Bang.
@Thorny: http://arxiv.org/abs/astro-ph/0607207 figure 14. Some more recent microlensing studies have been published, but of smaller samples so haven’t updated the percentages yet. Basically if you combine the MACHO and EROS samples, you are left with solar mass scale black holes making up no more than about 20% of the needed halo mass. It’s also very hard to invent a way to make primordial black holes prior to Big Bang Nucleosynthesis time scales without producing axions as a side product.
@John R Ramsden: Baryon Accoustic Oscillations – hard to get that 100 Mpc peak in the matter power spectrum without the early universe having a lot of dark matter. See http://astro.berkeley.edu/~mwhite/bao/ for a good description.
spaceman: I’ll hazard a guess. The reason very smart people can’t solve the problem is because they have no direct experimental observation of the stuff. They see all kinds of weird things that cry out for an explanation, and they can come up with plausible explanations. But there is no substitute for empirical evidence. None. And no amount of smarts will ever make up for that fact.
http://www.youtube.com/watch?v=b240PGCMwV0
Phillip Helbig: “If such objects were to make up most of the dark matter, they would have a much stronger microlensing signal than is observed.”
Doug: “you are left with solar mass scale black holes making up no more than about 20% of the needed halo mass”
——————————————————————–
Yes, dear colleagues, but even IF the microlensing inference of 20% turns out to be valid in the long run, and this inference is far from a final verdict, that is:
MACHOs = billions
“WIMPs” = ZERO (as in zilch, nada, goose-egg, nein)
Tell me. Is billions of “birds” in hand less of a scientific desiderata than a small herd of “unicorns” that might be just around the 1,343rd corner, or the 1,344th, or the 1,345th, or …?
Albert Z
Doug: thanks a lot! I have to wonder though whether a check with other galaxies would be appropriate, just to make sure that there isn’t a coincidence between our orbital velocity and that of the DM halo in the direction of the Magellanic Clouds that’s changing the expected lengths and frequency of microlensing events. Is anything like that done/published yet?
Thorny: a lot of work has been done on Andromeda, and a few attempts on some more distant galaxies – a summary can be found at http://arxiv.org/abs/0912.2667
You run into two problems as you go to other galaxies: 1) the lensing signal is strongest when the lens is half way between us and the object being lensed, but all the possible machos are either in our halo or the target galaxy’s and not between the two, and 2) you can’t resolve individual stars in a ground based observation, so are looking at a blend of stars, one of which is being microlensed during an event (same problem Hubble had in his Cepheid observations that caused him to overestimate H0). This makes you need to model the observations in a more statistical manner and ultimately need many more events to reach the same level of significance as the galactic studies (of course you get more stars per image, which helps).
Calm down, everyone. WIMPs of the sort that XENON and CDMS are sensitive to are simply one of very many possibilities of dark matter. The idea that we are in the middle of some kind of paradigm collapse for dark matter is ludicrous. In fact both cosmology and astronomical constraints (see Bullet Cluster) make the case for dark matter stronger now than it was just a few years ago—-namely, incontrovertible.
At best we’re getting indications that GeV-scale SUSY-inspired particles may not be the dark matter. If so there’s about 10^2 other possible types of dark matter to be ruled out still.
Sean , I agree there is no need for alarm. However the “WIMP miracle” argument which
everyone talks about in any pedagogical talk about WIMPs fixes the cross-section
to be about weak-interaction scale about 10^-43 cm^2 and slowly that space is been ruled out
over a larger mass range. so people should now be talking about super-wimps or very-WIMPs or ultra-WIMPS.
Also as someone pointed out even if 2 more orders of magnitude cross-sections are ruled out,
one can always invent models which evade all bounds, but it becomes fine-tuning
@14 Lab Lemming. The possibility of atmospheric contamination affecting dark matter experiments was discussed on Nature’s news blog at http://blogs.nature.com/news/thegreatbeyond/2011/03/fukushima_reactor_products_cou.html
Thank you Dr. Reich!
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Wow, this post really brought out the quacks. DM isn’t going anywhere. Just because a few possible WIMP models are being ruled out doesn’t mean there aren’t many more where they came from. Everyone should understand that theorists have a tendency to talk up the plausibility of the most detectable versions of WIMPs, inflation fields, etc. to 1.) help experimentalists get funding for not-sensitive-enough instruments, and 2.) to stake out territory predicting something that might actually get detected in their lifetime.
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