When you think of dark matter, you very likely think of a halo of diffuse, unseen mass whose gravitational influence is felt by everything within our galaxy, and every galaxy or cluster out there.
Our Milky Way, like most galaxies, is surrounded by an approximately spherical halo of dark matter. Our sun moves through this halo.
But what you might not consider is that this dark matter is consistently passing through Earth and every atom-and-molecule on it. Every once in a while, a lucky (or unlucky) dark matter particle strikes, say, a DNA molecule in your body, breaking its bonds and leaving an unmistakeable, destructive signature.
Sketch of the proposed experiment. An incoming dark matter particle kicks out a gold atom which goes on to break several strands of DNA.
Creatively, a new paper has the scoop on how we might use this exact phenomenon to experimentally, directly confirm the particle nature of dark matter! Sabine Hossenfelder has the full story.
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Dark matter. I talk about it a lot here for a number of reasons. These include:
the fact that it makes up about 85% of the mass…
What a clever use of DNA.
I wonder if this would work for Neutrinos?
Interesting concept but my first question was how they planned on differentiating DNA damage done by DM versus DNA damage done by neutrinos. Upon reading the paper, the answer may be that they can't tell the difference but I give them bonus points for honesty.
From the paper:
Studying WIMPs with mχ ∼ 100 GeV will be much more difficult with our setup. This is the mass where LUX and XENON are optimized, and presumably in their next runs
the bounds will approach the neutrino background that also produces elastic scattering events and will make WIMP searches to lower cross sections very difficult.
I'm still really unclear about what exactly we know or don't know about the properties of dark matter particles. They DO interact gravitationally, they DON'T interact electromagnetically, but do we know for sure whether or not they interact via the strong or weak nuclear forces? How well have we narrowed the rest mass of these particles? How well do we know their kinetic energy distributions in our galactic neighborhood? I know we have some ideas about some of this stuff ("cool" rather than "hot" dark matter, for example) and we must know a bit more if our detectors are finding anything (I can't imagine we could even predict purely-gravitational interactions at a short enough scale that they'd be observable) but I'm still not sure what the best experimental, or even theoretical, limits are.
@2: if you know the neutrino scattering background, you look for signals above that background.
Given that the background noise is orders of magnitude larger than the dark matter signal, that's a very tough thing to do. But in principle there's no reason you can't do it. In practice, there's three basic ways to deal with a large background: one, shield your equipment from the background (but not the signal). Two, take a lot more data (i.e., through many detectors, larger detectors, or by collecting data for more time). Three, find an energy window in which you expect a lot more signal than noise, and build a detector to take advantage of that window. I suspect the third has already been done, and they are already on to figuring out how to realistically do 1 and 2.
@BenHead #3: You ask some very good questions. I'm one of the collaborators on the SuperCDMS experiment (a dark matter search currently running in Soudan, MN, and planned to be upgraded to run at SNOLAB in Ontario). Let me try to answer them as briefly as possible :-/
"[D]o we know for sure whether or not they interact via the strong or weak nuclear forces?" Nope. Not at all. Experiments like CDMS, LUX, LZ, etc. _assume_ they will interact through the weak (or a weak-like) force, for two reasons. First, if they don't interact at all, we have no way to detect them, so why bother (this is the "looking under the lamppost" strategy). Second, the particle physics model of supersymmetry quite generically predicts a stable supersymmetric particle with a mass in the tens of GeV range, which would only interact very, very weakly with normal matter. Doing well understood calculations in the Standard Model can give you cross-sections and production rates for these particles, which just happen to be very close to what you would need in order to produce the observed dark matter density. This so-called "WIMP miracle" is the main reason such particles are favored as dark matter candidates.
"How well have we narrowed the rest mass of these particles?" Not much at all. The problem is (see above) that the mass, interaction cross-section, and local density are highly correlated. Since we don't have any reproducible observation of WIMPs, what the experiments can do is set limits, as contours in the cross-section vs. mass plane.
"How well do we know their kinetic energy distributions in our galactic neighborhood?" Quite well, just from gravitational considerations. Keep in mind that we're assuming WIMPs (see above), so the DM is cold. The particles are gravitationally bound into the halo, and their motions are thermalized. The fact that they're bound tells us what their speeds around the galactic center (okay, around the total mass at smaller radius than us) must be. Since their motion is randomized, we get a range of speeds for the DM through an Earth-bound detector, which is the composition of the Earth's rotation (slow), orbit (faster) and the Sun's galactic orbit (fast). Basically, it's about 220 km/s +/- 20%.
"Dark matter particles thus can damage the bond structure of molecules inside your body."
Premises: Dark matter exists; we don't know what it is, but we know that it exists.
Conclusion: Dark matter particles thus can damage the bond structure of molecules inside your body.
A textbook argument from Creation Science and Intelligent Design [sorry for the repetition].
I can't recall the correct name for an argument that starts with a premise based on its chosen conclusion then adds premises to support its conclusion, but I'm fully aware that these types of arguments are unworthy of consideration because they are not even wrong.
I find it very disconcerting that this is the second time in the first two months of 2015 that I have had to point this abject absurdity to bloggers on ScienceBlogs LLC.
I might be wrong, but I thought I read that dark matter isn’t found on earth. Which would be surprising since dark matter with dark energy are said to make up about 90% of the universe.
If dark matter isn’t found on earth then I guess we don’t have to worry about its possible carcinogenic effects.
@Michael Kelsey #5: Thanks so much for the detailed response! I'm definitely curious to see where the explorations of supersymmetry lead, since my understanding is that all supersymmetric models imply proton decay and that that hasn't been detected yet. Thanks again!
@See Noevo #7: Dark matter has not yet(*) been observed in an experiment. There are numerous running, under construction, and proposed experiments looking for it, with limits which are fairly stringent, but NOT inconsistent with the expected density in our local neighborhood.
If you have the ability to do simple high-school algebra and geometry, then you can work out an estimate for the density of dark matter in the solar system all by yourself. (See the Wikipedia article "Milky Way" for numbers and details, if you prefer).
Approximate the Milky Way as a thick disk with a diameter of 100,000 light years and a thickness of 2,000 light years. The total mass of the galaxy is about 10^12 solar masses. From those numbers, you can calculate the average density of the galaxy (hint, it is going to be MUCH, MUCH, MUCH smaller than the density of the Earth). Roughly 85% of that density is due to dark matter.
Forgot my footnote: (*) Two experiments, DAMA and CoGENT, have both published data which they claim constitute an observation of dark matter (specifically, of direct interaction of WIMP-like dark matter particles with their detectors). However, their results are not consistent with each other, and are also not consistent with non-observation limits set by numerous other experiments.
Dark matter, we should look around our sun, where the strongest gravitational field
@Peter #11: Been there, done that. Search arXiv for "solar WIMP capture":
http://arxiv.org/abs/1308.5897
http://arxiv.org/abs/1208.0827
http://arxiv.org/abs/1107.3182