The hunt for dark matter

Sep 17, 2009 by Anne Trafton
An inside view of a neutron detector in development at MIT's Laboratory for Nuclear Science. Pappalardo Fellow Jocelyn Monroe, is seen through the detector. Photo - Donna Coveney

(PhysOrg.com) -- In a basement laboratory at MIT, assistant professor of physics Jocelyn Monroe is making some final adjustments to her team's newest particle detector. In just a few months, the detector will be 1,600 feet underground in Carlsbad, N.M., searching for the elusive particles known as dark matter.

Dark matter — considered one of the most baffling mysteries in science — is believed to make up 20 to 25 percent of the universe, while visible matter makes up only four percent. However, has never been directly observed, and scientists aren't even exactly sure what kind of particles they are looking for.

Scientists theorize the existence of dark matter to explain observations that suggest there is far more mass in the universe than can be seen. Because dark matter does not absorb or emit light, it has thus far proven impossible to detect. "There's a lot of dark matter out there, and nobody knows the best way to look for it," says Monroe.

Identifying dark matter is a fundamental pursuit, says MIT physics professor Peter Fisher, who has been studying dark matter for 20 years. "If you go way back in time, to the sitting around the fire, one of the first questions humans ask themselves is, 'what is the world made of?'" he says.

Dozens of research groups around the world are racing to be the first to detect dark matter. A few have reported possible detections in the past year, but those findings are not widely accepted.

Monroe, Fisher and associate professor Gabriella Sciolla have devised a new way to look for dark matter, taking advantage of the prediction that the particles should approach Earth from a certain direction in space.

'Solid evidence'

Caltech researcher Fritz Zwicky first proposed dark matter in the 1930s as a way to explain discrepancies between the inferred mass and the light output of a cluster of galaxies. Zwicky found that the amount of light coming from stars in the Coma was about 100 times lower than would be expected from a cluster of its mass.

At first, other physicists assumed the missing mass must be gas or dust, but scientists have since embraced dark matter as a way to explain Zwicky's astronomical observations and others that don't add up. For example, spinning galaxies generate centripetal force that would tear them to shreds if not for the counteraction of gravity. However, there isn't enough visible matter in those galaxies to produce the necessary gravitational pull, so physicists theorize that dark matter makes up the difference.

"There is solid evidence that dark matter exists. What we're missing is direct observation of dark matter," says Sciolla, the Cecil and Ida Green Associate Professor of Physics.

Physicists have devised numerous ways to look for dark matter. Some experiments look for indirect evidence of dark matter by measuring gamma rays emitted when dark matter decays, while others try to capture the distinctive electronic traces that should be left behind when dark matter collides with other particles.

One major difficulty in looking for traces of collisions is that WIMPS (weakly interacting massive particles), one of the theoretical types of particles that scientists believe may make up dark matter, have very weak interactions with normal matter.

"We expect these particles to have interactions with protons and neutrons, but the probability of those interactions is very, very small," says Monroe.

WIMPs normally pass through regular matter without interacting at all. It is estimated that the density of dark matter (if the dark matter particle is 100 times heavier than a proton) is such that there are 10 WIMPs in a two-liter bottle of soda. Fewer than one of these WIMPs will interact with a nucleus in an entire year.

To make things even more difficult, these incredibly rare interactions can be easily overshadowed by neutron collisions, which occur much more frequently and produce a similar signature in the detector. To prevent neutrons from masking potential dark matter detections, physicists install their detectors far underground, in hopes of blocking neutrons hurtling toward Earth from outer space.

Because there are so many other possible events that could masquerade as dark matter interactions, solid evidence will be required for physicists to accept any potential dark matter detection.

"You have to make a convincing argument that you're not seeing something else, and that's very hard," says Fisher.

Dark matter wind

The MIT team's new dark matter detector (Dark Matter Time Projection Chamber) is expected to start running late this year or early next year, and is designed to look for the so-called "dark matter wind."

The disc-shaped Milky Way galaxy rotates through a spherical halo of stationary dark matter. So, just as there seems to be a wind blowing toward you when you stick your arm out the window of a moving car, "there should be an opposing wind of dark matter particles blowing opposite to the direction of our motion, because we're moving and it's not," says Monroe.

This "dark matter wind" approaches Earth from the direction of the constellation Cygnus. Incoming dark matter should collide with fluorine-rich gas inside the new detector and knock off fluorine atoms, which will recoil in the opposite direction.

The researchers can compare the direction of incoming particles with the location of Cygnus, which shifts position relative to Earth every 12 hours (just as the sun appears to set and rise). That should allow them to distinguish dark matter interactions from other electronic traces picked up by the detector.

Monroe is also working on a detector called Mini-CLEAN, which will be housed in an old coalmine about a mile underground in Sudbury, Ontario. The detector, expected to start collecting data late next year, contains 300 kilograms of liquid argon. When dark matter particles collide with an argon nucleus, the nucleus recoils, producing a burst of light.

A second detector, designed to pick up neutron collisions, will be placed next to the dark matter detector. That way, interactions that show up in both detectors can be dismissed, and those appearing only in the dark matter detector can be considered reliable.

How long will it take any of these detectors to pick up dark matter interactions? That depends on chance, as well as the size and sensitivity of the detector. The sensitivity of dark matter detectors has grown by several orders of magnitude since the earliest experiments, 20 years ago, and continues to improve.

However, the lack of results so far has led some physicists, including Fisher, to question whether they're even looking for the right thing. "After a while, there should be some hint, some indication, and there really just isn't anything," he says.

But the search continues, and some are hopeful that the first sighting is not too far off. "Maybe not this year, but I would bet a good bottle of champagne it will be found within the next 10 years," says Monroe.

Provided by Massachusetts Institute of Technology (news : web)

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User comments : 10

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NotAsleep
5 / 5 (1) Sep 17, 2009
This article fascinates me, I'd love to learn more. Can someone recommend good literature that can explain:

1) What "solid evidence" there is that dark matter exists
2) How we know what the electron trace is of a theoretical partical colliding with a real one
3) How we're basing the existence of one theoretical particle (dark matter) on the premise that it emits another theoretical particle (WIMP)
4) Why we've decided that there is a stationary "dark matter wind"

Seriously, I'd appreciate it if someone could point me in the right direction (not try to explain it here in a forum, as I'm sure has been done a million times before)
Alexa
1 / 5 (1) Sep 17, 2009
..what "solid evidence" there is that dark matter exists...
In science no "solid evidence" exist for anything. But there are many "less-solid" evidences, which leads to the same conclusion in many cases.

http://en.wikiped...evidence

We can add gravitational anomalies like Alais effect, Pioneer anomaly and some others. Situation is complicated by fact, most of dark matter is formed by composite effects (deformation of space-time due the omni-directional space-time expansion and less or more massive particles trapped inside this deformation).

Currently we're distinguishing so called cold dark matter formed by space/time deform and or very lightweight particles (neutrinos, axions) and hot dark matter, formed by heavier, yet weakly interacting particles (WIMPs, heavily ionized atom nuclei, particles of antimatter, etc...). This complicates formal models, because particle models of dark matter often doesn't play well with the another ones.
NotAsleep
5 / 5 (1) Sep 17, 2009
Alexa;
Interesting reading above. However, none of the reading prevents me from returning to my fundamental issue with Dark Matter: that the strongest evidence FOR it involves gravitational anomalies. It is much easier for me to convince myself that we don't have a true understanding of how gravity operates on a macroscopic level.

What I truly need is a lengthy, scientific document laying out why we're sure that information recieved from outside of our solar system hasn't been "tampered with" by forces beyond our understanding (thereby making this information fundamentally invalid). Until we "pace off" the distance from here to the center of the galaxy, I won't be convinced that we know the true distance from here to there... unless there's a good document out there that gives logical steps to get me there without a mess of theories and conjectures

So anyone got a good book for me?
Question
Sep 17, 2009
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GPhillip
4 / 5 (1) Sep 17, 2009
So if there is no dark mater, how do you explain gravity where there is no mass? There are places in the Cosmos where light from distant objects is bent, but there is no visible mass there. That is dark matter, whatever it is. The simplest explanation is it is some type of WIMP. That's not the only explanation, it's just the simplest.

In my own humble opinion, revising the laws of gravity and Theory of General Relativity to explain the anomalies is a much more complex solution. Invoking some new unproven theory is even more complex still. Generally science solves these problems by looking for the simplest solution first. While there is absolutely no proof that the WIMP solution is the correct one, if it's not, then we are all off down a rabbit hole where no one has been before. While that may sound like a great adventure, it's not the way science works.
brant
not rated yet Sep 17, 2009
So if there is no dark mater, how do you explain gravity where there is no mass? There are places in the Cosmos where light from distant objects is bent, but there is no visible mass there. That is dark matter, whatever it is.

In my own humble opinion, revising the laws of gravity and Theory of General Relativity to explain the anomalies is a much more complex solution.


Your assuming that the object your observing is lensed. There is no proof for that. Especially if you stick strictly to Einsteins predictions about lensing shape.

You dont have to revise relativity but only to bring back some form of aether. If you think about it dark matter and energy already are some form of basic energy.
Velanarris
5 / 5 (2) Sep 18, 2009
It is much easier for me to convince myself that we don't have a true understanding of how gravity operates on a macroscopic level.

Not only is it much easier to convince yourself of it, it's a defensible point seeing as we still don't comprehend gravity beyond our knowledge that it exists.
Question
Sep 18, 2009
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kasen
5 / 5 (1) Sep 18, 2009
Excuse the pun and potential ignorance. Wild stab in the dark here, but couldn't gravitational waves form interference patterns? If so, wouldn't the peaks act as gravitational anomalies? Something along the lines of anti-Lagrange points, on a galactic scale. Has this been considered as an alternative to dark matter?
omatumr
Sep 18, 2009
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Question
Sep 18, 2009
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Noumenon
4.8 / 5 (46) Sep 19, 2009
@kasen, interesting idea, the total gravitational energy of the system would remain the same, so the peaks would be compensated by troughs, right, ...or am I misunderstanding your point?
kasen
not rated yet Sep 19, 2009
Well, the troughs would either act as negative gravity points, or just as null gravity points. I suppose the latter would violate some conservation law, so it's negative gravity. Which, as a quick wiki search shows, is commonly known as dark energy. Nothing on gravitational waves, though.

When trying to visualise, I imagine hills in spacetime(or divergent density gradients, hehe). The problem is, as far as light is concerned, the effect would be the same as positive gravity, so I'm not sure how you could test this idea.

Maybe with a stream of massive particles, small enough not to interfere too strongly? Shot between two bigger sources of gravity, it should change direction alternately at different points, corresponding to peaks and troughs. Might be observable only at really large scales, though. Earth-Moon could do.
Tangent2
1 / 5 (2) Sep 22, 2009
I still believe that an article I saw on here recently is on the path to the right answer. That paper suggested that there is in fact a 5th force of nature that we haven't realized yet, and we are trying to make all of the theories fit with observations by making up things as we go along because everyone assumes that the 4 fundamental forces encapsulates everything. Maybe the 5th force is the missing link in all the equations. As those researchers pointed out, they didn't set out to discover or prove the existence of a 5th force, it just appeared in the math they were doing to reconcile dark matter with standard physics.