Dark matter is for WIMPs

The entire universe in blog form
Feb. 27 2008 12:29 PM

Dark matter is for WIMPs

What the heck is dark matter?

We know it's real: many, many independent observations indicate that the majority of matter in the Universe does not give off light that we can detect, but we can detect the effects of dark matter very clearly.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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We also know it's not made of normal matter, like anything made up of electrons, neutrons, or protons. The effects we see would be very different if it were, so it must be made up of some kind of exotic matter we don't have experience with, yet.

A leading contender for this stuff are WIMPs, or Weakly Interacting Massive Particles. It's a bit of a generic term for a class of subatomic particles that don't interact well with normal matter. They can pass right through you -- actually zillions of them can -- without having any effect on you whatsoever.

That's not to say that WIMPs don't interact at all. It's just that they affect us weakly. According to theory, every now and again a single WIMP will ping off an atomic nucleus of normal matter, making it resonate like a hammer hitting a bell. It's possible to detect that effect, and from that deduce the mass of the particles. That in turn will tell physicists a lot about the particle itself.

So lots of folks are looking for just that ringing. Caltech has an experiment running called Cryogenic Dark Matter Search, or CDMS. Deep underneath northwest Minnesota, 2400 feet (730 meters) down, is a laboratory that contains cryogenically cooled detectors, kept to within a hair's-breadth of absolute zero. Billions of WIMPs pass through the detector every second, but it's likely that only one or two per year will actually smack into a nucleus and shake it up.

New results just announced indicate that the mass of any purported WIMPs must be less than 100 times the mass of the proton. How do they know? Because they have been looking long enough that if any WIMP more massive than 100 times a proton existed, they'd have detected it by now. They haven't, so the mass must be less than that.

It sounds like a negative result, but that can still be pretty useful. In this case, it's consistent with theory, which poses the most likely WIMP mass at about 40 times the mass of a proton. As the experiments continue to run, and the detectors get better, they'll be able to nail things down to that mass range as well.

If and when they find their WIMP interactions, it'll be a HUGE day for astronomy. It's a little frustrating to know that dark matter is out there, to see it affecting whole galaxies and clusters of galaxies, but not know what the heck it is!

I bet that inside of a decade (probably sooner) we'll have either detected WIMPs, or ruled them out as a contender for dark matter. I'm not sure which I'd prefer. It would be incredible to have a new particle in our repertoire, especially since its existence would have been predicted by astronomy and not from the standard model of subatomic particles. It would also be fascinating to find out that there is something out there even weirder than WIMPs, which are plenty weird enough.

And while you're thinking on that, chew on this: it's only been in the past few decades that the very biggest thing of all -- the Universe -- has been tied to the smallest things of all -- subatomic particles. The very small and the very big are connected in a fundamental way, and it's only been through science that we've perceived that connection.

Some people like to say that science can't answer questions like why are we here, and what the Universe is all about. I think those people are wrong. By peeking behind the Universe's curtain, we're learning more about it every day. Big questions deserve big answers, and science is up to the task.

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