Dark matter detected?

The entire universe in blog form
April 21 2008 9:57 AM

Dark matter detected?

I'm not keeping up with all my email too easily here in the UK -- I've been on the road quite a bit -- but this one I have to jump on right away. A team of researchers in Italy are claiming to have directly detected dark matter particles.

Short background: we know dark matter exists. We also know it must be made up of particles that are very difficult to detect (or else we'd see them, of course). A leading contender are WIMPs, or weakly interacting massive particles, specifically a weird particle called an axion. We still have not directly detected and identified an axion in any particle experiments, though LHC (and more on that later, of course!) may bag one soon.

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Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death from the Skies! Follow him on Twitter.


DM particles form a cloud around and throughout the Milky Way. Although they don't interact with normal matter terribly well (hence the weakly interacting part of their name) they do sometimes slam into normal matter. You can build a detector to look for that interaction -- it would make little flashes of light -- but again you need to look very carefully.

The Italian experiment looked very carefully. One of the things they looked for was a modulation in the signal, a change over time. They wanted to see the number of flashes of light from DM hitting normal matter go up and down by a few percent, with a maximum in June and a minimum number in December. Why?

Imagine you are in a car, driving through a cloud of bugs. If you hold your hand out the window, a bunch of bugs will hit it (ewwww). Now if you throw a ball out the window into the direction of the car's motion, it will hit more bugs, because it's moving into the cloud faster. If you throw a ball behind you, then the ball will be moving slower relative to the bugs, and fewer bugs will hit the ball.

A similar thing is happening with the Earth. The whole solar system is orbiting the center of the Milky Way at about 250 kilometers per second. But the Earth is also orbiting the Sun. When the Earth is at one part of its orbit, its velocity (30 kps) adds to that of the solar system, but six months later it's headed the other way, and its velocity subtracts.

If the Earth is slamming into dark matter particles, then we should hit more when the Earth and solar system velocities are in the same direction, and hit fewer when the Earth is moving in the opposite direction of the solar system as a whole six months later. So not only should we see the number of hits go up and down every six months, but that oscillation must line up with the correct dates (June for the former, and December for the latter).

That is precisely what the Italian team found. Here are their results:

This is a little weird, so let me explain! The horizontal axis is time, measured in days. For the first 2700 days or so, they used an experiment that could detect the flashes, but after about 3200 days switched to an experiment that was a lot more sensitive. So you can see the data points (the gray circles) in the first half of the graph are a little ratty, but in the second half are much smoother. They then fit a smooth sine wave to the data points, to get the solid wavy line. The dashed lines are where the signal is at its max, and the dotted lines where it's at its min. The number of particles detected goes up and down once per year, as predicted, and the max is also on June 2, right when the Earth is "facing into the wind", so to speak.

Holy Haleakala.

I have to say, this is an incredible graph, and an incredible result. It needs to be confirmed independently, but it's already pretty convincing! The modulation of the signal is very strong (they are confident of it to the 99.99% level) and the phase -- its alignment in time -- is right on the money.

Now, this may not mean it's dark matter. It is certainly something interstellar extraterrestrial, though! The phase and alignment are too perfect to be anything else, I think. But dark matter is a very likely candidate.

I'll need to see what others in the field think of this (I happen to have Brian Cox handy, but I'll have to ask him later about this since he's busy). But it looks really solid. If true, the next step is to use even more sensitive detectors, and ones that can distinguish between different types of particles. In the meantime, as other detectors come online (like one at LHC meant to look specifically for axions), we are heading ever closer to solving one of the biggest mysteries in astronomy, if not in all of science. Dark matter makes up 23% of the stuff of the Universe, and 80% of all matter is dark matter. What the heck is it?

Soon enough, we may know.

Update: Dagnappit, I should read the other science blogs before writing mine! Cosmic Variance has this story as well, and is also skeptical of the result being dark matter, and also agrees that the modulation is real and probably something interesting. However, he's a bit more forceful about it than I am.