A Crash Course on Dark Matter

Have you ever wondered how we can calculate the mass of the Sun? We of course can’t go there and put it on a scale. So how on Earth do we find out?

The answer lies in Newton’s law of gravitation. According to it, the more mass an object has, the more it pulls others around it; the farther these are, the weaker the pull. If we want to know the Sun’s mass, then, all we need to find out is the force it exerts on some planet of known mass.

Now, how do we find that force? This part is a little trickier, but it can be explained using an example from everyday life. Have you ever spun a yoyo around? If you have, you’ll have realized that the faster it spins, the more force you need to do to keep it from flying off. In fact, there is a precise mathematical formula that relates the force to the speed, so that if you know one, you can find out the other.sun

So how do we find the mass of the Sun? We measure the speed at which some planet is spinning around it and use that to find the force binding both; from that, we calculate the mass of our star. Ta-dah!

This extremely simple technique can be put to good use for calculating other stuff besides the mass of the Sun. For example, we know the distance between the Sun and the center of our galaxy and we know how fast we are moving around it. Knowing this, we can estimate the total mass of our galaxy.

Of course, that is not the only way. Another possibility is to just count the amount of stars we see and multiply that number by the average mass of a star.

What’s funny is that, if you use both methods and then compare, you get different results. But not just a little different: you find out that, if you just count stars, your estimate is wrong by 100%!

English: The contents of the Universe as measu...

English: The contents of the Universe as measured with WMAP and computed by NASA/WMAP Science team. (Photo credit: Wikipedia)

Physicists, however, being the resourceful creatures they are, are not daunted by this. Instead, they put their analytical minds to work and start deducing what they can from the information they have. For example:

  • There must be some kind of particle we’re not seeing.
  • This particle has mass, since it pulls things gravitationally.
  • This particle does not interact with light or we’d be seeing some trace of it. No interaction with light means no electromagnetic interaction.
  • This particle must be absolutely everywhere and distributed relatively evenly. Otherwise we’d see unexplained movements towards “hot spots” instead of a general increase in the gravitational pull.

These new theorized particles cannot be seen and therefore are “dark.” Hence the name “dark matter.”

The idea of dark matter was first suggested 80 years ago by Jan Oort, but things didn’t advance much until recently, when people started building detectors hoping to catch one of these critters in the act. In the last 20 years these have sprouted like mushrooms: we have DAMA/LIBRA from Italy, CoGeNT, CDMS, Xenon-10, Xenon-100 and CREST. The last player in the game is LUX, who is the protagonist of today’s story.

But a little background first.

Dark matter detection is a relatively straightforward matter (no pun intended): even though dark matter particles are not subject to the electromagnetic interaction, they are still capable of hitting a nucleus and making it recoil. The idea, then, is simple: get a bunch of atoms, wait for one of them to be hit by something you can’t readily explain, count the occurrences and be done. Of course, in reality things are never that simple, since there are many sources of noise, so that separating actual recoils from random sources is more than a little hard.

However, it so happens that, since more than 10 years ago, DAMA/LIBRA has been observing a yearly fluctuation in their number of recoils. Their explanation for it is that they have detected a dark matter particle with a mass around 10 GeV. When they announced this, of course, nobody believed them. That is, until CoGeNT, which had a much smaller background noise, detected exactly the same. After that, people started getting excited: did we finally get a glimpse of the elusive dark matter particle?

English: CDMS (cryogenic dark matter search) p...

English: CDMS (cryogenic dark matter search) parameter space exclusion of DAMA result represented in green parameter space. (Photo credit: Wikipedia)

It turns out that no, we didn’t. LUX, a state-of-the-art detector in the US, just released a paper that completely eradicates any possibility that what DAMA and CoGeNT observed is actually a dark matter signal. The result is we’re more confused than ever, since nobody seems to have a clue about what’s causing the yearly fluctuation, but it’s certainly not dark matter.

That’s physics: new data comes in, people get excited and, at the end, the most boring scenario is usually the case. Surprises are few and far between but, when they come, are they worth it.

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9 thoughts on “A Crash Course on Dark Matter

    1. David Yerle Post author

      Really? I didn’t detect it… oh well. Yes, actually most physicists were convinced we were on the brink of finding the bloody dark matter particle and then LUX just came (two weeks ago or so) and ruined things for everyone. Oh well. It’s the Higgs Boson all over again…

  1. elkement

    Great concise summary! An off-topic question: Do you actually plan the word count of your posts in advance and/or cut a lot? Your posts always have the right length! (I can never tame my 2.500 word monsters…)

    1. David Yerle Post author

      Hi Elke, sorry for the late reply! (I’m apologising to everyone…) Well, what I do is write my posts using Microsoft Word. This way I can keep an eye on the length. My goal is usually to keep them around 1-2 pages, so when I reach one page I know that I need to cut them short. I normally don’t to a lot of trimming, but I do a lot of proof-reading and re-writing. Even so, I still miss plenty.


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