Friday, Jan. 27, 2012
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 Physics in a Nutshell

# Dark matter: Are you serious?

 This painting of the Milky Way galaxy is really quite misleading. Scientists believe that this beautiful spectacle makes up only a fraction of the mass of the galaxy.

We scientists have good reason to seriously believe that there is a totally invisible kind of matter in the universe that is about five times more common than the matter with which we are familiar.

As an analogy, that's like inviting a friend over for dinner with you and believing he brings five other invisible friends. Claims like this can get you committed.

On the other hand, there is strong reason to believe that we haven't lost our minds. The simplest evidence for the existence of dark matter comes from studying how fast galaxies rotate. The essential idea is identical to the following simple experiment: Tie a bucket containing sand to a string, and spin the bucket around. The string keeps the bucket moving in a circular path. If you spin too fast, the force will overcome the strength of the string. The string will break and sand will fly everywhere.

The same thing happens with galaxies. The gravity of the galaxy plays the role of the string, making the stars in the outskirts of the galaxy orbit in circular paths. If the stars move too fast, gravity isn't strong enough to keep the stars in their orbit and the galaxy would get ripped apart.

We measure the speed of stars as the galaxy spins and can estimate the amount of the mass in the galaxy by looking at the brightness of the galaxy and working out how many stars must be there. The result? Galaxies spin much faster than can be accounted for by the visible mass. These galaxies should tear themselves apart, but they don't.

The whole idea is that simple. We're forced to conclude one of a handful of options is true. First, perhaps we don't understand gravity as well as we thought. Second, perhaps our understanding of inertia, which governs how hard the star tugs on the metaphorical string of gravitation, is incomplete. The third option is that perhaps there is more matter in the galaxy than we think.

A recent observation of the collision of two clusters of galaxies, much like two flocks of ducks passing through one another, gave the strongest evidence so far that the third option was correct. It appears that a form of matter exists that experiences gravitation but doesn't give off light. This form of matter is called dark matter.

We don't know what dark matter is, but a combination of observations and calculations on how galaxies form gives a few hints. This matter is likely to be a bunch of subatomic particles surrounding the galaxies and spanning the entire cosmos. This invisible matter weighs about five times more than the matter we can see with our optical and radio telescopes.

We don't know the nature of this subatomic matter; however, we have some ideas of how we might make this matter in particle colliders like the Tevatron and the LHC. In separate approaches, at Fermilab and across the world, we're building detectors that might be able to directly observe the dark matter thought to permeate our entire galaxy.

Dark matter is one of the leading physics mysteries of the 21st century. We've explored the options carefully, engaging in a vigorous debate throughout our studies. It's a fascinating example of science in action, and I can only begin to scratch the surface of explanation here. The prospects for unraveling this interesting intellectual conundrum sometime soon are fairly high. This stuff is totally cool...

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—Don Lincoln