My Top 10 Science Stories of 2006 – #1: Dark Matter in the Bullet Cluster



Dark Matter Really Exists

What’s wrong with this picture?

By Davide Castelvecchi

The universe has been hiding stuff from us. We know there has to be a lot more in the sky than we can see: Astronomers have calculated that the known matter — mostly stars and interstellar gas — should only be about one-seventh of the universe’s matter. No one has yet managed to catch any of the invisible, or “dark,” matter, let alone make it in the lab, and skeptics have wondered whether it exists at all. But when a new discovery was announced last summer, it finally felt like the universe had to stop pretending. Suddenly, it was, like, come on!

If no one has seen dark matter, why did anyone come up with the idea in the first place? It was because of some very odd things going on in the sky. Take stars. In most galaxies, stars move too fast — so fast you would think they’d literally fly off on a tangent. To explain this, astronomers guessed that galaxies are in fact more massive than they look, so they exert a stronger gravitational pull on stars. The added mass would come from yet-undiscovered kinds of particles that are invisible and can zip through stars and planets like a ghost through a wall. The nature of the dark-matter particles has long been regarded as one of the biggest mysteries in physics.

But not everyone was convinced. If galaxies’ gravitational pull is so strong, a few skeptics reasoned, could it be that we don’t understand gravity itself? After all, we expect galaxies to exert a certain pull based on physics equations that we have only tested directly at much smaller scales. For all we can see, on the scales of galaxies and clusters of galaxies, those equations seem to need some mending. So let’s try that, instead of coming up with contrived explanations based on hypothetical, invisible stuff. Until this August, that skeptical stance seemed perfectly reasonable to me. But then, something changed.

Enter the bullet cluster, a cluster of galaxies about 4 bllion light years away. The bullet is really two clusters, caught dashing away from each other after having flown trough each other head-on like swarms of fruit flies. During the two clusters’ encounter, most of the stars in each of their galaxies just kept going their way. But the galaxies’ interstellar and intergalactic gas did not. The two clusters swept each other clean.

The bullet cluster. In red: X-ray imaging of intergalactic gas, where most of the luminous matter is. In blue: the stretching of space estimated from gravitational lensing. Credit: NASA/CXC/CfA/STScI/Magellan/U.Arizona

Using a NASA X-ray telescope, a team of astrophysicists led by Douglas Clowe, of the University of Arizona in Tucson, imaged the clouds of gas left behind by the two clusters (in red in the picture). They estimated that these mounds of cosmic debris outweigh the stars in the two clusters by 5 to 1. So you would think that this spring cleaning would have made the two clusters look a lot lighter. But when the team calculated their masses using a separate method, they found otherwise. The clusters still appeared a lot more massive than the clouds of gas they left behind.

The team figured that out by measuring how the clusters distort the fabric of space, an effect predicted by Einstein’s general relativity. This effect, called gravitational lensing, is similar to how the fabric of a hammock stretches when someone sits on it. We can’t directly see the canvas of space, but the stretching makes galaxies behind the two clusters look slightly warped. Team member Marusa Bradac, of Stanford’s Kavli Institute for Particle Astrophysics and Cosmology, wrote software that analyzed the distortions in the images of hundreds of foreground galaxies; she calculated that the distortions around the galaxy clusters (pictured in blue) were a lot more pronounced than those around the displaced clouds of gas.

From the mischievous universe’s point of view, the evidence was embarrassing. It was trying to hide a huge amount of dark matter, but we saw a bulge where the matter was sitting. “Gravitational lensing is great,” Bradac said at a talk last November, when I was visiting Stanford. “It doesn’t care whether matter is luminous [as opposed to dark] or not.”

A computer simulation of the collision of clusters by Stanford graduate student John Wise

While Clowe’s team can’t quite claim to have observed dark matter directly, alternatives to it now seem in very bad shape. Any theory that replaces the known equations of gravity would have to predict the distortion of space, but explain why cleaned-up galaxy clusters somehow would distort space less than the more-massive heaps of intergalactic gas they left behind.

The reasoning behind this result seems complicated, but here’s the basic idea. Suppose you are lying down in your backyard, underneath a hammock. Overhead, your grandmother is sitting on the hammock, with a little girl on her lap. Their weight stretches the hammock under Granny’s butt.

Now Grandma picks up the little girl and lays her to her side. The girl’s weight makes a second bulge in the hammock. Surprisingly, the weight seems to have mostly shifted to where the girl is. The bulge under your grandma’s butt has become a lot less pronounced, while the little girl’s butt makes a deeper one. At this point, you would guess, the simplest explanation is that the girl is holding something in her lap, something really heavy — in fact, even heavier than Grandma herself.

In most galaxies, we observe the combined masses of a heavier old lady (interstellar gas) with a small girl on her lap (the stars) plus an added, invisible object more massive than the rest (dark matter). In the bullet cluster, the grandmother and the girl are separated, but the hammock (the fabric of space) shows that most of the original mass was indeed dark matter.

The team’s results also tell us some very substantial information about the seemingly insubstantial stuff: The dark matter doesn’t interact with itself. The two clouds of dark matter kept flying in their trajectories, instead of slowing down through friction like the intergalactic gas. In other words, there were hardly any collisions between particles of dark matter belonging to the two clusters. Dark matter, of course, doesn’t interact with regular matter either, or as Sean Carroll once put it, “Most of the universe can’t even be bothered to interact with you.”

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