Since the 1930s, astronomers have recognized that the universe contains far more matter than can be explained by all the stars and planets and galaxies that can be observed.
Now, scientists at SLAC, studying a collision of galaxy clusters three billion light years away, have confirmed the existence of the invisible dark matter that gives the universe its gravitational heft.
“We had predicted the existence of dark matter for decades, but now we’ve seen it in action. It’s groundbreaking,” says astrophysicist Marusa Bradac, of Menlo Park and Slovenia.
Dr. Bradac, who is a postdoctoral fellow at the Kavli Institute for Astrophysics and Cosmology (KIPAC) at SLAC, did a computer simulation of the collision of two clusters of galaxies in what is called the “Bullet Cluster,” possibly the largest explosion since the Big Bang launched the universe we know.
“The simulation shows how the collision happened and what happened afterwards,” Dr. Bradac says during an interview at the Kavli Institute, which just opened last spring. “All the gas (and stars) stayed in one place, and the dark matter kept going.”
Weird stuff
Dark matter has proved extraordinarily difficult to pin down, because the evidence of its existence is indirect, Dr. Bradac explains. We know it’s there because clusters of galaxies are far more massive than their visible parts. Yet it doesn’t shine or give off light or heat or any signal we can detect; it interacts with other matter only through gravity.To confirm the existence of dark matter, scientists used the technique of gravitational lenses. These are based on an effect, first discovered by Einstein, that a massive object can bend the light from galaxies behind it. “With gravitational lenses, we can measure the mass that bends the light,” says Dr. Bradac. “We still can’t ‘see’ it because it doesn’t shine. But we can see its effects.”
The latest discovery comes from a huge collaboration of scientists, using four major telescopes, including the Hubble Space Telescope and the orbiting Chandra X-ray Observatory. They mapped the distortions caused by the bullet cluster in the positions of galaxies in the background.
The result showed four separate clumps of matter: two large clumps of dark matter speeding away from the collision, and two smaller clumps of luminous matter trailing in their wake. “I actually measured the gravitational lens effect and discovered where the dark matter is positioned,” says Dr. Bradac.
“These measurements are compelling,” adds KIPAC Director Roger Blandford of Menlo Park. “The direct demonstration that dark matter has the properties inferred on the basis of indirect arguments shows that we are on the right track in our quest to understand the universe.”
Dark matter and beyond
Dark matter is only part of the mysteries of the universe that scientists are probing; past observations have shown that only a very small percentage of the mass in the universe — maybe 5 percent — can be explained by regular matter.“A universe that’s dominated by dark stuff seems preposterous, so we wanted to test whether there were any basic flaws in our thinking,” says Doug Clowe of the University of Arizona, one of the leaders in the study. “We believe these results prove that dark matter exists.”
From there, it gets even more confusing. Dr. Bradac estimates that about 25 percent of the universe is dark matter, 5 percent is luminous matter, and 70 percent is dark energy.
“Dark energy is even more mysterious,” Dr. Bradac says. She refers to theories suggesting that dark energy is a form of material that exerts negative pressure that causes the universe to expand faster and faster, rather than collapsing.
This kind of mind-stretching thinking makes Dr. Bradac love her time at SLAC. She comes from Slovenia, part of the old Yugoslavia, and earned her Ph.D. in astrophysics from the University of Bonn in Germany. She is here on a three-year fellowship.
“It’s so exciting here,” she says, especially the interface with scientists studying such varied subjects. She hopes the particle physicists, working with SLAC’s accelerators, will be able to detect some effects of dark matter.
“Dark matter particles barely see each other; they interact only gravitationally,” Dr. Bradac says. “We tell particle physicists about properties they are trying to detect. The accelerator people will be trying to detect dark matter particles.
“Bridging the gap between particle physicists and astrophysicists — that’s great.”
