The Huge Surprise: Dark Matter
For all of the answers found on Mauna Kea, new questions keep arising. “One of the exciting things about astronomy,” says Kudritzki, “is that you find things that surprise you. And you have many times an observed fact, but you don’t have an explanation.” In other words, it’s perfectly common to discover something and not have the faintest idea what it is. Such is the case with dark matter.
Astronomers are making numerous
fundamental new detections using the
telescopes on Mauna Kea. Spiral galaxies
led astronomers to the discovery of "dark
matter," a mysterious mass that makes up
some 90 percent of the universe.
Photo: Canada-France-Hawaii Telescope
The first suggestions of its existence came with Fritz Zwicky in the 1930s and Vera Rubin in the 1960s. They looked at the huge velocities and centrifugal forces pushing and ripping at galaxies and wondered why the galaxies didn’t just come apart. Could it be, they theorized, that there was some hidden matter holding them together?
It was a controversial idea. If it were true, astronomers reasoned, then one way to prove it would be to look at the movement of light. Matter, Einstein had proved, warps light because everything with mass has a gravitational pull. When light approaches matter, it bends in response to that pull, an effect astronomers call “gravitational lensing.” They started looking for it.
At the same time, in the 1980s, astronomers at Mauna Kea’s Canada-France-Hawai‘i Telescope (CFHT) were using the telescope’s wide-field imaging camera to do a comprehensive deep survey of massive clusters of galaxies. And they found … gravitational lensing. They watched the way the light from background galaxies was deformed into a thin arc by a cluster of galaxies in the foreground, knowing it was impossible that the visible matter in those foreground galaxies had sufficient mass to bend light to such an extent. Looking at those stretched arcs, the astronomers realized that they were literally in the dark.
“What we know now is that when you look at a picture of a galaxy, what you see—the visible matter—is only 10 percent of the real matter in the universe,” says Kudritzki. “The other 90 percent is ‘dark matter.’ Here we are, doing astronomy over hundreds of years, very proud of our discoveries, and then we had to learn that we missed 90 percent. Dark matter is the dominant force that keeps the universe together. It’s everywhere.”
But what is it? Nobody has a clue.
“Dark matter is just an artificial term,” says Armandroff. “It’s matter that’s there that doesn’t really interact much with the baryonic matter we’re familiar with: protons, electrons, neutrons. There are theories that explain dark matter, but we don’t know what particle it is yet. That’s a major area of astronomy and physics right now. Could dark matter be neutrinos, subatomic particles that don’t interact with much? It doesn’t look like it. Keck is trying to get clues from the way the dark matter is distributed, but we can’t do it alone. It’s going to take particle physics, really. Astronomy and physics are intersecting very strongly right now.”
“Without astronomers, physicists wouldn’t have a job,” laughs Kudritzki. “We make the detections, and they build the accelerators and start the theories.”