PASADENA, CALIFORNIA — The universe has just presented humanity with yet another blow to our collective ego. It's not enough that the earth isn't the center of the universe, or even that we're just so VERY tiny and insignificant in the big picture. No, now it turns out that we (meaning the earth, the sun, and all the stars and galaxies out there) aren't even made of the stuff that makes up over 90 percent of the universe. And just what the vast majority of the universe is made of is a major mystery.
What I'm talking about is called dark matter. The term has been around for a while, but in the last few years, it has taken on a whole new importance. The story begins with the discovery of dark matter which, at first, was only called "dark" because it was stuff that didn't shine like a star. Forty or 50 years ago, the only way astronomers could study external galaxies was to take pictures of them with giant telescopes like Mount Wilson or Palomar. The lovely, visible-light images showed galaxies full of twinkling, glowing stars. And since stars were the only things we could take pictures of, we automatically assumed that they were what made up the majority of a galaxy's mass. Without a lot more thought, astronomers happily began to estimate the masses of galaxies based on the amount of starlight we could focus through our telescopes into our warm, soft, limited eyes.
The first inklings of a problem with this picture started with Vera Rubin. Rubin, among other astronomers, had set out to measure the rotation rates of galaxies. All spiral galaxies, including our own, rotate around their centers. The stars mix and swirl around a little, but, for the most part, everything orbits in the same direction in a slow, stately dance. Rubin hoped to use her observations to better understand the structure and behavior of galaxies, but she hardly expected any major surprises.
But, at the end of her research, instead of understanding galaxies better, Rubin could scarcely explain how galaxies existed at all. According to her measurements, galaxies were spinning too fast. Not just a little too fast, but well over twice what their spin rates should be, given their calculated masses. So, what's the wrong with a few dizzy galaxies? The problem was that the stars should have obtained escape velocity. In other words, the stars were moving so fast that the gravity of the galaxy shouldn't have been able to hold the stars together. Ever seen a clown spin a plate on a stick? Try giving the clown a plate full of peas and you begin to get the idea. The galaxies were spinning so fast that the stars should have been flying off in all directions.
Since galaxies do manage to stay together, Rubin made the straightforward conclusion that they must have more mass than previously thought. Since this mass must be something other than stars, the search was on to find out just what this "missing mass" or "dark matter" really was.
At first the ideas weren't all that strange; there were plenty of things that would be hard to see in a visible-light telescope. Maybe there were vast clouds of dust and gas drifting between the stars. Some astronomers suggested there might be billions of giant planets like Jupiter roaming undetected around the galaxy. Others thought the mass might be hiding inside black holes. But the persistent thorn in astronomer's sides was the sheer amount of stuff that seemed to be hiding. Rubin's observations suggested that we weren't just missing a little matter, but the majority of the mass of the universe.
As technology got better, we began to use invisible light to search for the missing mass. Lots of things don't give off any visible light (including all those things mentioned above), but radiate quite happily in other wavelengths of light. And yes, when we turned our new instruments to the sky, we found lots of stuff we'd overlooked before. Infrared telescopes took pictures of hundreds of giant rogue planets, unassociated with any star or solar system, meandering around the galaxy. Vast clouds of cold dust and gas, in some cases stretching hundreds of thousands of light years farther than the "edge" of galaxies (judging from where the stars stop) have been imaged with radio waves. Even black holes, which by definition don't give off any light, can be sniffed out with sensitive X-ray space telescopes like Chandra.
But even with all the new and wonderful stuff we were finding, we weren't even coming close to resolving the dark matter dilemma. If anything, things were getting worse. Looking farther out into the universe, we began to see more and more evidence that something profoundly strange was going on. Weird, smeared-out galaxies began to appear in the sensitive cameras of the Hubble Space Telescope. Called "gravitational lenses" or "Einstein rings," these ghostly images are caused by the light of distant galaxies being warped and bent by a massive gravitational field (you can see a picture of one at http://hubble.stsci.edu/gallery/showcase/exotica/e5.shtml. These observations floored me when I first saw them. There seem to be concentrations of gravity out there strong enough to warp space itself into a giant lens, making it seem like you're looking at distant galaxies through the bottom of a glass bottle. But the gravity wasn't associated with anything we could see, in any wavelength of light. What's was going on?
It seems that dark matter is nothing as simple and familiar as giant planets or cold dust. The final word on this comes from recent measurements of the microwave background radiation. All of space is immersed in a diffuse, cool bath of microwaves, actually the remnant heat of the Big Bang. Astronomers have been interested in these microwaves for some time, as they show us a picture of the universe as it was about a hundred thousand years after the beginning of the universe (the Big Bang), 15 billion years ago. And we've been able to measure some pretty amazing things. Fluctuations in the microwave background have allowed astronomers to measure the total amount of baryonic matter in the universe. Baryonic matter is a fancy word for regular matter anything made up of atoms (protons, neutrons, etc). Humans, planets, stars, everything we can touch or stand on, taste or smell, is made of atoms. And current measurements suggest that baryonic matter makes up less than 10 percent of the universe.
Think about that for a second. I'm fairly certain that as creatures made of baryonic matter, we couldn't interact with dark matter in any way other than gravity. I don't think you'd be able to touch it, I doubt you could contain it in a physics laboratory. All our laws of physics, all our theories about the beginning and end of the universe have been based on the behavior of baryonic matter. If 90 percent of the universe is in some kind of shadow-form that we can't understand, what effect must that have on the evolution of the universe?
The only thing we know about dark matter is that is exerts gravitational force. Not only does dark matter glue galaxies together, it may have a profound effect on the large-scale structure of galaxy clusters. For years now we've observed that galaxies are not randomly distributed around the sky, but seem to fit into a vast foam-like structure that fills the known universe. This large-scale structure has always been a mystery to us, as the microwave background tells us that matter in the early universe was distributed smoothly and randomly. There just shouldn't have been enough time for the galaxies to form such huge, well-organized structures. But maybe we haven't been looking at the right kind of matter. Perhaps the dark matter in our universe had structure early on, and has been pulling the rest of us baryonic folks into shape.
My final thought on dark matter comes from several recent lectures I've been to at the national astronomy conventions. It's highly theoretical and still pure conjecture, but it makes my hair stand on end.
There are some interesting coincidences between what astronomers are finding out about the distant universe, and what high-energy particle physicists are finding in their laboratories. There's something special about gravity. It doesn't behave like the other forces of nature, and it may exist in more than our three dimensions of space. Dark matter acts like gravity that doesn't have any matter associated with it, and it may be just that. In these higher spatial dimensions, there may be other universes existing in parallel to our own. According to particle physicists, it may be possible for gravity to move through these higher dimensions, binding parallel universes together. The dark matter we observe may really be (and I have to admit I just love things like this) a parallel universe interacting with our own. Or universes. If this turns out to be true, we may some day use measurements of dark matter to produce maps of other universes. And far from being a blow to our egos, dark matter may turn out to be an astoundingly powerful tool in our understanding of what's out there.