Astronomically Speaking #5, published in the York University Gazette, February 2000
It’s the semester home-stretch: Reading Week has come and gone, the summer can’t be too far away. That Mountain Equipment Co-Op catalogue with the teal fleece outerwear and kevlar canoes has just been delivered to the front door. A camping trip would be nice …
One of the greatest pleasures of a canoe trip in northern Ontario is to quietly steal away from the evening campfire and set oneself adrift, lie in the bottom of your canoe and gaze into the pristine night sky. If the moon is below the horizon, you see the arc of our own Milky Way galaxy.
The sheer number of stars can be overwhelming.
There are perhaps a hundred billion stars in our rather typical galaxy. And there are a trillion or so galaxies. So, if you do the math ... that's a lot of stars! To imagine how many, think of all the grains of sand on all the beaches on earth.
Well, it’s more than that.
Yes, that’s a lot of stuff — but to Astronomers and Cosmologists, it’s not enough. Many astronomical observations suggest that we can only see a fraction, perhaps just ten percent, of the matter in the Universe. The rest is invisible. Hidden. Ephemeral. We have convincing evidence it’s there, we just don't know where: it’s dark.
For this reason it's been called ‘Shadow’, ‘Missing’, or 'Dark' Matter.
Where do you get evidence for something you can’t see? You look for its effect on the stuff you can see. Though it may seem pretty quiet in the night sky, every single star, every galaxy and every nebula, is moving. Their motion is governed by a single physical law, the law of Gravity, that says every bit of matter attracts every other bit of matter. The way stars and galaxies move depends on the gravitational forces from other stars and other galaxies, as well as any other matter that may be lying about.
You probably have a mental image of our Milky Way Galaxy, a bit like a great stellar swirl, a pinwheel of light. Surrounding the swirl, though, is a giant halo of stars. Astronomers have measured the speed of the stars in this halo, and to their surprise, they find they’re going too fast. Adding up all the mass in all the stars in our galaxy can not create enough gravitational force to explain the high speed of those stars.
On a larger scale, our galaxy itself is also moving. Galaxies are not spread uniformly throughout the universe; they tend to clump together in bigger groups called clusters. Our galaxy is just one of many in our local cluster, known as 'The Local Cluster'.
I know. Not a terribly creative bunch, these astronomers.
Anyway. In the 1980s, researchers were amazed to discover that between the clusters of galaxies are enormous bubble-like areas of nothingness, great voids where no galaxies exist at all. The galaxies within a cluster are all orbiting each other, and if their speed is measured, once again they seem to be going way too fast for the amount of matter nearby.
Now, all this is related to the ultimate fate of the universe. At the current time, almost fourteen billion years after the Big Bang, the universe is still expanding and all the galaxies (or clusters of galaxies) are rushing away from each other. Now, the force of gravity slows this expansion down, and there are three possible fates for the universe. First, if there was an enormous amount of matter in the universe, after the initial expansion everything would collapse back in on itself: The Big Bang followed by The Big Crunch. Conversely, if there was only a little matter, the universe would continue expanding forever. The final result would be a dilute, rather uninteresting universe.
Given with the amount of luminous matter (stuff we can see) in the cosmos, we’d expect the latter scenario. But upon measuring the expansion rate of the universe, we find ourselves precariously perched between cataclysmic collapse and a runaway expansion towards a cold, desolate cosmic darkness. Even at this largest of scales, once again we find evidence for missing matter.
So what is it, this mysterious hidden stuff? That's just it: we don't know. It could be that the universe is filled with ‘failed’ stars that just weren’t massive enough to light up.
Or maybe neutrino mass is the answer. Neutrinos, those ghostly particles that are constantly streaming out of the sun, were also produced in huge quantities in the Big Bang. If neutrinos have mass — an open possibility at this point — then all that extra matter might solve the problem.
For many physicists, the smart money is on a quite exotic suggestion: that galaxies could be shadowed by a strange form of matter that is only perceptible to us though gravity. This 'shadow matter' would tend to clump where our galaxies clump, because of the gravitational attraction with ordinary matter.
Such strange ideas have been suggested by modern theories of particle physics. Which leads to the rather haunting possibility that the eventual fate of the universe is controlled by the stuff of ghosts.