North Hawaii News Articles from CFHT

Astronomers use CFHT to reveal Dark Matter

In 1915, Albert Einstein produced the third of his three major contributions to theoretical physics: the General Theory of Relativity. The main purpose of this theory was to improve the explaination of how gravity works. The previous theory, that of Isaac Newton, explained gravity as a force between two objects, and worked very well for most situations. However, by the end of the 1800s, it was possible to measure planetary motions with enough precision to demonstrate small inaccuracies in certain situations. Einstein's new theory explained gravity not as a force, but as a distortion of space and time. This theory had a certain mathematical beauty, and more importantly, it explained the discrepancies that had been found. To this day, General Relativity has passed even the most stringent measurements made.

One of the interesting side-products of Einstein's new theory was the suggestion that even light is subject to the effects of gravity. So, for example, if light from a distant star passes close to the sun, the gravity of the sun will bend the path of the light like a lens. If you could watch the star as the sun passes nearby, it will appear to move slightly. Careful measurements of the star's position will tell you that it appears to have moved.

One of the first important tests of General Relativity came just a few years later, in 1919. Astronomers travelled to the island of Principe, off the West coast of Africa, to measure stars in the vicinity of the sun during a total eclipse (the only time it is possible to see stars close to the sun!). Their measurements showed that the stars appeared to have been moved, by exactly the amount predicted by General Relativity. The conclusion: gravity bends light like a lens bends light.

Just 14 years after this triumph of General Relativity, a Swiss astronomer named Fritz Zwicky made some measurements that had important implications related to the topic of gravity. He was studying a group of galaxies called the Coma Cluster. The galaxies in this group orbit each other in paths that are determined by the total mass of the cluster. Zwicky measured the velocities of these galaxies and found that they moved much quicker than expected. The implication was that there was much more mass in the cluster than expected.

Astronomers had long assumed that the stars and gas in a galaxy, or a group of galaxies, accounted for essentially all of the mass. It is fairly easy to determine the mass of stars, and then derive the total mass of a galaxy from the total number of stars you see (or more accurately, from the total amount of light emitted by all of those stars). In other words, at that time, astronomers assumed that the mass of a galaxy was due to objects we could see and study. Zwicky's observations gave the first hint that this is not the case.

This important discovery was almost completely ignored for several decades, because astronomers assumed that they just were not doing a good job of accounting for the mass of the stars and gas. However, in 1970, Vera Rubin and Holland Ford, two American astronomers, brought the problem to the forefront. They proved conclusively that the mass discrepancy could not be simply due to fainter stars -- the extra mass had to be due to something else. Astronomers have coined the term 'Dark Matter' to describe the missing mass. In the years since, observations have shown ever more strongly that there is Dark Matter and that there is lots of it: at least 90% of the universe must be invisible.

However, exactly what that 'Dark Matter' might be has proven very elusive. There are some strong reasons to believe that a large fraction of the Dark Matter is not even what we consider 'normal' matter - the electrons, protons and neutrons that make up everything we can touch or see. It might be some kind of unusual, hitherto undetected sub-atomic particle, or it might be that certain kinds of particles we already know about have more mass than we thought. Or it might be small black-holes that weigh less than the Earth, formed at the beginning of the Universe. And these are just some of the less exotic suggestions that scientists have proposed! In fact, it is not even clear where the Dark Matter is located. We know that 'Light Matter' is grouped together in galaxies, which are in turn grouped in clusters of galaxies and chains of clusters. It could be that the 'Dark Matter' is at the same places, mixed-up with the stars and galaxies we see. Or, it could be filling the voids between galaxies. There could even be dark galaxy-sized clumps of Dark Matter that we can't see. Until now!

Just recently, a team of astronomers headed by Yannick Mellier have used the Canada-France-Hawaii Telescope and the new CFH12K camera to measure for the first time the distribution of Dark Matter on very large scales in the universe. (see here for their web page) They did this by using the fact we discussed above, that gravity bends light. The idea goes like this: Imagine a very distant galaxy. If there is a large clump of Dark Matter between us and the galaxy, the light from that galaxy will be bent, just slightly, by the gravity of the intervening Dark Matter. As a result, the image of the galaxy will be slightly distorted. This is a very weak effect - in fact, it has the name 'Weak Lensing'. It takes very accurate measurements of thousands of galaxies to demonstrate this effect. Several teams around the world have been trying for the past few years to measure a sufficiently large number of galaxies sufficiently well. The results recently demonstrated by Yannick Mellier and his collaborators are the first to show this weak lensing effect in a convincing way.

The team headed by Yannick Mellier have succeeded in large part because of a new camera, the largest of its kind, which was installed at the Canada-France-Hawaii Telescope in January 1999. With this camera, call CFH12K, the astronomers have measured 200,000 galaxies in a large area of the sky to show the consistent distortions that result from the weak lensing due to the Dark Matter between us and those very distant galaxies. The other crucial ingredient in this group's success is the excellent observing environment of Mauna Kea. Mauna Kea consistently provides astronomers with some of the best images of the sky, with the least distortion from the atmosphere. Only with the high-quality images possible from Mauna Kea could Mellier and his collaborators measure the galaxy shapes carefully enough to detect the weak lensing effect. This work, and more like it to come, will hopefully allow astronomers to unravel the mystery of Dark Matter.

Eugene Magnier