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