Dark matter: imagining the invisible

Ken Tapping's astronomy column from the Dominion Astrophysical Observatory

Even today, my favourite science fiction movie is “Forbidden Planet”. This old movie, dating back to the 1950’s has probably stood the test of time because it is based upon a good solid source, Shakespeare’s play “The Tempest”.

One of the principal characters in the movie is a monster of almost infinite power, and which also happens to be invisible. We only sense its presence by seeing its footprints appear in the ground and seeing things being knocked over as it passes by.  This evidence is enough to convince us that something is there even though we cannot see it and don’t know what it is.

That is pretty well the situation with dark matter. It is totally invisible; we only (know) it’s there because of its gravitational tugging at the things we can see: the stars and huge clouds of gas and dust making up galaxies. From the extent of this tugging we know there is a lot of this invisible material: roughly five times as much dark matter as “ordinary matter” – the stuff we’re made of. It plays a major role in the workings of galaxies and the universe as a whole, and we can’t see it. That is, until recently, when a team of astronomers used the Canada France Hawaii Telescope to make detailed maps of the observable effects of the dark matter. They then used them to image the things causing them.

One of the predictions made in Einstein’s “Theory of Relativity” is that light is deflected by gravity, just as it is bent by a prism or focussed by a lens. For small things like us, or the Earth for that matter, the effects are tiny, but for large things like galaxies or huge clumps of dark matter, the effects are easy to detect. This process is used in what has become known as “gravitational lensing”.

The gravitational field of a large lump of matter, like a galaxy, acts like a lens, focussing the light of objects lying beyond it. Essentially it gives us a rather crude telescope with an objective lens the size of a galaxy. We have used this technique to look at objects lying far beyond our galactic “lens”. Alternatively we can search the sky for “images” formed by gravitational lenses due to objects we cannot see, like lumps of dark matter.

The Canada France Hawaii Telescope is located on top of mount Mauna Kea, in Hawaii, where the observing conditions are often excellent. This made it possible for the astronomers to record high quality images. They recorded four images over a year, each covering an area of sky equivalent to the amount of sky your hand would cover when held at arm’s length. These images captured about ten million galaxies. On its way to us, their light was bent and focussed by lumps of dark matter. The images show dark matter forming lumps and threads, with voids that look empty.

This important discovery was made using an instrument that came into service during the 1970’s, which makes it venerable by the standards of modern scientific research. However, technical improvements and upgrades are making it possible to take this inherently good telescope and keep it on the research front line for a long time to come.

Jupiter dominates the southern sky during the night. Venus, brighter and whiter, is spectacular in the Southwest after sunset. Mars rises around 10 p.m., Saturn at1a.m. The Moon will be new on the 22nd.

Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton.