Astronomers believe that 90% of our universe is composed of dark matter, a mysterious substance that we can’t directly see. Add to that dark energy, a force that seems to be accelerating the expansion of the universe and the picture you get is of a very strange universe much of which we don’t currently understand. To scientists such as Dr Stefan Keller of the Research School of Astronomy and Astrophysics, this is precisely what makes the present era so interesting. “This is a really exciting time in astronomy and one of the key things in understanding phenomena such as dark matter and dark energy is now being able to study the transient universe. By that I mean phenomena like supernovae and gamma-ray bursts that last a few days or even a few seconds.”
Of course one of the great difficulties in studying such transient phenomena is knowing when and where to look. For years amateur astronomers have reported novae and supernovae in galaxies they happened upon whilst looking at the stars. However, to catch all such events early and also to include objects too faint and distant to be within the reach of amateur telescopes, a more scientific approach is needed. This is the relatively new domain of the survey telescope. Robotic instruments that constantly map and remap the sky looking for changes.
One such survey telescope is SkyMapper, a newly commissioned 1.3m, f4.7 telescope at the ANU Siding Spring Observatory in NSW. Dr Keller is one of the lead scientists on the project. “SkyMapper will be the first survey telescope to create a detailed digital map of the southern skies,” Dr Keller says, “but the way this instrument is designed makes if far more powerful than a simple robotic camera.”
SkyMapper has six filters that automatically move in and out over its massive CCD. Each filter is designed to select specific wavelengths of light so that in effect the spectral signature of every one of the trillions of stars photographed will also be recorded. This will enable its temperature and spectral type to be logged along with the position and brightness data. “This is one of the really unique and powerful things about SkyMapper, we will not only build up a record of where and how bright each object is at a given time, but also spectral information that may reveal changes that would otherwise be missed.
SkyMapper will photograph an area of sky then do the same again the following night. The next photograph will be a week later, then a few months, then years. The reason for this rather odd sounding logarithmic time scale is that it enables scientists to examine variations on many time scales. For example, optical correlations to gamma-ray bursts may last only seconds, asteroids move over days, novae come and go in weeks and other phenomena such as the movement of stars within our galaxy can be observed over years.
“One of the exciting things for me is the contribution SkyMapper data will make to the study of dark matter,” Dr Keller says, “We know that dark matter exists from simple Keplarian dynamics – in other words by watching the way stars orbit within galaxies we can tell that there’s about ten times as much mass there than we can account for with luminous matter. And we’re pretty confident that the maths is right because it’s exactly the same as that which predicts the motions of planets within our own solar system so accurately.”
But what could this dark matter be? It can’t be something like dust because if the universe were filled with such fine material it would obstruct the starlight. It could be in the form of exotic subatomic particles that posses comparatively large mass but only interact with ordinary matter extremely weakly, if at all. In order to account for all the dark matter we infer, millions of such particles would be pouring through our bodies every second.
An alternative theory would be that dark matter is ordinary matter stuffed into literally dark objects such as black holes and dead stars. Although it wouldn’t be possible to see objects like small black holes or neutron stars directly, their gravity would bend the light of stars they passed in front of. A recent RSAA project known as the MACHO (Massive Compact Halo Object) survey, monitored the light from the millions of stars in the two Magellanic clouds that orbit our own galaxy. The idea was that if our own galaxy had countless massive invisible objects orbiting it they would pass in front of the very dense star fields of the Magellanic clouds. It should then be possible to see the gravitational lensing of each black hole distort the star images as it passed by. However after almost ten years, no such lensing was seen. As part of its routine work, SkyMapper will continue to look for such micro-lensing though at this point, the black hole theory isn’t looking strong.
Investigation of the exotic particle explanation for dark matter is largely down to particle physicists and machines such as the Large Hadron Collider (LHC) near Geneva. “Obviously as scientists we wait for the results before we draw conclusions,” Dr Keller says, “But for what it’s worth, my hunch is that dark matter won’t turn out to be an exotic particle. I’m hoping for something quite unknown and much more exciting – to me at least!”
Another handle on the illusive nature of dark matter may come from watching the motions of stars within our own galaxy. The majority of such stars circle in the vast whirlpool of the spiral arms and core of the galaxy. But there are just a few that don’t and those may be the ones to yield clues about how the galaxy formed and what role the dark matter may have had in that. “There are a class of stars known as the halo population that are amongst the oldest in the galaxy. As their name suggests they exist in a large spherical halo around the galaxy in three dimensions and occasionally their orbits bring them right through the core. If we can get a handle on the distribution and motions of these stars, we’ll be a long way towards better understanding galaxy formation and the dark matter that’s so influential in the process.” Dr Keller explains.
However to study such stars in detail, one must first know which ones of the trillions out there they are. That’s where SkyMapper comes in. Because it can identify the temperature and spectral type of each star, it has no trouble in identifying the residual halo stars by their spectral “age”. Over the coming years, astronomers can then calculate their orbits from the tiny movements known as proper motion, that appear on SkyMapper images taken years apart. Once all this data is fed into computer models, it should give scientists like Dr Keller, vital clues as to the real nature of our galaxy and how it and the universe at large formed. And that may well lead to us finally getting a handle on dark matter.