General Meeting Report June
Speaker Dr Randall Wayth, University of Melbourne.
Subject “Gravitational Lensing Probes Dark Matter”.

The year 2005 has been declared the world-year-of-physics (WYP2005) and physics societies around the world are presenting major programs of celebration and outreach. This commenced in Australia with the 2005 AIP Congress held at the ANU between 31 January and 4 February with the theme “Physics for the Nation”, and everyone is encouraged to share ideas and resources to make this an outstanding year of celebration. Dr. Randall Wayth’s talk, and the subject of the talk tonight, certainly fits into that category. Chris Johnston, in giving his vote of appreciation at the end of the evening’s talk (commenting on the ubiquitous nature of Dark Matter), made the point succinctly: Certainty in science is a very elusive thing. 100 years after the theory of relativity has been created we are still talking about confirming or disproving it. Not that anyone is seriously talking about abandoning Einstein’s masterpiece in total (many aspects of the theory have been in daily use by science, technology and industry for decades and have been confirmed beyond any doubt), just the extend of it, the predictions the theory makes into the unknown. We know the theory breaks down at the infinitesimal small, the quantum end of matter. Could it also have limits as we approach cosmological infinity?

The bending of light (gravitational lensing) was one of the first confirmed predictions of the General Theory of Relativity and has become today a valuable tool in cosmology. Dr Randall Wayth is using the effect to analyse the shape and composition of giant galaxies and their Dark Matter halos at the edge of the visible Universe. There a foreground galaxy bends the light of a more distant object behind it to form circular rings or ring-segments in the shape of its halo around the centre of mass. Dark Matter (matter that does not emit any measurable form of radiation) is, in one form or another, assumed to make up 25% of the “missing gravitational mass” of the universe. “Missing” meaning here something unseen, undetected, whose presence would explain the odd rotational behaviour of galaxies and galactic clusters that individually appear to move too fast to be held together by gravitational attraction from visible matter alone. Computer simulations of “cold dark matter” have been enormously successful in explaining the large scale structure of the Universe, but make only generic predictions on distribution and density of Dark Matter within and around galaxies. It is important to test those predictions and make them more specific.

How can you see something invisible? Gravity, of course, and that’s what Dr Wayth’s project is: to use gravitational lensing to probe for mass distribution in lens galaxies, looking for concentrations of the elusive Dark Matter in the halos of giant galaxies. Barry Adcock, during question time, brought up the interesting point from Newton’s physics, that is: matter in the form of a hollow sphere does not gravitationally affect a collection of matter inside that sphere, meaning if Dark Matter was mainly concentrated in a halo around galaxies or clusters then the motion of their individual components could not be dominated by it. At this stage various computer models of the lensing galaxy’s composition are used in an effort to match theory to the pictures obtained from the end of the Universe.

Dr.Randall Wayth, when he isn’t lecturing in Astronomy at the Melbourne University, can also be found on world tour with his lovely wife Cathy, and in the Australian Virtual Observatory testing the limits of Wikipedia.