March General Meeting
Speaker Professor Richard Ellis from Caltech
on the subject: Gravitational Lensing, Nature’s Giant Telescopes
Professor Ellis works primarily in observational cosmology, addressing issues related to the nature of the world model, the origin and evolution of galaxies, the growth of large scale structure and the nature and distribution of dark matter. He is enthusiastic about the use of new instruments and observational opportunities when they further the progress in these areas. The list of committees he is involved with reads like a Who’s Who of Astronomy and Astrophysics. The ASV is indeed fortunate to have speakers like Richard Ellis address our meetings on their pet subjects and their cutting edge science.
For a long time Gravitational Lensing was thought to be nothing more than a pipe dream; its effect too feeble to be of any practical use and only of academic interest to astronomers. Anticipated by Newton in 1665, theoretically proposed by Einstein in 1916, experimentally confirmed by Eddington in 1919 and promoted by Fritz Zwicky in 1936, it seemed destined to remain forever just beyond our reach. But lately advances in telescope designs have made possible a dramatic increase in their size and light-gathering power to a stage where they can now be used to investigate cosmological distances at which gravitational lensing becomes effective. After almost 400 years the field of gravitational lensing has finally come of age. It has risen from academic obscurity to become a significant tool in modern cosmology. Segmented mirrors and Adaptive Optics, in which active mechanical and electronic focussing on a known point source (a guide star) in a terrestrial telescope’s line of sight is used to continuously compensate for atmospheric distortions, has now made a picture quality possible that may in time exceed the sub-arcsecond resolutions achieved so far only with the Hubble Space Telescope. (An arcsecond is the equivalent of the width of a human hair seen at a distance of 10metres, or the ability to resolve the two headlights of a car at a distance of 200km). Powerful yellow Lasers are used to create artificial “guide stars” by exiting sodium atoms naturally present in a layer at an altitude of about 90km. Computer crunching power can now handle the massive date collected on the components of three dimensional wedges in space (such as the 2dF Galaxy Redshift Survey) to produce and animate shapes of gravitationally bound collections of galaxies that may act as gravitational lenses for object behind them.
By studying the lines of maximum light deflection within the lensing material, predictions can be made not only of the shape of the light-source behind the lens, but also of the amount and distribution of any Dark Matter contributing gravitationally to the lensing effect. In astrophysics and cosmology, dark matter is not directly observable, it does not emit or reflect enough electromagnetic radiation to be detected directly (such as WIMPs, weakly interacting massive particles), whose presence can be inferred only from gravitational effects on visible matter. Inference of dark matter can explain much of the perplexing galactic behaviour without invoking changes to Newton’s famous Inverse Square Law. Proponent of a rival system, called Modified Newton Dynamics (MOND), claim that the same results can be obtained by assuming that gravity, when it falls below a certain strength, becomes inversely proportional to distance, not to its square.
The first giant arc confirming gravitational lensing was found in the galactic cluster Abell 370 in 1987. The cluster acts as a foreground lens distorting the light from a single galaxy in the background into circular segments. Spectroscopic analysis of the ring segments confirms the common origin. Today these so-called Einstein Rings are a familiar sight to astronomers. While their shape varies with the spatial alignment between the lens and the source, from rings to arcs to spots, all of them enable us to magnify distant parts and secure our first glimpse of the earliest cosmic light sources. Gravitational lensing may be the only precise probe to study the contribution that Dark Matter makes to cosmological evolution, with the additional promise of one day uncovering the nature of Dark Energy.
It was not only the details and the technique of gravitational lensing that Richard Ellis imparted to the audience. The mysteries of the Cosmological Constant, the nature and distribution of Dark Matter, the study of Cosmic Shear, the new generation of giant telescopes and the history of pioneers on whose shoulders today’s achievers stand, all blended together under Professor Ellis’ expert handling into a powerful, informative presentation.
The vote of thanks was given by Ross Berner, Cosmology Section, and the tinkling in the present lent some reality to the mysterious dark matter captivated in the two bottles in the bag. Alfred