Because of the blurring effect of the atmosphere on optical
telescopes, the astronomers use high resolution radio telescopes - the
Very Large Array in New Mexico and the MERLIN array in the UK (see
the main MERLIN WWW page at
http://www.merlin.ac.uk/) - to pick out gravitational lens
systems. Only about one in every five hundred distant radio sources
(galaxies and quasars) is lensed and so thousands of radio sources
have to be searched to have a good chance of success. The British
team, working together with an international team of colleagues,
have now found thirteen such systems - more than doubling the number
The radio picture produced by MERLIN (Figure
1), which allowed the system to be recognised in the first place,
shows only part of a ring. The reason is that, while the source of r
adio emission is embedded in the distant galaxy, it is not exactly
aligned with the lens galaxy. The ``optical'' picture produced
by the Hubble (Figure 1) is actually in
the infra-red region of the spectrum taken with the NICMOS camera.
The wavelength used is about twice that of red light. The infra-red
emission from the distant galaxy is more extended than the radio
emission. Some of it comes from directly behind the lens galaxy and
hence a complete ring is formed.
Unlike the lenses with which we are familiar, in spectacles for
example, a gravitational lens can produce not one but several images
of a given object; these images may be highly distorted and
magnified. Whereas a conventional glass or plastic lens has a simply
curved shape the analogy with a gravitational lens is a piece of glass
shaped like the base and stem of a wine glass with the bowl cut
off. Even without breaking the glass the ring effect can easily be
seen by tipping the glass and looking at a mark on a piece of paper
(or a table cloth) through the base.
The way in which a gravitional lens produces multiple images,
including the special Einstein ring case, is illustrated in the
explanatory diagram (Figure 2).
Why study gravitational lenses?
By studying this and other gravitational lenses astronomers can not
only measure the masses and shapes of distant galaxies, including any
``dark'' matter which will not show up in the optical or radio
pictures, but also can measure Hubble's constant which is related to
the time elapsed since the Big Bang.
Einstein's ``greatest blunder'' refers to the elusive Cosmological
Constant. This describes the strength of the long-range repulsive
force he introduced into the General Relativity equations in
1916. Other astronomers soon showed, however, that this force was not
needed to explain the properties of the Universe as it was then
known. Einstein ruefully wrote ``away with the cosmological
term''. But like a genie, once released it has proved hard to put away
and many astronomers now invoke the Cosmological Constant to account
for modern observations of the distant universe.
A Universe in which the Cosmological Constant is not identically zero
has different geometrical properties to one governed solely by
gravity. Counting gravitational lenses, in other words counting the
number of lines-of-sight ``blocked'' by intervening galaxies, is
acknowledged to be the best way of measuring the geometry of the
Universe at large distances. By the end of this year we expect to be
able to place the best limit so far on the Cosmological Constant.