Discovery of the Double Quasar
of the predictions of Einstein's Theory of General Relativity
is that a massive object will distort the space around it forming
what is called a Gravitational Lens. This causes light
and radio waves from an object behind to follow curved paths
in its vicinity, producing multiple or distorted images of the
same object. In 1972, the Mark IA (now Lovell) telescope made
a survey in which 800 radio sources were discovered. Follow-up
observations were used to find their precise positions and thus
allow optical identifications to be made. One source, 0957+561,
coincided with a close pair of faint blue objects. These were
observed with an optical telescope in 1979 and found to be two
images of the same distant quasar - the first gravitational
lens (left). Since then, with MERLIN playing a key role, many
other lenses have been discovered, and it appears that approximatly
one distant radio source in 500 is split into multiple images
due to lensing by a foreground galaxy. If this galaxy is precisely
aligned with the remote quasar, an "Einstein Ring" can be formed,
several of which have now been observed (below).
lenses can provide a way of measuring the size of the universe.
The paths through space by which we see the multiple images
will be different and the ratio of their lengths can be determined
from the geometry of the image. If the source is variable, one
can determine the difference in path lengths and so calculate
the distance of the quasar. This, combined with its redshift,
leads to a measurement of Hubble's Constant. Values of between
61 and 69 km/s/Mpc are currently being found.
is a collaborative effort led by the Jodrell Bank Observatory
and involving institutes in the USA and the Netherlands. A total
of over 15,000 radio sources have been investigated from which
19 new lenses have, so far, been found.
Einstein first tried to apply his General Theory of Relativity
to what he thought was a static, unchanging universe, he had
to incorporate a term called L, the cosmological constant. This
represents a repulsive force which can overcome gravity at great
distances. When it was realised that the Universe was expanding
this term was no longer required and was soon dismissed by cosmologists.
Einstein said that it had been the "greatest blunder" of his
life! However it is beginning to appear as if this repulsive
force might exist after all, and the CLASS census of the percentage
of remote radio sources that have been gravitationally lensed
will soon tell us where the truth lies!
For more information see the Gravitational Lens Research Group pages
Diagrams showing how multiple images and Einstein Rings are
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