Gravitational Lensing
Gravitational lensing is a phenomenon when the gravitational pull of a massive galaxy deviates the light coming from a star behind it and acts as a lense by creating the image of the star.
It is a cosmological phenomenon in which the light from an illuminating body such as a star, quasar or galaxy, bends due to the presence of a huge gravitational field such as a massive galaxy on the way. The mass that deflects the light is called the deflector or cluster mass distribution according to Einstein’s theory of relativity. The gravitational lens sometimes creates multiple images. Gravitational lenses have been observed since 1979 when Walsh et al. first discovered a double image quasar in 0957+561.
The gravitational lensing can be classified on the basis of the power of lens to bend the light:
Strong lensing: The strongest bending of light is when the lens is very massive and the source is very close to it. In this case light can take different paths to the observer and the multiple images can be seen. The first example of a double image was found in 1979, of a quasar. If the source varies with time, the number of images can also vary.
Weak lensing: In many cases the lens is not strong enough to make multiple images but the source can still be distorted: both stretched (shear) and magnified (convergence). Lensing can be used to measure mass both dark matter and the luminous matter. Weak lensing is a complementary way to measure luminous masses within the galaxy.
Micro lensing: The bending is so little that multiple images are not seen but still the source appears brighter due to bending of light. Due to this effect, sometimes the sources which are too dull become brighter and visible.
Gravitational lensing helps scientists to see distant objects they could never have hoped of seeing with current available instruments and learn more about young universe. Imagine a quasar at very large distance (several light years) from earth. If a galaxy lies in between earth and quasar and is perfectly symmetric with respect to the line between earth and quasar, we would actually see a ring of quasars. It is called an “Einstein’s ring”. If the galaxy is off-center with respect to the line of earth and quasar then the two light paths would be different distances around the galaxy. This makes the twin images be formed at different distances from the actual quasar. Till now many partial Einstein rings have been discovered.
Published in Science Reporter, March 2008 issue