October 17, 2013
An international team of astronomers has found the most distant gravitational lens yet — a galaxy that, as predicted by Albert Einstein’s general theory of relativity, deflects and intensifies the light of an even more distant object. The discovery provides a rare opportunity to directly measure the mass of a distant galaxy. But it also poses a mystery: lenses of this kind should be exceedingly rare. Given this and other recent finds, astronomers either have been phenomenally lucky — or, more likely, they have underestimated substantially the number of small, very young galaxies in the early Universe.
Light is affected by gravity, and light passing a distant galaxy will be deflected as a result. Since the first find in 1979, numerous such gravitational lenses have been discovered. In addition to providing tests of Einstein’s theory of general relativity, gravitational lenses have proved to be valuable tools. Notably, one can determine the mass of the matter that is bending the light — including the mass of the still-enigmatic dark matter, which does not emit or absorb light and can only be detected via its gravitational effects. The lens also magnifies the background light source, acting as a “natural telescope” that allows astronomers a more detailed look at distant galaxies than is normally possible.