Cosmological lenses
Strange as it may sound, gravitational lensing elsewhere in the cosmos was mostly ignored for decades. Only in 1979 and as a consequence of the strange behavior of a quasar by the unremarkable name of
0957+561 did it come back to the limelight. Quasars are now known to be extremely distant galaxies harboring supermassive black holes at their cores as usual, but in a feeding frenzy thus pouring gigantic amounts of radiation visible from afar. What happened was a pair of strikingly similar quasars proved to be in fact a double image of the same object, one resulting from the straight path taken by photons from source to observer, and the second resulting from photons which originally streamed slightly offset in angle with respect to the former, but whose path was deflected by the gravitational influence of a closer object, in this case a massive galaxy. From the Earth vantage point, the illusion was of a second source located in an offset position resulting from the extension of arriving light rays straight away instead of bending, just as we see the sky as reflected by a water body and below the horizon in the case of a mirage. In a mirage, light rays are bent by the gradient in index of refraction caused in time by the gradient in air temperature induced by hot sunlit surfaces like sand or concrete.
That the images belonged to the same object was deduced from the fact variations in the intensity (quasars usually exhibit variable light output) matched each other except for a time delay, the time it took light to travel the longer distorted path.
Several new spectacular examples have been found ever since, being the Hubble Space Telescope with its exquisite, deep sight, one of the most prolific explorers.
Gravitational lensing does not merely reduce to generate cloned images from the same object. Depending on the distribution and optical geometric arrangement, it also serves to amplify or enhance images of objects that due to their intrinsic brightness and distance should probably remain undetected with current technology. This is how galaxy Abell 1835 IR 1916 was recently discovered, at a red shift (distance) so great it could not possibly have been observed. This clump one hundredth the size of the Milky Way was detected because it was undergoing a brisk starbirth phase, as much for being behind closer Abell 1835 galaxy cluster to act as a handy natural telescope. We are seeing Abell 1835 IR 1916 as things were when the universe was about 3.5% its current age.
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