Gravitational lenses are a fascinating phenomenon predicted by Einstein’s theory of general relativity. When a massive object (such as a galaxy) lies between us and a distant light source, its gravity bends the light from the source, acting like a giant magnifying glass. This produces an image of the background source that can be magnified, distorted, or even duplicated. Gravitational lensing is one of the most powerful tools in astronomy, providing insights into everything from the nature of dark matter to the size and shape of the universe itself.
The first gravitational lens was discovered in 1979, when astronomers observed multiple images of a distant quasar (a bright point-like object powered by an accreting supermassive black hole). The quasar appeared to be surrounded by four distinct images, arranged in a perfect square. This “Einstein Cross” quasar became one of the most famous astronomical objects, and helped confirm that gravitational lensing was real.
Since then, astronomers have used gravitational lenses to study all sorts of astronomical phenomena. One particularly important application is using them as “cosmic telescopes” to study very distant objects that would otherwise be too faint to observe directly. For example, by studying gravitationally-lensed quasars we have been able to measure the size and shape of our universe with unprecedented accuracy. We have also used them to discover some of the most distant galaxies ever seen and to study how these galaxies evolve over time.
Gravitational lenses can also be used to probe the distribution of dark matter in galaxies and clusters of galaxies. Dark matter is an invisible form of matter that makes up about 27% of all mass in our universe; it does not emit or absorb light, so we cannot see it directly with telescopes. However, its gravity affects how visible matter behaves; for example, it determines how fast stars orbit around galactic centers and how strongly galaxies are clustered together on large scales. By measuring these effects using gravitational lensing techniques we can indirectly map out where dark matter is located within cosmic structures like galaxies and clusters