Gravity distorts spacetime and so we can see distant objects that would otherwise be very difficult to distinguish here from Earth. This is called the “gravitational lens”, an effect predicted by Einstein and which can be perfectly observed in a new image from the Hubble Space Telescope.
In the center of the image (below) is a shiny, nearly perfect ring; around it, there are four glowing dots, which outline another two central dots that glow golden. What we actually see is an “Einstein ring”: the perfect alignment of the gravitational lens between the Hubble and the image that was captured.
How many galaxies do you see in this ring?Source: ESA / Hubble and NASA, T. Treu; J. Schmidt / Reproduction.
But the reality is a little different from what we observe, due to the phenomenon of the gravitational lens: the bright spots are not six galaxies, but three: two in the center of the ring and a quasar behind them. The distorted and magnified light from the quasar as it passes through the gravitational field of the central galaxies creates the ring with four bright dots around the central galaxies.
As the mass of the two foreground galaxies is very high, this causes a gravitational curvature of spacetime around the pair. Any light traveling through this spacetime follows this curvature and enters the telescopes smudged and distorted — but also magnified.
The gravitational lens was suggested by Isaac Newton in 1704 and quantified by Albert Einstein in 1936. It is a very useful tool for probing the far and near ends of the Universe: anything with enough mass can function as a gravitational lens. It could be two galaxies, as observed by Hubble, or huge clusters of galaxies, which produce a wonderful jumble of light spots from the many objects located behind them.
But that’s not all gravitational lenses can do. The strength of such a lens depends on the curvature of the gravitational field, which is directly related to the mass around it. Thus, gravitational lenses can allow us to weigh galaxies and galaxy clusters, which can help find and map the universe’s dark matter — the mysterious and invisible source of mass that generates additional gravity and remains unexplained.
Gravitational lenses can also help find objects closer within the Milky Way that would be too dark for us to see any other way, such as stellar-mass black holes.
Astronomers have even been able to detect “rogue” exoplanets — without a host star — roaming the galaxy, cold and alone, from the magnification that occurs when these exoplanets pass between us and distant stars. These gravitational “microlenses” can also detect exoplanets in other galaxies.
Did you like the image? You can download a wallpaper-size version of the photo of distant galaxies taken by the Hubble telescope from the website of the European Space Agency (ESA).