From April 2005 issue
When the gravity of a foreground object distorts and magnifies light from the background, images surround the object responsible for the lensing.
The galaxy cluster Abell 1689, located 2.2 billion light-years away in the constellation Virgo, contains trillions of suns, but most of its material cannot be seen. Nevertheless, this invisible mass affects space, bending light from individual galaxies behind the cluster into many curved, blue arcs. This image is approximately 2 million light years wide. Credit: NASA/ESA and ACS science team
When a gravitational lens magnifies a background galaxy, why do we not see the lensing object as we do through an optical lens?
robert hertrick
pittsburgh
Gravitational lensing is one of the more beautiful and unusual phenomena that physicists have predicted and astronomers have observed. When the gravity of the foreground galaxy distorts and magnifies the light from the background galaxy, the result is either a single Einstein ring or multiple images of the object.
In all cases, these images surround the object responsible for the lensing. So, if we think of the lensing galaxy as a kind of magnifying glass, why doesn’t the image appear superimposed? It is as if we were looking through a magnifying glass but seeing an image 3 feet to the side of it.
In fact, the image is visible inside the lens – it’s just that the lens is invisible. Light does not need to pass through a galaxy to be deflected by its gravity. Take, for example, this beautiful image of the galaxy cluster Abell 1689 captured by the Hubble Space Telescope. Gravity binds the cluster’s bright, yellow galaxies, and the density of galaxies maps the cluster’s gravitational depth well, roughly telling us where the invisible gravitational field is located. (X-ray views, which reveal extremely hot gas within the cluster, provide another way to “see” the gravitational field.) The blue-white arcs between the yellow galaxies are lensed images of background objects visible within the gravitational lens.
Of course, not every instance of lensing in the universe is gravitational. Just as light rays are bent when they interact with electrons in a piece of glass, astronomers can see lensed objects in X-rays when their light interacts with electrons in the dust of a foreground galaxy. The resulting haloes, analogous to Einstein’s rings produced by gravitational lensing, were first observed in 1983. Of course, if you can’t see the dust itself, such haloes will – at first – be as mysterious as those created by gravity. Princeton’s Bruce Drain and Nick Bond have suggested using geometric relationships in the lensing of these haloes to determine the cosmological distance scale, something astronomers have been doing with gravitational lensing for some time.
Treatises on gravitational lensing say that the images produced are not true images in the optical sense: their light is not focused on a single point of the observer. But our telescopes have no problem gathering light and focusing themselves.
simon dedeo
Princeton University
This question and answer was originally published online April 1, 2005.
