During a routine Hubble Space Telescope scan for an Einstein ring — a galaxy with a bright ring around it — astronomers noticed a double ring, the first of its kind. The team led by Raphael Gavazzi and Tommaso Treu report their discovery online (http://arxiv.org/abs/0801.1555; 2008).

A single Einstein ring is a consequence of gravitational lensing, whereby a massive galaxy (the lens) bends the light from a more distant galaxy (the object) along the same line of sight, or optical axis. Rather than focusing the light, as lenses do, the galaxy in the foreground has a focal line that creates mirages of the object, so we see the deflected light as a ring around the lens instead of a spot. (When the alignment is not perfect, arcs will appear instead of a full ring.) Acting as a magnifier, the lens amplifies the brightness of the object.

Credit: © NASA, ESA, R. GAVAZZI AND T. TREU

Albert Einstein himself made the calculation for lensing by a single star, which didn't yield an easily observable ring, but Fritz Zwicky was the first to propose that galaxies can also act as gravitational lenses. In fact, any massive object can bend space–time. It's a way to observe the unobservable: we can't see a black hole or dark matter, but we can see the effect they have on other objects.

In turn, the lensing provides a way for astronomers to determine the mass distribution of the lens galaxy. For a double ring, which is produced by light from not one but two distant galaxies beyond the one we can see, the situation is even better. The galaxy in the middle acts as an additional lens for the most distant one (to create the outer ring). Of course, the compound lens is more difficult to model, but it provides a unique method of measuring the total mass of small distant galaxies.

At 3 billion, 6 billion and approximately 11 billion light-years away, the galaxies in the double Einstein ring are cosmologically distant. A natural question then is whether we can obtain constraints on cosmological parameters. According to Gavazzi et al., the uncertainties are currently too large for any meaningful interpretations. However, they calculate that fifty such double-source lens systems would lead to measurements of the matter density of the Universe and the equation of state of dark energy with unprecedented accuracy (10%). With several planned space missions expected to reveal tens of thousands of single Einstein rings and tens of double Einstein rings, that goal may not be so far away.