Wing V · the map roomRoom IV of IV
Inv. no. V.4 · the ring
Gravitational lenses
Mass bends the path of light. Put a heavy galaxy almost exactly in front of a farther one and the light of the far one comes round it on more than one side. The far galaxy gets smeared into arcs; with a near miss it splits into two or four copies; with a perfect line-up it becomes a ring. Einstein worked it out and did not expect anyone to see one. Today hundreds are known, and the mass they give away is mostly dark matter.
Behind the lens
Fig. I. A far galaxy (blue) seen past a near one (gold), every pixel traced back along its bent ray. The dashed circle is the Einstein radius; the faint outline, where the far galaxy really is. Drag to move it.
- Einstein radius
- Off the line by
- Made brighter
- What you see
Every pixel here is a ray traced backwards: from your eye, past the lens, where it is bent towards the lens's centre, and on to wherever it lands among the far galaxies. Light that would have missed you on either side is bent back towards you, so one galaxy shows up in more than one place. Far off the line, the galaxy is only a bit stretched. Get closer and a second, fainter copy shows up on the other side. Right behind the lens, the copies meet in a ring at the Einstein radius. Make the lens a little elongated and the ring breaks into a cross of four.
A lens also gathers light, the way a magnifying glass does: the images together are brighter than the galaxy would be without it. Astronomers use the biggest lenses, whole clusters of galaxies, as natural telescopes to see galaxies too far and faint to see otherwise.
Weighing a cluster
Fig. II. A cluster of galaxies in front of hundreds of far ones. Their stretched shapes, lined up round the centre, are what a lensing map is made from.
- Mass bending the light
Give the cluster only the mass in its stars and hot gas and the far galaxies behind it are hardly touched. Give it the mass its arcs actually call for and they are pulled into long thin arcs, all curving round the centre. Real clusters show the second picture. Measured by how much they bend light, clusters weigh five to ten times more than their stars and gas. The difference is dark matter.
Far from the centre the stretching is too slight to see in any one galaxy, but averaged over thousands it is still there. That is weak lensing, and survey telescopes now use it to map where the dark matter sits.
Who worked it out
Where it already matters
Starlight bent by the Sun, 1919. During a total eclipse, Arthur Eddington's expeditions measured stars near the darkened Sun shifted outwards by about 1.6 to 2 arcseconds, matching the 1.75 that general relativity predicted: twice what Newton's gravity would give. The next day Einstein was in the newspapers.
One star lensing another, 1936. Einstein worked out that one star could lens another into a ring, and judged the chance of ever seeing one very small. A year later Fritz Zwicky argued that whole galaxies would make far better lenses, and that they could be weighed with them.
The Twin Quasar, 1979. Two quasars a few arcseconds apart with exactly the same spectrum turned out to be one quasar seen twice: the first gravitational lens found. In 1985 the Einstein Cross followed, a quasar seen four times round a nearby galaxy.
A supernova on repeat, 2014–2015. A supernova behind a cluster showed up four times as a cross. The models of the cluster's mass predicted one more copy, delayed by about a year along a longer path. It appeared on schedule.
Webb's first picture, July 2022. The first image from the James Webb Space Telescope was a cluster ringed with lensed arcs of galaxies far beyond it. The European Euclid telescope, launched in 2023, is measuring the weak stretching of over a billion galaxies to map the dark matter in three dimensions.