Gravity as a lens

This is a real photograph of a crowd of galaxies called Abell 370, billions of light years away. The yellowish blobs belong to the crowd. The thin blue streaks and arcs do not. They are galaxies much further off, lying behind it, and the gravity of the whole crowd has bent their light on its way to us and smeared them out of shape.

Here is how that happens. Mass bends the space round it, and light crossing that space follows the bend. Light from a far galaxy that would have gone past us on either side is turned inwards and arrives after all. We look back along the way the light came in, so we see the galaxy in places where it is not, sometimes more than once.

When the far galaxy, the near one and the Earth sit almost exactly in a line, the light comes round every side at once and the far galaxy is drawn out into a ring. This one is called the Cosmic Horseshoe. The orange galaxy in the middle has about ten times the mass of the Milky Way. The blue ring is a galaxy far behind it, seen as it was about 3 billion years after the Big Bang.

When the line-up is a little off, the ring breaks into separate copies. The four bright dots here are one single quasar, the blazing centre of a far galaxy, seen four times over. Its light set out about 10 billion years ago. The faint glow in the middle is the galaxy doing the bending, whose light has taken only about half a billion years to get here.

Bending the light also gathers it. Light that was heading wide of the Earth is turned towards us, so the thing behind looks brighter than it has any right to. In 2022 Hubble picked out a single star, nicknamed Earendel, whose light set out 12.9 billion years ago. A cluster of galaxies in front of it makes it look a thousand times brighter or more. Without that, no telescope could have seen it.

A single star can do the same on a small scale. When one star drifts in front of another, the one behind brightens and fades again. If the nearer star has a planet, the planet adds a short flicker of its own. That flicker is how 292 planets had been found by 11 November 2026, by the count in NASA’s Exoplanet Archive.