What is a gravitational lens?

A gravitational lens is a distortion of space that bends light from objects behind it. Anything with mass – the Earth, the Sun, black holes, you – pulls at space around it. Another object with mass feels that distortion, and as a result, both tend to fall towards one another. We call this force gravity.

Light does not have mass. But light travelling through the distorted space slows down. This is exactly what happens when light passes through glass. Just like a glass lens is thicker in the middle and thinnest at the rim, space is most distorted nearest the mass and less so further out. Gravitational distortions of space therefore act like lenses do – they magnify sources behind it. This can happen when a star passes at some distance behind another, or when a distant galaxy is seen behind a galaxy in the foreground. Like with a telescope, it allows us to see things that otherwise would be too dim. The magnified images look stretched out, though. While this may look weird, it makes them easy to find. Lensed galaxies are then powerful tracers of the mass that makes up the lens. This is one way in which to map dark

matter, which doesn’t give off light at all.