A lamp is hidden behind a large aquarium filled with water. You see not one light source but several reflections and refractions — the water bends the path of the rays. In much the same way, a massive galaxy acts as a giant lens: it bends the rays of a distant quasar (the bright core of a galaxy), and several copies of it appear in the sky.

Light from each copy reaches us by its own path, and the time difference can be days or even months. This delay depends on how strongly the lens curves spacetime and on the distance to it. The distance, in turn, is linked to the expansion rate of the universe — the very Hubble constant that is hotly debated today. By measuring the delays, one can compute this rate directly, in a purely geometric way.

The authors took seventeen recently discovered systems and ran them through a unified modeling pipeline. They reconstructed the mass distribution in the lensing galaxy and predicted how much it would delay the light. The main result is a ranking: six systems promise an error of no more than three percent, five up to seven, three up to twelve, and three are still too finicky for precise work. It is the first eleven that will become targets for telescopes in the coming years.

In essence, astronomers have calibrated a set of cosmic rulers. All that remains is to be patient and wait for the light from the different quasar copies to tell us itself how fast our world is expanding, just like the lamp behind the aquarium gradually reveals its story.