Amanda Weltman

Light from a distant star travels to us for billions of years, and during that time the universe has time to expand. How can we check that we correctly understand this journey? Astronomers use two 'rulers': supernova explosions, which shine with a known brightness, and fast radio bursts — short, powerful pulses of radio waves from other galaxies.

Imagine two neutron stars, compressed to the size of a metropolis yet weighing one and a half Suns each. They whirl in an unrestrained gravitational dance, drawing together and flying apart every 4.4 hours. At periastron — the point of closest approach — only a couple of light-seconds separate them, and their speed reaches three and a half million kilometers per hour.

Somewhere deep in space, a radio beacon flashes for a fraction of a second, and then goes dark. Such signals are called fast radio bursts. They come from other galaxies and last from microseconds to milliseconds.