Second only to black holes, neutron stars – incredibly dense star remnants – are the densest objects in the universe. When neutron stars collide, they create ripples in the fabric of space and time in a way that we can detect on Earth.
We can then use these ripples to measure one of the universe’s most fundamental but elusive properties – how fast it is expanding. This is called the Hubble constant, and scientists have pursued and debated it for decades.
