In August 2019, the LIGO and Virgo detectors registered ripples in spacetime from the merger of two objects in a binary system. One weighed 23 solar masses — a typical black hole. The other weighed 2.6 solar masses. Too heavy for a neutron star (the ultra-dense remnant of an exploded star) but too light for a black hole. A mystery.
The solution lies in the very fabric of space. Instead of a perfectly smooth sheet, picture it as a grainy surface — like the grain of photographic film. This graininess has a direction: it's easier to move along it than across it. In such a "grainy" geometry, known as modified gravity, a neutron star gets extra support. It's like walking on sand: the grains provide traction, allowing you to carry a heavier load without sinking.
The story of neutron stars began with predictions by Fritz Zwicky and Subrahmanyan Chandrasekhar, and their existence was confirmed by Jocelyn Bell Burnell, who discovered the first pulsar — a rapidly spinning neutron star. According to stellar evolution theory, they are born in the fury of supernovae — explosions of dying massive stars. Normally, if the remnant's mass exceeds a limit, it collapses into a black hole. But the graininess of space changes the rules.
Scientists took data from four known pulsars and computed: in smooth space, the maximum mass of a neutron star would be 2.01 solar masses. But with graininess, the limit jumped to 2.67 solar masses. The star also got even denser — its radius shrank from 11 to 9.5 kilometers. So an invisible graininess, unnoticeable in everyday life, decides the fate of the densest matter in the Universe. The mystery object from GW190814 finds its rightful place, and future observations will test whether space truly is grainy.
Finsler geometry, on which this grainy gravity is based, helps engineers design distortion-free lenses and even explains how ants find the shortest path.