In August 2019, the LIGO and Virgo detectors picked up gravitational waves from the merger of two compact objects in a binary star. One weighed 23 solar masses — a standard black hole. The other came in at 2.5–2.67 M⊙. Too heavy for a neutron star (an ultra-dense supernova remnant made mostly of neutrons), too light for a black hole. An object that shouldn't exist.

The solution may lie not in exotic matter but in the microstructure of spacetime itself. If, under gravity's magnifying glass, it's not perfectly smooth but possesses a tiny directional graininess, then a neutron star gains extra support. The idea goes back to Bernhard Riemann but took shape in the 20th century as Finsler geometry. Unlike Riemannian geometry, where distance is fixed by coordinates alone, here it also depends on direction. It's like a rough surface: moving along the grain is easier than across it. In physics, such anisotropy arises from a small vector field ξ(r) added to the metric.

The authors took Randers-type modified gravity and applied it to neutron stars. They employed a spherically symmetric model with a linear equation of state, pinning down free parameters with data from four well-studied X-ray pulsars (LMC X-4, Vela X-1, EXO 1785-248, and Cen X-3). In standard general relativity, such a star would top out at 2.01 M⊙. But introducing microscopic graininess shot the mass ceiling up to 2.67 M⊙. The radius shrank from 11 to 9.5 kilometers. The star became denser, and its compactness M/R nearly doubled. The moment of inertia grew too, making the object more resistant to spin.

As early as 1934, Fritz Zwicky predicted neutron stars as the endpoint of supernova explosions, while Subrahmanyan Chandrasekhar found the mass limit for white dwarfs beyond which collapse ensues. Decades later, Jocelyn Bell Burnell discovered the first pulsar, a rapidly spinning neutron star, confirming their reality. Today, stellar evolution paints a picture where these objects are born in the fire of supernovae, their final fate determined by mass.

Thus, the microscopic graininess of space, imperceptible in everyday life, becomes the crucial scaffold for the densest matter in the universe. What seemed an anomaly — the mysterious object from GW190814—now finds a legitimate spot in the roster of compact objects. And perhaps the next step is to see whether other gravitational surprises lurk in this grainy fabric.