When a massive star runs out of fuel, it explodes as a supernova, leaving behind a neutron star or, if the mass is great enough, a black hole. Its gravity is so strong that not even light escapes. According to Einstein's theory, inside it forms a region where space and time are curved to infinity — this was also proven by Penrose. Previously, calculations relied on the cumbersome solution of Schwarzschild.

Now a simpler recipe has been found: the star's density — like a list of ingredients — uniquely determines the 'dish' at the end. Taking into account not just ordinary matter but also quantum corrections, dark energy, and magnetic fields, the density immediately gives the type of black hole. This approach quickly weeds out gravity theories: if the corrections in the formulas appear as integer powers, the model is serious; if as fractional ones, it's more like a temporary patch. When telescopes pick up ripples in spacetime from a collapse or the glowing disk of gas around a black hole, we'll check whether the fateful point of infinite density vanishes at the center.