Detectors of gravitational waves (conceived by Einstein and captured by instruments of Weiss) detected how a black hole of 23 solar masses swallowed an object of 2.6 solar masses. Too light for a black hole and too heavy for a neutron star — it hung in the 'mass gap' pointed out by Chandrasekhar. The whirlpool gave a clue: after the supernova, a plasma disk spun around the black hole. Like foam in a funnel, it broke into clumps. One tore away and spiraled for 60 days, losing energy, until it fell — a kilonova flared, and in it, like an alchemical cauldron, heavy elements were forged (nucleosynthesis).

Computer models (numerical simulations) confirmed the scenario. But the real surprise: the two-month delay linked the gravitational waves to a distant galaxy. Thus, for the first time, cosmic expansion was measured directly — about 70 km/s per megaparsec. The very stellar evolution measured this interval more precisely than any clock.