Near-Extremal Black-Hole Formation Observed from Null Infinity

We identify near-extremal black-hole formation directly from observables at future null infinity. Fully nonlinear evolutions of the spherically symmetric Einstein–Maxwell–charged scalar system in 3+1 dimensions form black holes with charge-to-mass ratios up to 0.993 from regular, horizon-free, superextremal initial data. Quasinormal frequencies and redshift decay independently recover the near-extremal charge-to-mass ratio. Critical mass scaling is recovered at null infinity and charge scaling at the apparent horizon. The charged scalar Newman–Penrose quantity obeys an asymptotic balance law that replaces the conservation law of the neutral theory.