The blackest thing in the universe turns out to be, very faintly, warm.

The object that couldn't have a temperature

A black hole is supposed to be perfectly black — a one-way door that swallows light and hands nothing back. For most of the twentieth century that was the whole story: whatever falls in stays in, and the horizon only ever grows.

That last clause is where the trouble started. In 1972 Jacob Bekenstein, then a graduate student, noticed that a horizon's area behaves suspiciously like entropy — both can only ever increase. He argued this was no accident: a black hole genuinely has entropy, fixed by its surface area. But anything with entropy has a temperature, and anything with a temperature radiates. A black hole would have to glow. Which seemed impossible, because nothing gets out.

Hawking's reluctant discovery

Stephen Hawking was skeptical of Bekenstein's claim, and in 1974 set out to do the quantum mechanics near a horizon properly. The mathematics changed his mind. Out of the calculation came a faint, steady trickle of particles — what we now call Hawking radiation.

The cartoon version, the one Hawking himself offered: empty space is never quite empty. Pairs of particles — one matter, one bit of antimatter — constantly flicker into being, borrow a little energy, and pay it back by annihilating an instant later. Right at the horizon, occasionally one partner falls in while the other escapes. The escapee carries off energy, so to balance the books the black hole must surrender a sliver of its mass. Bit by bit, it pays for its own glow.

It is only a cartoon — the real derivation is about quantum fields stretched across curved spacetime, not billiard balls — but the bookkeeping is exact. A radiating black hole shrinks.

A black hole is not eternal. It leaks, slowly, then finally all at once, until nothing is left but the light it gave away.

Slow beyond all imagining

Before you picture the sky raining black holes: this is staggeringly gradual. A hole the mass of the Sun has a temperature of about sixty billionths of a degree above absolute zero — far colder than the faint afterglow of the Big Bang that still fills space. So today such a hole absorbs more warmth than it emits, and grows. Only in the deep future, once the universe has cooled below even that whisper, will real evaporation begin — and then it drags on for something like 10⁶⁷ years, against the mere 14 billion the cosmos has managed so far.

Smaller holes run hotter and die faster, finishing in a final flash of radiation. None small enough to have popped yet has ever been found.

The bill left unpaid

Hawking's glow came with a deeper problem. The radiation seeping out looks featureless — the same regardless of what fell in. So when a hole finally evaporates to nothing, where did the information about everything it swallowed go? Quantum mechanics insists information can never be destroyed. A black hole appears to shred it.

This is the information paradox. Hawking argued for thirty years that the information was simply lost, then in 2004 conceded a famous bet and switched sides. The lean now is that it does escape, hidden in correlations too subtle for us to fully read — but nobody can yet show exactly how.

The blackest object we know of asked physics its sharpest question. We are still, decades on, working out how to answer it.