Ask Ethan: Do evaporating black holes ever lose their event horizons?

Here in our Universe, there are few objects more awe-inspiring or mysterious than black holes. First theorized to exist back in the late 18th century, they represent a region of space where so much mass and energy is confined within such a small volume that, from within it, nothing can escape: not even light itself. Black holes are bounded by an event horizon, and as more and more mass falls into the black hole, the black hole grows, with its event horizon increasing in size as it happens.

But at some point — even if it’s very far into the future — the Universe will become more evolved, more isolated, and more sparse in terms of both matter and energy. Beyond a certain point, black holes will cease to effectively grow, and once that happens, a different process will begin to dominate: Hawking radiation, which is the quantum process by which black holes decay. As the decay progresses, the black hole will lose mass and its event horizon will shrink.

Will that ever allow us to see what’s inside of it? That’s the question of Artur Gouveia, who asks: “I’d like to know what happens to a black hole which receives no matter inflows from an accretion disk and keeps on losing mass from Hawking radiation. Does there arrive a moment when the mass is not enough to keep light from going out?

What happens at that moment? Does an object become visible? Is it a neutron star or an even denser object?

Have these ends of black holes been observed in the universe?” It’s a great and wondrous question, and stretches the limits of what’s conventionally known about physics today. Although it’s well beyond the limits of anything we can actually observe and measure today, here’s what we expect to occur. One of the most important contributions of Roger Penrose to black hole physics is the demonstration of how a realistic object in our Universe, such as a star (or any collection of matter), can form an event horizon and how all the matter bound to it will inevitably encounter the central singularity.

Once an event horizon forms, the development of a central singularity is not only inevitable, it’s extremely rapid. . Jarnstead/Royal Swedish Academy of Sciences; annotations by E. Siegel Forming a black hole is actually easier than you might think.