The Beginning to the End of the Universe: How black holes die

Black holes are areas of space-time where gravity rules supreme: A black hole’s gravitational pull is so powerful that nothing, not even light, can escape. They range in size from stellar-mass black holes with masses ranging from five to 100 times that of the Sun to supermassive black holes with masses exceeding a billion solar masses.

Astronomers currently believe that supermassive black holes exist at the center of most galaxies. (A major exception to this norm is M33, which appears to lack a center supermassive black hole despite being the third largest member of our Local Group.)

The cosmos is currently in its Stelliferous Era, when stars and galaxies are constantly being born. The chemicals needed to produce these things will eventually run out, and the stars in the night sky will gradually fade, leaving just black holes as the universe’s occupants.

Even black holes will die one day. When they do, these monsters will not go quietly into the night. A burst of fireworks will light up the universe in the final moments of each black hole, heralding the end of the era.

Using the Event Horizon Telescope, scientists accomplished the impossible, capturing an image of a black hole. This historic image shows the shadow of the supermassive black hole at the heart of the Messier 87 galaxy. EHT Collaboration
Cheating death
Black holes thrive by consuming the gas and stars that surround them, and their gluttony is what gives them away. They are frequently surrounded by accretion rings of material that they have torn apart and pulled close, like water swirling down a drain. As material approaches the black hole, it begins to travel faster and faster, piling up around it.

Friction among the dust generates heat, causing the accretion disk to flare and shape the black hole’s shadow — or event horizon. “It wants to hide, but it does a really bad job of it sometimes,” says Sheperd Doeleman, a Harvard University black hole researcher and the director of the Event Horizon Telescope, which took the first photo of a black hole in 2019.

Besides giving a black hole away, the event horizon is also the key to a black hole’s death.

Nothing can escape the grasp of these gluttonous monsters, thus anything that crosses the horizon of a black hole is gone forever. That is, at least, what our current understanding of gravity suggests. However, this so-called point of no return ignores quantum mechanics. (Physicists are still working to develop a unified theory of quantum gravity.) Stephen Hawking demonstrated in 1974 that, from a quantum standpoint, escape from a black hole is possible, but very slow.

Latent energy (quantum fluctuations) in the fabric of space-time may give rise to pairs of virtual particles all over the universe. If such a pair is created right on a black hole’s event horizon, or boundary, one particle may fall in while the other barely escapes, taking with it a negligible amount of energy and thus mass from the black hole. This theoretical phenomenon is known as Hawking radiation. Rick Johnson is an astronomer.

While empty space may appear to be devoid of energy, it isn’t; quantum mechanics shows that the energy of a vacuum fluctuates slightly over time. These fluctuations appear as pairs of particles — a particle and an antiparticle — that appear and disappear across the universe. Because energy cannot be formed from nothing, one particle will have positive energy while the other will have negative energy.

These particle pairs almost always annihilate one another. However, if the particles appear at the event horizon of a black hole, the particle with negative energy may fall into the black hole while the particle with positive energy escapes.

The black hole then looks to have radiated a particle away. With his equation E = mc2, Einstein demonstrated that energy and mass are proportional. As a result, the forsaken particle’s negative energy actually takes mass from the black hole, causing it to shrink.

However, don’t anticipate a black hole to disappear very soon. A black hole takes an astonishingly long time to lose all of its mass as energy via Hawking radiation. A supermassive black hole would take 10100 years, or a googol, to completely vanish. “The entire age of the universe [is] a fraction of [the time] it would take,” says Priyamvada Natarajan, a researcher at Yale University who probes the nature of black holes. “As far as we’re concerned, it is eternity.”

Death throes
The mass of a black hole strongly influences how long it lives. The longer it takes for a black hole to evaporate, the larger it becomes. “In that sense, [a black hole] can cheat death by growing,” Doeleman says.

He likens the process to an hourglass, with the sand at the top indicating how much time a black hole has left. A black hole continues to add sand to the hourglass of its life by devouring more stars and gas, even as individual particles drip out. “As long as there is material around [to eat], the black hole can keep resetting its clock,” Doeleman says. As the cosmos ages, the material surrounding a black hole will eventually run out, and its doomsday clock will start ticking.

As a black hole evaporates, it gradually shrinks, and as it loses mass, the pace of particles escaping increases until all of the remaining energy exits all at once. In the final tenth of a second of a black hole’s life, “you will have a huge flash of light and energy,” Natarajan says. “It’s almost like a million nuclear fusion bombs going off in a very tiny region of space.”

By Earth’s standards, that’s a lot, far more than all of the world’s nuclear arsenals combined. Not very much in astronomical terms. The most powerful supernova ever recorded (ASSASN-15lh) was 22 trillion times more explosive than the dying seconds of a black hole.

The closing fireworks of a black hole are the same no matter how small or large it is. The only difference is the time it takes for a black hole to explode. When a black hole consumes its last meal, all that remains is for the sand grains to tumble down until there is nothing left.