What can destroy a black hole?

Black holes are the densest and most powerful objects known to our species. Nothing escapes them once it gets too close and crosses the event horizon. Even light cannot get away.

Everything that falls in only makes the black hole bigger and stronger, adding to its mass. Like some kind of Borg collective from Star Trek, all the distinctiveness of individual matter is added to the black hole and erased from our universe.

Ever since the idea of the black hole was proposed, however, people have asked: can anything defeat these monsters?

One answer, which famed late theoretical physicist Stephen Hawking came up with in the 1970s, was: time. All black holes radiate energy from just outside their event horizons. This Hawking radiation carries mass away from the black hole back into the universe. Some recent papers even theorize that this radiation carries the information contained in any objects that fell in back out of the black hole in a scrambled form. That means that if you dropped a copy of Shakespeare’s greatest works into a black hole, billions or trillions of years in the future, the lines “To be or not to be” might emerge in the form of radiation. The smaller the black hole gets, the more radiation it releases. Eventually, the radiation carries away so much of the black hole that it vanishes.

The second contender to defeat the black hole is called phantom dark energy. This hypothetical state is when the amount of negative pressure from dark energy exceeds matter density. Negative pressure is tension that causes the universe to want to pull itself apart. Too much of this and the universe will enter a runaway state called a Big Rip where space expands so fast that not even atoms can stay together.

In a phantom dark energy state, black holes are literally ripped apart, evaporating far faster than their natural release of Hawking radiation and vanishing in moments rather than aeons.

A third hypothetical contender would be a collapse of false vacuum. One theory about our universe is that matter formed out of a quantum state called false vacuum. False vacuum means that empty space has more energy in it than nothing. Nothing would be a true vacuum. Hypothetically, if a bubble of true vacuum ever appeared anywhere in the universe, it would expand outward at the speed of light, gobbling up everything in its path like the Great Nothing from The Neverending Story. That includes black holes. The true vacuum would enter the event horizon just like any other matter, but upon encountering the singularity, it could cause it to collapse just like ordinary matter. The physics here is a little iffy, however, because we lack a theory of quantum gravity.

There is a considerable literature on how black holes might cause the false vacuum of our universe to decay into a true vacuum, so it works both ways.

The fourth contender, and probably the most likely in our universe, is the ordinary expansion of the universe. Even without a phantom dark energy state, normal dark energy can sap the strength of a black hole faster than if that black hole lived in static, unchanging universe. On the other hand, this isn’t guaranteed. For example, we may be living in something called a de Sitter universe, a universe with constant curvature. This constant curvature, which would be the source of dark energy, would simply modify the black hole without destroying it, and the two, black hole and universe, would evolve into something called a Schwarzschild-de Sitter black hole and universe.

One of the interesting features of a Schwarzschild-de Sitter black hole and universe is when the black hole and universe are the same size. This is called a Nariai spacetime, and in that case, you have no singularity, and the universe’s cosmological horizon, which defines the boundary of the observable universe, and the black hole event horizon are the same size. This is the biggest black hole that can be in a de Sitter universe. It is possible that we could be living in a Nariai spacetime, in fact, since our universe has about the same density as a black hole with an event horizon the same size as the observable universe.

De Sitter space is a very special kind of space, however, where black holes and the universe easily coexist. In other kinds of universes, black holes' event horizons simply expand along with the universe. Only in the case of phantom dark energy, however, does this cause the black hole to vanish.

Given that the recent Dark Energy Survey Instrument (DESI) has found some evidence of phantom dark energy states occurring about 5 billion years ago, it would be interesting to observe if black holes or other gravitating bodies might have been ripped apart in that period.

It turns out that black holes are not invincible after all.