Astrophysical viability of extremal black holes

Black holes are the most extreme objects in our universe, regions where gravity is so strong that nothing, not even light, can escape. For decades, physicists have been fascinated by a theoretical version of these objects called "extremal" black holes. These are not just any black holes; they are the absolute limit of what a black hole can be. Imagine a black hole that has been charged with as much electricity as physics allows, or spun as fast as possible without falling apart. In these extreme states, the inner and outer boundaries of the black hole merge into a single surface, and the object reaches a temperature of absolute zero. While these objects are mathematically beautiful and play a crucial role in theories about how gravity and quantum mechanics might fit together, a fundamental question has remained unanswered: do they actually exist in our real universe, or are they just mathematical ghosts that cannot survive in nature?

A new study by researchers Chiara Coviello and Ruth Gregory takes a fresh look at this question, asking whether an extremal black hole could persist in the messy, material environment of our actual cosmos. The researchers argue that while these objects might exist in a perfect vacuum, the reality of our universe makes their survival impossible. They show that the very forces that would create such a black hole also contain the mechanisms to destroy it almost instantly, or at least prevent it from ever forming in the first place. Their work suggests that if you were to find a black hole in our universe, it would almost certainly be slightly less than the maximum possible charge or spin, never quite reaching that perfect, frozen extremal state.

To understand why these objects are so difficult to sustain, one must first consider how a charged black hole interacts with its surroundings. In the vacuum of deep space, a black hole with a massive electric charge might seem stable. However, our universe is not empty; it is filled with a sea of particles and fields. The researchers focused on a process known as Schwinger discharge, a quantum effect where a strong electric field can spontaneously rip pairs of particles out of the empty vacuum. Think of the electric field around a charged black hole as a tightly stretched rubber band; if it is stretched hard enough, it will eventually snap, creating a pair of particles that fly apart. One of these particles carries away the black hole's charge, effectively neutralizing it.

For a long time, physicists believed that this discharge process would only happen for very small black holes, where the electric field is incredibly intense. They assumed that massive black holes, like the supermassive ones found at the centers of galaxies, would have electric fields too weak to trigger this effect. Coviello and Gregory revisited this calculation with a more careful eye. They realized that previous estimates had overlooked a crucial factor: the sheer volume of space around the black hole where this discharge could happen. Even if the rate of particle creation is slow in any single spot, the vast area surrounding a massive black hole means that the total number of particles being created is enormous. When they accounted for this volume and the specific probability of the event occurring, they found that the discharge is far more efficient than previously thought.

Their calculations revealed a startling threshold. For an electrically charged black hole to survive without losing its charge, it would need to be incredibly massive—more than 100 trillion times the mass of our Sun. To put this in perspective, the largest black holes we have ever observed are only about 36 billion times the mass of the Sun. This means that the most massive black holes in our universe are still far too small to escape this discharge process. Even if a black hole were somehow created with the perfect amount of charge, the electric field would immediately begin to strip that charge away by creating electron-positron pairs from the vacuum, returning the black hole to a neutral state long before it could be observed.

The researchers also considered a second, even more potent mechanism for discharge that occurs in the real environment of a galaxy. Black holes are rarely surrounded by pure vacuum; they are often embedded in clouds of gas and dust, primarily made of hydrogen. The electric field of a charged black hole is strong enough to rip electrons off these neutral hydrogen atoms, a process called ionization. Once the atoms are ionized, the resulting plasma of charged particles rushes toward the black hole, neutralizing its charge even faster than the vacuum process. When the team calculated the time it would take for a black hole to lose its charge through this ionization, they found that the required mass for stability was even higher than the vacuum limit. The conclusion is inescapable: any electrically charged black hole in our universe, no matter how large, will quickly lose its charge and cease to be extremal.

The study also examined the possibility of magnetic black holes. Unlike electric charges, which are common, magnetic charges (or magnetic monopoles) have never been observed directly, though many theories of the early universe predict they should exist. If a black hole were to capture a huge number of these magnetic monopoles, it could theoretically become a magnetically charged extremal black hole. The researchers investigated whether such an object would be stable. They found that while magnetic fields do not trigger the same rapid discharge as electric fields, these black holes are unstable in a different way. If the black hole is too small, the magnetic field lines from the monopoles would leak out through the event horizon, causing the black hole to shed its charge.

To form a stable magnetic black hole, the object would need to be massive enough to contain a vast number of monopoles—perhaps a million or more. The researchers then looked at how such a black hole could form. They analyzed the conditions of the early universe, specifically the period of rapid expansion known as inflation. Their calculations showed that the probability of a black hole spontaneously appearing with enough volume to capture the necessary number of monopoles is vanishingly small. The universe simply does not produce these objects in the quantities or sizes required to make them stable. Even if they did form, the conditions needed to create them would likely have been erased by the expansion of the universe.

Finally, the team turned their attention to rotating black holes. Unlike charged black holes, which are rare in nature, almost all observed black holes spin rapidly. The question here is whether a black hole can spin fast enough to reach the extremal limit. The researchers suggest that even if a black hole gets very close to this limit, it would likely lose its spin over time. They propose that the background hum of gravitational waves rippling through the universe could interact with a spinning black hole, sapping its angular momentum. While this process would be slow, it acts as a constant brake, preventing the black hole from ever reaching or maintaining that perfect, frozen state of extremality.

The overall picture painted by this research is one of a universe that naturally resists the formation of extremal black holes. Whether through the rapid discharge of electric charge, the instability of magnetic fields, or the slow draining of spin, the laws of physics seem to conspire to keep black holes just a step away from the extreme limit. This does not mean that extremal black holes are impossible in a mathematical sense, but it strongly suggests that they are not a feature of our physical reality. For scientists hoping to use these objects as a testing ground for the deepest laws of quantum gravity, the news is sobering: the perfect extremal black hole may be a beautiful idea, but it is likely an object we will never find in the sky. The black holes we observe are likely near-extremal, but never quite there, forever hovering just below the threshold of perfection.