An exploding black hole could reveal the foundations of the universe

Physicists have long believed that black holes explode at the end of their lives, and that such explosions happen—at most—only once every 100,000 years. But new research published in Physical Review Letters by physicists at the University of Massachusetts Amherst has found a more than 90% probability that one of these black-hole explosions might be seen within the decade, and that, if we are prepared, our current fleet of space and earthbound telescopes could witness the event.

Such an explosion would be strong evidence of a theorized but never observed kind of black hole, called a “primordial black hole,” that could have formed less than a second after the Big Bang occurred, 13.8 billion years ago.

Furthermore, the explosion would give us a definitive catalog of all the subatomic particles in existence, including the ones we have observed, such as electrons, quarks and Higgs bosons, the ones that we have only hypothesized, like dark matter particles, as well as everything else that is, so far, entirely unknown to science. This catalog would finally answer one of humankind’s oldest questions: from where did everything in existence come?

We know that black holes exist, and we have a good understanding of their life cycle: an old, large star runs out of fuel, implodes in a massively powerful supernova and leaves behind an area of spacetime with such intense gravity that nothing, not even light, can escape. These black holes are incredibly heavy and are essentially stable.

But, as physicist Stephen Hawking pointed out in 1970, another kind of black hole—a primordial black hole (PBH), could be created not by the collapse of a star but from the universe’s primordial conditions shortly after the Big Bang. PBHs, like the standard black holes, are so massively dense that almost nothing can escape them—which is what makes them “black.”

However, despite their density, PBHs could be much lighter than the black holes we have so far observed. Furthermore, Hawking also showed that black holes have a temperature and could, in theory, slowly emit particles via what is now known as “Hawking radiation” if they got hot enough.

“The lighter a black hole is, the hotter it should be and the more particles it will emit. As PBHs evaporate, they become ever lighter, and so hotter, emitting even more radiation in a runaway process until explosion. It’s that Hawking radiation that our telescopes can detect,” says Andrea Thamm, co-author and assistant professor of physics at UMass Amherst.

Yet, while we should be able to, no one has ever directly observed a PBH.

“We know how to observe this Hawking radiation,” says Joaquim Iguaz Juan, a postdoctoral researcher in physics at UMass Amherst. “We can see it with our current crop of telescopes, and because the only black holes that can explode today or in the near future are these PBHs, we know that if we see Hawking radiation, we are seeing an exploding PBH.”

Though physicists since Hawking’s time have thought that the chances of seeing an exploding PBH are infinitesimally slight, Iguaz Juan notes that “our job as physicists is to question the received assumptions, to ask better questions and come up with more precise hypotheses.”

The team’s new hypothesis? Get ready now to see the explosion. “We believe that there is up to a 90% chance of witnessing an exploding PBH in the next 10 years,” says Aidan Symons, one of the paper’s co-authors and a graduate student in physics at UMass Amherst.

In its work, the team explores a “dark-QED toy model.” This is essentially a copy of the usual electric force as we know it, but which includes a very heavy, hypothesized version of the electron, which the team calls a “dark electron.”

The team then reconsidered long-held assumptions about the electrical charge of black holes. Standard black holes have no charge, and it was assumed that PBHs are likewise electrically neutral.

“We make a different assumption,” says Michael Baker, co-author and an assistant professor of physics at UMass Amherst. “We show that if a primordial black hole is formed with a small dark electric charge, then the toy model predicts that it should be temporarily stabilized before finally exploding.”

Taking all known experimental data into account, they find that we could then potentially observe a PBH explosion not once every 100,000 years as previously thought, but once every 10 years.

“We’re not claiming that it’s absolutely going to happen this decade,” says Baker, “but there could be a 90% chance that it does. Since we already have the technology to observe these explosions, we should be ready.”

Iguaz Juan adds, “This would be the first-ever direct observation of both Hawking radiation and a PBH. We would also get a definitive record of every particle that makes up everything in the universe. It would completely revolutionize physics and help us rewrite the history of the universe.”