If you like classic two-for-one monster movies such as King Kong vs. Godzilla, then a new paper combining two bêtes noires of pseudoscientific scaremongers—mini black holes and the collapse of the vacuum—may appeal to you. Physicists working with the world's biggest atom-smasher—Europe's Large Hadron Collider (LHC)—have had to reassure the public that, even if they can make them, mini black holes, infinitesimal versions of the ones that form when jumbo stars implode, won't consume the planet. They've also had to dispel fears that blasting out a particle called the Higgs boson will cause the vacuum of empty space to collapse. Now, however, three theorists calculate that in a chain reaction, a mini black hole could trigger such collapse after all.
Come out from under the bed; there's a big caveat. If this could have happened, it would have long before humans evolved. "The thing you mustn't say is, ‘Shock, horror! We're going to destroy the universe!’" says Ian Moss, a theoretical cosmologist at Newcastle University in the United Kingdom and an author of the paper explaining the result. Rather, he says, the message is that some unknown physics must enter to stabilize the vacuum—encouraging news for physicists searching for something new. Still, Moss acknowledges that the paper could be taken the wrong way: "I'm sort of afraid that I'm going to have [prominent theorist] John Ellis calling me up and accusing me of scaremongering."
Stability of the vacuum is a real issue. Ever since the discovery of the long-predicted Higgs boson in 2012, physicists have known that empty space contains a "Higgs field," a bit like an electric field, that is made of Higgs bosons lurking "virtually" in the vacuum. Other fundamental particles such as the electron and quarks interact with the field to gain their mass. However, particle physicists have calculated that, given their current standard model of the known particles and the Higgs boson's measured mass, the Higgs field may not be in its stable, lowest energy state. Rather, it could achieve a much lower energy by taking on much higher strength. That energy-saving transition should inevitably cause the vacuum to collapse and wipe out the universe.
So why hasn't that collapse happened? It turns out that to get to the lower energy "true vacuum" state, the Higgs field would have to get through an enormous energy barrier through a process known as quantum tunneling. That barrier is so big that it would likely take many, many times the age of the universe for the transition to occur. So, theorists generally agreed that the Higgs field is "metastable," temporarily stuck in a "false vacuum" state, and that although the collapse is a problem in principle, practically it's nothing to worry about.
But now, Moss and theoretical physicists Philipp Burda and Ruth Gregory of Durham University in the United Kingdom contend that argument falls apart when you mix in mini black holes—microscopic regions of space where gravity is so strong that not even light can escape. That’s because a mini black hole acts like a "seed" that can trigger formation of a bubble of true vacuum in a sea of false vacuum, just as a bit of grit can trigger the formation of a bubble of steam in boiling water, as they explain in a paper in press at Physical Review Letters.
Without such a seed, a bubble of true vacuum would inevitably shrink. That's because, even though the vacuum within the bubble has lower energy than the vacuum outside the bubble, the bubble wall at which the two meet has very high energy. So the bubble can lower its total energy by growing smaller and disappearing. For a bubble with a tiny black hole inside, however, it’s a different story. The black hole’s gravity can shift the energy balance, Moss explains, so that any bubble beyond a certain very small size could instead lower its energy by growing. Within a fraction of a second, the bubble would then expand to consume the entire visible universe, Moss says.
Those black holes have to be small, Moss and colleagues argue, and they could conceivably come from two sources. They could be "primordial" black holes lingering since the birth of the universe. Or they could be microscopic black holes created within particle collisions such as those at the LHC.
So should we worry? No, Moss says. The fact that the universe has been around 13.8 billion years shows that primordial black holes will not trigger such a collapse, he says. As for black holes at the LHC, even if they can be created they also won't create havoc, he says. The proof of that comes from cosmic rays, which crash into the atmosphere and create even higher energy particle collisions than the LHC can. So even if such collisions spawn black holes, the black holes don't trigger vacuum collapse, Moss says, or the cosmos would have vanished long ago.
The real point, Moss says, is that theorists can no longer shrug off the problem by assuming that the collapse of the vacuum would take a hugely long time. By showing that—according to the standard model—the collapse should happen quickly, the paper suggests that some new physics must kick in to stabilize the vacuum.
Others aren't so sure the argument is persuasive. The theorists make a number of questionable assumptions in their mathematics, says Vincenzo Branchina, a theorist with Italy's National Institute for Nuclear Physics at the University of Catania. John Ellis, a theorist at King's College London, questions the consistency of the calculation. For example, he says, it assumes that the standard model holds true to very high energy scales. However, he notes, the only way the LHC can make a mini black hole is if the standard model conks out and space opens up new dimensions at much lower energy, he says. Still, both Branchina and Ellis say that based on other arguments, they suspect that something does make the vacuum stable.
As for the presentation of the argument in the new paper, Ellis says he has some misgivings that it will whip up unfounded fears about the safety of the LHC once again. For example, the preprint of the paper doesn’t mention that cosmic-ray data essentially prove that the LHC cannot trigger the collapse of the vacuum—"because we [physicists] all knew that," Moss says. The final version mentions it on the fourth of five pages. Still, Ellis, who served on a panel to examine the LHC's safety, says he doesn't think it's possible to stop theorists from presenting such arguments in tendentious ways. "I'm not going to lose sleep over it," Ellis says. "If someone asks me, I'm going to say it's so much theoretical noise." Which may not be the most reassuring answer, either.