Although our universe seems stable because it has existed for 13.7 billion years, many experiments show that it is at risk; it is walking on the edge of a very dangerous cliff. And all because of the instability of a single fundamental particle: the Higgs boson.

In a new study that my colleagues and I have accepted for publication in Physical Letters B, we show that some of the models of the early universe that include objects called lightweight primordial black holes are unlikely to be correct, because they have already given rise to the Higgs boson that will bring about the end of the cosmos.

The Higgs boson is responsible for the mass and interactions of all known particles. This is because the mass of particles is a result of the interaction of elementary particles with a field called the Higgs field. We know the field exists because the Higgs boson exists.

You can think of this field as a completely still water bath that we are immersed in. It has the same properties everywhere in the universe. This means that we observe the same masses and interactions throughout the cosmos. This uniformity has allowed us to observe and describe the same physics over several thousand years (astronomers often look back in time).

But the Higgs field is not in its lowest possible energy state. This means that it could theoretically change state by falling to a lower energy state at a certain location. But if that were to happen, the laws of physics would fundamentally change.

Such a change represents what physicists call a phase transition. This is what happens when water turns into vapor and bubbles form in the process. A phase transition in the Higgs field would similarly create low-energy space bubbles with completely different physics.

Inside such a bubble, the mass of electrons and their interactions with other particles would change abruptly. The protons and neutrons that make up the atomic nucleus, which is made up of quarks, would suddenly switch places. In fact, no one who experienced such a change would likely be able to report it.

Constant risk
Recent measurements of particle masses at the Large Hadron Collider (LHC) at CERN suggest that such an event is possible. But don’t panic: it could happen just a few billion billion years after we retire. That’s why it’s common in the hallways of particle physics departments to say that the universe is not unstable but “meta-stable,” because the end of the world isn’t coming anytime soon.

The Higgs field needs a good reason to bubble. Thanks to quantum mechanics, the theory that governs the microcosm of atoms and particles, the energy of the Higgs boson is always fluctuating. And it’s statistically possible (although unlikely, which is why it’s taking so long) for the Higgs boson to bubble occasionally.

However, in the presence of external energy sources such as strong gravitational fields or hot plasma (a form of matter composed of charged particles), the story changes: the field can borrow this energy to form bubbles more easily.

So, while there is no reason to expect the Higgs field to form many bubbles today, a big question in the context of cosmology is whether the extreme conditions shortly after the Big Bang could have triggered such bubble formation.

However, when the universe was so hot, although there was enough energy to create Higgs bubbles, thermal effects also stabilized the Higgs boson, changing its quantum properties. So this temperature cannot cause the end of the universe, which is probably why we are still here.

Primordial black holes

In our new work, we show that there is a single heat source that consistently causes this bubbling (without the compensating thermal effects seen in the early days after the Big Bang). These are primordial black holes, a type of black hole that emerged in the early universe as a result of the collapse of extremely dense regions of spacetime.

Unlike ordinary black holes formed by the collapse of stars, primordial black holes can be very small, weighing less than a gram.

The existence of such lightweight black holes is predicted by many theoretical models that explain the evolution of the universe shortly after the Big Bang, including some inflationary models that propose that the universe exploded to very large proportions after the Big Bang.

But proof of this existence comes with a major caveat: Stephen Hawking showed in the 1970s, via quantum mechanics, that black holes slowly evaporate by emitting radiation across their event horizon (the point beyond which even light cannot escape).

Hawking showed that black holes act as heat sources in the universe, and that their temperature is inversely proportional to their mass. This means that lightweight black holes are much hotter and massive black holes evaporate more quickly.

In particular, if primordial black holes weighing less than a few thousand billion grams (less than 10 billion times the mass of the Moon) had formed in the early universe, as many models suggest, they would have evaporated long ago.

In the presence of the Higgs field, such objects would behave like impurities in a fizzy drink; they would help the liquid to form gas bubbles, adding energy to it due to the effect of gravity (due to the mass of the black hole) and the temperature of the surroundings (due to Hawking radiation).

When primordial black holes evaporate, they heat up the universe locally. They will develop in the middle of hot spots that can be much hotter than the surrounding universe but still colder than typical Hawking temperatures. Using a combination of analytical calculations and numerical simulations, we show that the presence of these hot spots will cause the Higgs field to decay continuously.

But we’re still here. That means such objects almost never exist. In fact, we must eliminate all cosmological scenarios that predict their existence.

Unless, of course, we find evidence of their past existence in ancient radiation or gravitational waves. It would be even more exciting if we did. That would mean there is something we don’t know about the Higgs boson, something that keeps it from bubbling up in the presence of evaporating primordial black holes. They could actually be entirely new particles or forces.

Either way, it’s clear we have much more to discover about the universe, at both the smallest and largest scales.