Although our universe may seem stable, having existed for a whopping 13.7 billion years, several experiments suggest that it is at risk – walking on the edge of a very dangerous cliff. And it’s all down to the instability of a single fundamental particle: the Higgs boson.
In new research by me and my colleagues, just accepted for publication in Physical Letters B, we show that some models of the early universe, those which involve objects called light primordial black holes, are unlikely to be right because they would have triggered the Higgs boson to end the cosmos by now.
The Higgs boson is responsible for the mass and interactions of all the particles we know of. That’s because particle masses are a consequence of elementary particles interacting with a field, dubbed the Higgs field. Because the Higgs boson exists, we know that the field exists.
You can think of this field as a perfectly still water bath that we soak in. It has identical properties across the entire universe. This means we observe the same masses and interactions throughout the cosmos. This uniformity has allowed us to observe and describe the same physics over several millennia (astronomers typically look backwards in time).
But the Higgs field isn’t likely to be in the lowest possible energy state it could be in. That means it could theoretically change its state, dropping to a lower energy state in a certain location. If that happened, however, it would alter the laws of physics dramatically.
Such a change would represent what physicists call a phase transition. This is what happens when water turns into vapour, forming bubbles in the process. A phase transition in the Higgs field would similarly create low-energy bubbles of space with completely different physics in them.
In such a bubble, the mass of electrons would suddenly change, and so would its interactions with other particles. Protons and neutrons – which make up the atomic nucleus and are made of quarks – would suddenly dislocate. Essentially, anybody experiencing such a change would likely no longer be able to report it.
Constant risk
Recent measurements of particle masses from the Large Hadron Collider (LHC) at Cern suggest that such an event might be possible. But don’t panic; this may only occur in a few thousand billion billion years after we retire. For this reason, in the corridors of particle physics departments, it is usually said that the universe is not unstable but rather “meta-stable”, because the world’s end will not happen anytime soon.
To form a bubble, the Higgs field needs a good reason. Due to quantum mechanics, the theory which governs the microcosmos of atoms and particles, the energy of the Higgs is always fluctuating. And it is statistically possible (although unlikely, which is why it takes so much time) that the Higgs forms a bubble from time to time.
However, the story is different in the presence of external energy sources like strong gravitational fields or hot plasma (a form of matter made up of charged particles): the field can borrow this energy to form bubbles more easily.
Therefore, although there is no reason to expect that the Higgs field forms numerous bubbles today, a big question in the context of cosmology is whether the extreme environments shortly after the Big Bang could have triggered such bubbling.
However, when the universe was very hot, although energy was available to help form Higgs bubbles, thermal effects also stabilised the Higgs by modifying its quantum properties. Therefore, this heat could not trigger the end of the universe, which is probably why we are still here.
