The ultimate vacuum stability of our universe may rest on the masses of two fundamental particles, the Higgs boson – which inhabits all space and time – and the top quark. The latest measurements of these masses reveal that our universe is metastable, meaning it may persist in its current state essentially forever…or not.
Our universe has not always been the same. In the early moments of the Big Bang, when our cosmos was only a fraction of its current size, energies and temperatures were so high that even the fundamental rules of physics were completely different. Specifically, physicists believe that at some point the four forces of nature (gravity, electromagnetism, strong nuclear and weak nuclear) were merged into one unified force.
The nature of this unified force remains a mystery, but as the universe expanded and cooled from its initial state, the forces became separated from each other. First came gravity, then the strong nuclear, and finally the electromagnetism and the weak nuclear force separated from each other. This last step we can recreate in the lab. In our most powerful particle colliders, we can achieve the energies necessary to – temporarily, at least – recombine these forces into a single “electroweak” force.
Each time the forces split, the cosmos underwent a drastic phase transition, populated by new particles and forces. For example, the unified electroweak force is carried by a quartet of massless particles, but the electromagnetic force is carried by a single massless particle, the photon, while three massive particles carry the weak nuclear. If these two forces hadn’t separated, then life as we know it, which depends on electromagnetic interactions to glue atoms together into molecules, simply wouldn’t exist.
The universe hasn’t undergone such a reshuffling of fundamental forces in over 13 billion years, but that doesn’t mean it isn’t capable of playing the same tricks again.
The decisive Higgs boson
The current stability of the vacuum depends on the ultimate degree of separation of the electroweak force. Did this split bring the universe to its final lowest energy ground state? Or is it just a pit stop on the road to its future evolution?
The answer comes down to the masses of two fundamental particles. One is the Higgs boson, which plays a major role in physics: its existence triggered the separation of electromagnetic and nuclear forces billions of years ago.
In the beginning, when our universe was hot and dense, the Higgs remained in the background, allowing the electroweak force to reign unimpeded. But once the universe cooled beyond a certain point, the Higgs made its presence known and interfered with that force, creating a separation that has been maintained ever since. The mass of the Higgs boson determined when this separation occurred, and it regulates the “strength” of this separation today.
But the Higgs plays another major role in physics: by interacting with many other particles, it gives these particles mass. The strength with which a particle connects to the Higgs governs the mass of that particle. For example, the electron barely talks to the Higgs, so it gets a light mass of 511 MeV. At the other end of the spectrum, the top quark interacts the most with the Higgs, making it the heaviest object in the Standard Model of particle physics, weighing 175 GeV.
In particle physics, particles constantly interact and interfere with all other types of particles, but the strength of these interactions depends on the mass of the particles. So when we try to assess anything involving the Higgs boson – like, say, its ability to maintain separation between the weak electromagnetic and nuclear forces – we also have to pay attention to how other particles will interfere with it. this effort. And since the top quark is by far the largest of the bunch (the next largest, the bottom quark, weighs only 5 GeV), it’s essentially the only other particle we have to worry about.
When physicists first calculated the stability of the universe, determined by the ability of the Higgs boson to maintain separation from the electroweak force, they knew neither the mass of the Higgs itself nor that of the top quark. Now, yes: the top quark weighs about 175 GeV and the Higgs about 125 GeV.
Plugging these two numbers into the stability equations reveals that the universe is… metastable. This is different from stable, which would mean there is no chance of the universe splitting up instantly, but also different from unstable, which would mean it has already happened.
Instead, the universe is balanced in a rather precarious position: it can remain in its current state indefinitely, but if something were to disturb spacetime in the wrong way, then it would transform into a new ground state. .
What would this new state look like? It’s impossible to say, because the new universe would feature new physics, with new particles and new forces of nature. But it’s safe to say that life would be different, if not completely impossible.
Worse, it may have already happened. Certain corners of the cosmos may have already begun to transition, with the bubble of a new reality expanding outward at the speed of light. We wouldn’t know he hit us until he had already arrived. Sleep well!