This is the way the world ends: not with a bang, but with a quantum vacuum decay of the ground state of the universe to its true minimum.
The universe underwent radical phase transitions in the past. These transitions eventually led to the division of the four fundamental forces of nature and the panoply of particles we know today. All of that occurred when the universe was less than a second old, and it has been stable ever since.
But it might not last forever.
A divided cosmos
To understand the stability of the universe, first we need to talk about phase transitions. Phase transitions are when a substance undergoes a rapid, radical transformation. They happen all the time. You boil water, and it transforms from a liquid into a gas. You cool that same water, and it turns into a block of ice.
Perhaps the most exotic phase transitions are those that happen to quantum fields. Quantum fields are the fundamental building blocks of the universe. Every kind of particle — say, a photon or an electron — is really just a local manifestation of an underlying field. That field soaks all of space and time like bread dipped in olive oil. The way those fields interact and communicate with each other makes up the forces and physics of our existence.
That existence is based on four fundamental forces: gravity, the weak force, electromagnetism and the strong force. But it hasn't always been this way. In the earliest moments of the cosmos, those forces were united. As the universe expanded and cooled, the quantum fields underwent phase transitions, splitting apart one by one.
The last phase transition occurred when the electromagnetic force split from the weak force. That splitting gave rise to the photon and the W and Z bosons, the carriers of those two forces.
Since that event, which happened when the universe wasn't even a second old, everything's been stable — no more splitting, no more phase transitions. The four forces of nature went on to shape and sculpt the evolution of the cosmos for billions of years.
As far as everything looks, it's all stable — for now, anyway.
Not so stable after all
The stability of the universe is tricky to measure. Sure, it's been over 13 billion years since anything as interesting as a phase transition has occurred. Yes, 13 billion years is a really long time, but in the world of quantum fields, anything can happen.
Our best bet at probing the stability of the universe is through the mass of the Higgs boson. The Higgs is a very interesting field; its presence in the universe is what separated the electromagnetic force from the weak force and what maintains that split today. Without the Higgs boson, those forces would merge right back together.
In quantum physics, the more massive an entity is, the more unstable it is. Massive particles quickly decay into lighter ones, for example. So, if the Higgs is very massive, it might not be as stable as it seems, and it might decay into something else someday. But if the Higgs is light enough, it's likely to hang out forever, and there's nothing more to say about the future of the quantum fields of the universe.
Measurements of the Higgs have found that its mass puts the universe smack in between the "really, honestly stable" and "Oh no, it looks a little unstable" regimes. Physicists call this state "metastable" — a situation that is stable for now but could quickly deteriorate if something were to go wrong.
Searching for stability
The apparent metastability of the quantum fields of the universe is a little unsettling. Although it could mean that the universe could persist for billions, even trillions, of years without anything going wrong at all, it could also mean that the universe is already beginning to transform. All it would take is one little shake in the wrong direction, in some random patch of the universe, where the Higgs falls apart and the underlying quantum fields find a new, more stable configuration. That region of "new" universe would then propagate outward at nearly the speed of light through the "old" universe.
This kind of phase transition is called a “false vacuum decay.” It references the idea that the vacuum of our universe is a “false” one — it’s not as stable as it might appear, and it will someday decay into something new.
By the time we received any information that the phase transition was upon us, it would already be happening.
What would be on the other side of that new universe? It's impossible to say. It might be totally mundane, with the new quantum fields looking exactly like the old quantum fields and nothing amiss. It could be just a slight adjustment, like a little tuning to the nature of dark energy or a slight adjustment to the masses of neutrinos. Or, it could be radically different, with a universe filled with brand-new forces, fields and particles — which would make life (and chemistry and atomics) as we know it impossible.
Of course, we're not even 100% sure about the metastability criterion. We know that the Standard Model of particle physics is incomplete. A complete version could rewrite our understanding of quantum fields and where the "stable-unstable" line is drawn.
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