Physicists Find a 'Cosmic Lockdown' That Can Trap the Universe in a False Vacuum
A Portsmouth-led model shows that a quantum field's constant contact with its surroundings can all but shut off the tunneling that would carry it into a deeper, radically different energy state.
One of the more unsettling ideas in modern physics is that the universe might not be sitting in its lowest-energy state. If it is perched in a "false vacuum," a quantum fluke could in principle tip it into a deeper one, rewriting the properties of matter and the forces that hold it together. A new study suggests that the universe's own crowdedness may be what keeps that from happening.
In a paper in the Journal of Cosmology and Astroparticle Physics, Robson Christie, Jaewoo Joo, Greg Kaplanek, Vincent Vennin and David Wands built a simplified model of a quantum field in an expanding universe that has two possible resting states: a shallow valley, the false vacuum, and a deeper one, the true vacuum. "When we talk about a vacuum in cosmology, we do not mean completely devoid of energy," said Wands, a professor at the University of Portsmouth's Institute of Cosmology & Gravitation. "A vacuum is rather a state in which a field sits at a minimum of its energy."
The question matters most for the Higgs field, which gives mass to the particles of the Standard Model. Some calculations based on the measured masses of the Higgs boson and the top quark suggest the Higgs may sit in a false vacuum, with a deeper minimum at very large field values. In classical physics, a field stuck in a shallow valley stays there unless it gets enough energy to climb out. In quantum mechanics, it can tunnel straight through the barrier. "In principle, a transition to that deeper minimum would take the universe into a radically different state," said Christie, the study's first author.
Most tunneling calculations treat the field as perfectly isolated. Real fields are not. "We know, however, that perfect isolation is an idealization," said Kaplanek, a researcher at Syracuse University. The team added an environment made of other fields interacting with the main one, which causes decoherence, the same process that makes quantum computers so hard to build, where contact with surroundings destroys delicate superpositions and pushes a system toward ordinary classical behavior.
They found two things. First, the environment does little to decide which valley the field falls into early on. That choice depends mainly on whether the field is heavy or light compared with the Hubble rate, the speed of cosmic expansion. Heavy fields keep up with the expansion and usually settle into the true vacuum. Light fields cannot keep up and have a real chance of getting stuck in the false one.
Second, once the field has settled, decoherence locks it in place. Because the environment keeps gathering information about which valley the field is in, the coherent superposition needed for tunneling is destroyed, and the jump to the other vacuum is strongly suppressed. The authors call it "cosmic lockdown" and describe it as a form of the quantum Zeno effect, in which a system that is constantly monitored struggles to change state. "You do not need a conscious observer," Kaplanek said. The environment does the watching.
The model is deliberately simple and does not directly compute the fate of the Higgs field. But it suggests that estimates of vacuum decay built on isolated fields may be missing a stabilizing effect that comes from the universe simply being full of other things.
