Portsmouth-led cosmologists find decoherence can keep quantum fields trapped in false vacua

Most tunnelling calculations start from a field with no neighbours. Many of them treat the field as completely isolated ^[7]. "We know, however, that perfect isolation is an idealization," said Greg Kaplanek, a Syracuse University researcher and one of the authors ^[7]. He compared it to quantum computing, where engineers shield stored information from the environment. "We go to enormous lengths to protect the quantum information stored in these machines from the environment, because even weak interactions with it can quickly alter the quantum state. Something similar happens in cosmology: A field is never really alone," he said ^[8].

The direction of the result follows from what tunnelling is. A classical system in the shallower valley can reach the deeper one only if it has enough energy to climb the barrier between them. A quantum state can extend past the barrier, leaving a small probability that it appears on the other side ^[11]. Decoherence is the loss of superposition through contact with the environment, and it makes a system behave more like an ordinary classical one ^[10]. Suppression is the direction I would expect. A field pushed toward classical behaviour loses the one route out that did not require climbing ^[10][11].

The stakes depend on what cosmologists mean by a vacuum. "A vacuum is rather a state in which a field sits at a minimum of its energy. The true vacuum is the lowest possible minimum, but there can also be local minima, which we call false vacua," said David Wands, a professor at the University of Portsmouth's Institute of Cosmology & Gravitation ^[3]. If a field sits in one of those local minima, the question is whether it can leave. For the Higgs, the answer matters a great deal. "In principle, a transition to that deeper minimum would take the universe into a radically different state, in which the structure of matter and the forces that govern it would be altered," said Robson Christie of Portsmouth's School of Mathematics and Physics, the study's first author ^[6].

The study is not directly about the Higgs field. It uses the Higgs as a concrete example of a field trapped above a lower-energy state ^[5]. The phys.org report does not say how large the suppression is, what coupling strengths produce it, or how it compares with the rate for an isolated field. Without those figures, "effectively locking" ^[1] is a qualitative result from a simplified model. A reader cannot tell whether the Higgs sits in the regime where it would apply.

In my view, the finding matters first as a point about method. A tunnelling rate computed for a perfectly isolated field describes a field that, by the authors' account, never exists ^[7][8].