https://jnews.sissamedialab.it/en/cosmic...tum-fields
PRESS RELEASE: The vacuum is not always so empty. "When we talk about a vacuum in cosmology, we do not mean completely devoid of energy," explains David Wands, Professor at the Institute of Cosmology & Gravitation at the University of Portsmouth. "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."
We can picture this as a landscape made up of valleys of different depths: the true vacuum is the deepest one, while the others are false vacua. Something sitting in one of these depressions can remain "trapped" there even if, somewhere else, a lower-energy state exists.
This is exactly what can happen to quantum fields, fundamental physical objects that permeate the Universe. A classical field is something that has a value at every position in space, like a magnetic field, whose strength changes from point to point. Quantum fields behave in a similar way, and their excitations appear as particles.
In a new study published in JCAP, Robson Christie, Jaewoo Joo, Greg Kaplanek, Vincent Vennin and David Wands used a simplified model to investigate what determines which vacuum a field may end up in within an expanding Universe.
The case of the Higgs field. To understand why this question matters, the authors themselves point to the case of the Higgs field. Its vacuum value contributes to giving mass to the particles of the Standard Model — the theory that describes the known elementary particles and three of the four fundamental forces — and helps determine the structure of low-energy physics.
According to some calculations based on the Standard Model, it is possible that the Higgs field does not sit in the lowest possible energy state, but in a false vacuum, while at very large field values a second, deeper minimum may exist.
The study is not directly about the Higgs field, but uses it as a concrete example of what can happen when a field becomes trapped in a local minimum even though a lower-energy state is available.
“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,” explains Robson Christie, a researcher at the School of Mathematics and Physics at the University of Portsmouth and first author of the study.
Such a transition can be made possible by a quantum phenomenon. Let us return to the picture of vacua as valleys separated by mountains. In classical physics, a system sitting at the bottom of the shallower valley can reach the deeper one only if it has enough energy to climb over the mountain between them. In quantum mechanics, by contrast, the state of the system can extend beyond the barrier, leaving a small probability that it will appear on the other side: this is quantum tunnelling.
No field is ever truly isolated. In their work, Christie and colleagues built a simplified model to understand how the environment affects the evolution of a field. Many calculations of tunnelling treat the field as completely isolated.
“We know, however, that perfect isolation is an idealisation,” explains Greg Kaplanek, a researcher at Syracuse University, New York. In reality, fields continuously interact with other fields and with what surrounds them — in other words, with their environment. “Think, for example, of quantum computers: 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.”
Interactions with the environment produce a phenomenon known as decoherence. A quantum system can exist in a superposition of different possibilities: using the analogy of a coin, it is not simply heads or tails, but a quantum state that includes both possibilities at once. Interaction with the environment makes it increasingly difficult to keep this superposition, causing the system to behave more and more like an ordinary classical system.
In the authors’ model, the environment is represented by other fields interacting with the main field. The latter can also initially be in a quantum superposition involving both vacua. One of the surprises of the study, however, is that the environment does not appear to play a decisive role in the initial choice of vacuum.
Light fields and heavy fields. What matters more is whether the field is “light” or “heavy” relative to the Hubble scale, that is, relative to the rate at which the Universe is expanding.
“A field that is heavy compared with the Hubble scale can quickly adjust to the changes as the Universe expands,” Christie explains, “and in this case it is highly likely to move towards the true vacuum, the deepest energy minimum.”
Something different happens for lighter fields. “If the expansion is too rapid compared with the dynamics of the field, the system cannot keep up with the changes,” Christie continues. “In this case there can remain a significant probability that the field will also end up in the false vacuum.”
In other words, the initial choice between the true and false vacuum is influenced mainly by the relationship between the field’s own dynamics and the rate of cosmic expansion.
Cosmic lockdown. So let us imagine a light field that has ended up in a false vacuum. What happens next?
In a perfectly isolated quantum system, tunnelling towards the other minimum would still be possible. But in the authors’ model, interaction with the environment produces decoherence and destroys the quantum properties needed to maintain a coherent superposition between the two vacua.
“The interesting thing is that it is not primarily the environment that decides where the field will end up,” explains Kaplanek. “Once the field has localised in one of the two minima, however, decoherence tends to keep it there. Tunnelling towards the other vacuum is strongly suppressed.”
This is the phenomenon the authors call “cosmic lockdown”: a kind of lock that stabilises the field in the vacuum it has reached. The authors interpret it as a manifestation of the quantum Zeno effect: under certain conditions, a quantum system that is continuously monitored can have much more difficulty moving from one state to another.
Of course, no one is literally observing the field in the model. “You do not need a conscious observer,” Kaplanek explains. “The environment continuously gathers information about the state of the system. This process destroys the coherence between the two possible vacua and makes tunnelling from one to the other much more difficult.”
Good news for our Universe? The work remains a simplified model and does not show that our current Higgs vacuum is protected by cosmic lockdown. The result does, however, suggest an interesting principle: interactions with the environment can make a false vacuum more stable by suppressing tunnelling towards another state.
“If a mechanism of this kind were relevant in more realistic cosmological situations, then it could help stabilise a field that is already sitting in a false vacuum,” Wands concludes. “But understanding how far this can be applied to the Higgs field will require more realistic models.”
If our Higgs field really is sitting in a false vacuum, then cosmic lockdown points to at least one reassuring possibility: interaction with its surroundings could help make a transition to a radically different state even more difficult.
