Quantum Decoherence Can Lock Cosmological Fields Into False Vacuums

A new theoretical study suggests that quantum decoherence - the process by which fields lose their quantum coherence through interactions with their surroundings - can suppress tunnelling between vacuum states in an expanding universe, effectively trapping fields where they are rather than letting them find their lowest energy state.

The research, conducted by a team including Greg Kaplanek of Syracuse University and David Wands of the Institute of Cosmology and Gravitation at the University of Portsmouth, used a simplified cosmological model to investigate how environmental interactions shape which vacuum state a quantum field settles into.

In cosmology, a vacuum does not mean an absence of energy. "When we talk about a vacuum in cosmology, we do not mean completely devoid of energy," Wands explained. "A vacuum is rather a state in which a field sits at a minimum of its energy." Fields can occupy either true vacuums - absolute energy minima - or false vacuums, which are local but not global minima.

Classical physics would predict that a field in a false vacuum eventually tunnels through the energy barrier to reach the true vacuum. Quantum mechanics complicates this picture by allowing fields to maintain superpositions across multiple vacuum states simultaneously.

The study found that environmental interactions - couplings to other fields and to the surrounding cosmos - introduce decoherence that tends to fix the field in whichever vacuum state it currently occupies. "Perfect isolation is an idealisation," Kaplanek noted. "Fields continuously interact with other fields and with what surrounds them." The result is a kind of cosmic lockdown.

Crucially, the researchers found that the environment does not primarily determine where the field ends up. Instead, decoherence reinforces the state it already occupies. "The interesting thing is that it is not primarily the environment that decides where the field will end up," Kaplanek said. "Decoherence tends to keep it there."

The analysis also reveals a dependence on field mass relative to the Hubble scale. Lighter fields are more susceptible to becoming trapped in false vacuums, while heavier fields are more likely to track the true vacuum as the universe expands. This has potential implications for understanding the Higgs field and the cosmological constant.

The findings are relevant to long-standing puzzles in cosmology, including why the universe appears to occupy the vacuum state that it does and whether fields such as the Higgs are stable over cosmological timescales.

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