What happens in the seemingly empty space between galaxies? According to cosmologists, that ‘empty’ space may not be empty at all.
New research published today explores how the environment surrounding quantum fields in the early universe can effectively trap them in a particular state, a phenomenon researchers have dubbed “cosmic lockdown.”
Quantum fields are invisible entities that exist throughout the Universe. They underpin everything we see around us, with their vibrations appearing as the particles that make up matter.
One famous example is the Higgs field, which plays a crucial role in giving particles their mass. Physicists have long wondered why some fields appear to settle into one state rather than another, and whether they could one day transition into a different state with dramatic consequences for the Universe.
To investigate, researchers from the University of Portsmouth developed a simplified model to study how quantum fields behave as the Universe expands.
Professor David Wands, from the University’s Institute of Cosmology and Gravitation, explained that in physics a vacuum is not completely empty.
“When we talk about a vacuum in cosmology, we do not mean completely devoid of energy. 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.”
A useful way to picture this is as a landscape of valleys. The deepest valley represents the lowest energy state, while shallower valleys represent higher-energy states. A field can sometimes become trapped in one of these higher valleys even though a lower-energy state exists elsewhere.
The team wanted to understand what determines where a field ends up and whether it can later move to a different state.
The image shows a true vacuum on the left and a false vacuum on the right
Lead author Dr Robson Christie, from the University of Portsmouth’s School of Mathematics and Physics, said the answer appears to depend largely on how quickly the field can respond to the expansion of the Universe.
“A field that is heavy compared with the Hubble scale can quickly adjust to the changes as the Universe expands, and in this case it is highly likely to move towards the true vacuum, the deepest energy minimum.
“If the expansion is too rapid compared with the dynamics of the field, the system cannot keep up with the changes. In this case there can remain a significant probability that the field will also end up in the false vacuum.”
The researchers found that interactions with the surrounding environment do not appear to play a major role in determining which state a field initially chooses. However, once a field settles into a state, those interactions become much more important.
The environment causes a process known as decoherence, which strips away some of the quantum behaviour that would otherwise allow a field to switch between states. As a result, the field becomes effectively locked into its chosen state.
The researchers describe this stabilising effect as “cosmic lockdown”.
The phenomenon is related to the quantum Zeno effect, in which frequent interactions can prevent a quantum system from changing state. Rather than requiring a conscious observer, the effect emerges naturally because the surrounding environment continually interacts with the field.
When we talk about a vacuum in cosmology, we do not mean completely devoid of energy. 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.
The findings do not prove that the Higgs field itself is protected by cosmic lockdown, but they suggest a possible mechanism that could help stabilise fields throughout the Universe.
Professor Wands said: “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. But understanding how far this can be applied to the Higgs field will require more realistic models.”
The researchers say their work provides new insights into how quantum physics and the evolution of the Universe are connected and could help shape future investigations into the fundamental forces that govern reality.
The study was carried out by University of Portsmouth researchers Dr Robson Christie, Dr Jaewoo Joo and Professor David Wands, together with collaborators Greg Kaplanek from Syracuse University and Vincent Vennin from École Normale Supérieure in Paris.
The paper is published in the Journal of Cosmology and Astroparticle Physics (JCAP).
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