The story begins with the physics of cosmic rays themselves. Most primary cosmic rays are protons, electrically charged particles whose trajectories are bent by Earth’s magnetic field before they ever reach the atmosphere. When they arrive, they collide with atomic nuclei in the air and generate cascades of secondary particles. Among these secondaries are muons, short-lived charged particles produced at high energies that travel at nearly the speed of light and penetrate deep into the atmosphere. As Sunil Gupta of the Tata Institute of Fundamental Research (TIFR) in Mumbai, one of the authors of the study, explains, the cosmic rays detected at ground level consist predominantly of these highly penetrating muons. The GRAPES-3 facility includes a 560-square-metre muon telescope that records around four billion muons every day, providing an extraordinarily dense stream of data.

Crucially, those muons are not evenly balanced in electric charge. Because the primary cosmic rays are mostly positively charged, positive muons are more abundant in the atmosphere than negative ones. Physicists quantify this imbalance with the muon charge ratio, the number of positive muons divided by the number of negative ones, which normally exceeds one. That small excess is what allows GRAPES-3 to do something it was never originally designed to do. Inside a thundercloud, electric charges become separated, producing intense electric fields and large potential differences. When muons cross these regions, the field acts on positive and negative particles in opposite ways, generally slowing positive muons while accelerating negative ones. If the two charges were present in equal numbers, these effects would cancel almost perfectly, leaving no detectable change in the total muon flux.

Hari Haran Balakrishnan of TIFR, first author of the new study, puts the point starkly: if nature provided equal numbers of positive and negative muons throughout the telescope’s field of view, the thunderstorm phenomenon simply could not be observed with the current setup. Because positive muons dominate, however, each thunderstorm leaves a small but measurable imprint on the total muon count, and from that imprint researchers can infer the electrical potential inside the cloud. Gupta describes muons as an ideal gift for these kinds of studies, likening them to an electric current flowing through the atmosphere. In an earlier analysis, the team reported a striking result: a potential of 1.3 gigavolts measured in a single thunderstorm, a figure that dwarfs anything obtained by conventional means.

The idea that thunderstorms might generate potentials on that scale is not new. As early as the 1920s, the physicist Charles Thomson Rees Wilson, who would later win the 1927 Nobel Prize in Physics, suggested that thunderclouds could reach such enormous voltages. Direct measurements, however, had previously topped out at only around 130 million volts, more than an order of magnitude lower. The difficulty is practical as much as conceptual: sampling the interior of a large, rapidly evolving storm generally requires instruments carried by aircraft or weather balloons into or near active weather, a hazardous and spatially limited approach. Muons invert the problem entirely. They pass through the atmosphere continuously, they can be monitored from the safety of the ground, and they sample the whole volume of the cloud along their paths.

But the earlier GRAPES-3 observations carried an unexpected puzzle alongside their headline result. Between April 2011 and December 2020, the telescope detected 487 thunderstorm events, and their distribution across the sky was strikingly lopsided. Fully 81.5 percent of the events appeared in the eastern part of the instrument’s field of view, while only 13.7 percent appeared in the west, a difference of almost six to one. The obvious suspicion was local weather: perhaps storms in this part of India genuinely favour the east. An independent array of instruments that monitors the atmospheric electric field said otherwise. Actual thunderstorms, measured directly, showed no comparable preference for the eastern sky, which meant the asymmetry had to originate in the detector rather than in the atmosphere.

The answer lies in an effect that atmospheric and cosmic-ray physicists have known about for nearly a century: the east-west effect. Because cosmic-ray protons are charged, Earth’s magnetic field filters their arrival directions. Lower-energy positive particles arriving from the east are screened out more strongly than those arriving from the west, so the mix of cosmic rays, and therefore of secondary muons, depends on direction. This directional threshold, known as the geomagnetic cutoff, varies significantly across GRAPES-3’s field of view. The consequence is that the muon charge ratio, the very quantity that makes thunderstorm sensing possible in the first place, is itself direction-dependent, and the telescope is therefore more sensitive to thunderstorms in some parts of the sky than in others.

To test whether this could account for the observed lopsidedness, the researchers turned to computer simulations that model the production and propagation of muons through the atmosphere. The simulations showed that the balance between positive and negative muons changes markedly with direction: the muon charge ratio is about 1.37 in the east but only 1.14 in the west. That difference, they found, is more than enough to explain the event counts. As Gupta emphasizes, it is not that the voltage is different; thunderstorms to the east are not necessarily more electrically powerful than those to the west. Rather, the larger imbalance between positive and negative muons arriving from that direction makes the telescope more responsive to their electrical effects. The simulations confirmed the mechanism in both directions: when the direction-dependent charge ratio was included, a strong east-west asymmetry emerged, closely resembling the observations, and when the same ratio was imposed in both directions the asymmetry disappeared entirely.

The resolution of the puzzle matters well beyond bookkeeping for one detector. Understanding this directional sensitivity is essential if GRAPES-3 is to serve as a reliable, calibrated probe of thunderstorm electricity, because any inferred potential must account for how efficiently the instrument responds along each line of sight. With that correction in hand, the technique offers something no balloon campaign can match: continuous, ground-based monitoring of the largest electric fields in nature, drawn from a particle flux that arrives whether or not anyone is watching. The broader significance is considerable, since thunderstorm electrification is linked to lightning initiation, to terrestrial gamma-ray flashes, and to the still-uncertain role of storms in the global atmospheric electrical circuit. What began as an instrument for studying particles from the cosmos has ended up illuminating one of the most familiar and, remarkably, still one of the most mysterious phenomena on our own planet. The same muons that carry messages about distant astrophysical sources now carry, quite literally, the voltage of the storm overhead.

Subject of Research: Using cosmic-ray muons to probe electric fields and potentials inside thunderclouds

Article Title: From the cosmos to thunderstorms

Article References: From the cosmos to thunderstorms. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: cosmic rays, muons, thunderstorms, GRAPES-3, electric fields, geomagnetic field, muon charge ratio, atmospheric electricity, lightning, particle physics, TIFR, JCAP

A new study published in the Journal of Cosmology and Astroparticle Physics by Robson Christie, Jaewoo Joo, Greg Kaplanek, Vincent Vennin and David Wands used a simplified cosmological model to investigate what determines which vacuum a field may end up in within an expanding Universe. The stakes of this question are illustrated by the most famous example the authors themselves invoke: the Higgs field. The vacuum value of the Higgs field 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 it 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 it uses the Higgs 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. Classically, such a transition would require the field to climb over the energy barrier separating the two minima, like a ball rolling out of a shallow valley and over a mountain before it can descend into a deeper one. Quantum mechanics changes this picture fundamentally. The state of a quantum system can extend beyond the barrier, leaving a small but nonzero probability that the system will simply appear on the other side. This phenomenon, quantum tunnelling, is the mechanism by which a field in a false vacuum could, in principle, escape to the true vacuum, and it has long been a central concern in calculations of vacuum stability in the early and late Universe.

What distinguishes the new work is its treatment of isolation. Many standard calculations of tunnelling treat the field as completely cut off from its surroundings, an idealisation that the authors argue is physically misleading. “We know, however, that perfect isolation is an idealisation,” says Greg Kaplanek, a researcher at Syracuse University in New York. In reality, fields continuously interact with other fields and with whatever surrounds them, in other words with their environment. Kaplanek draws an analogy from laboratory physics: “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.” These interactions produce decoherence, the process by which a quantum system that can exist in a superposition of different possibilities gradually loses the ability to maintain that superposition and begins to behave more and more like an ordinary classical system. Using the analogy of a coin, a quantum state is not simply heads or tails but includes both possibilities at once, and interaction with the environment makes it increasingly difficult to preserve such a combination.

In the model constructed by Christie and colleagues, the environment is represented by other fields interacting with the main field under study. The main field can initially be in a quantum superposition involving both vacua, so the framework allows the researchers to track how the competition between the two minima evolves when the field is coupled to its surroundings. One of the surprises of the study is that the environment does not appear to play a decisive role in the initial choice of vacuum. Instead, what matters far more is whether the field is light or heavy relative to the Hubble scale, the characteristic rate at which the Universe is expanding. This comparison between the intrinsic dynamics of the field and the speed of cosmic expansion turns out to govern which minimum the field is likely to settle in during the expansion of space.

“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.” Heavy fields, in this sense, have enough time during each doubling of cosmic scale to relax toward their lowest-energy configuration, so the expansion does not outpace their internal dynamics. Something quite 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, rather than by the details of the environmental coupling. A rapidly expanding Universe can effectively freeze a light field before it has had the opportunity to find the deepest valley in its energy landscape.

The second half of the story concerns what happens after a light field has landed in a false vacuum. In a perfectly isolated quantum system, tunnelling towards the deeper minimum would still be possible, however slowly. 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,” Kaplanek explains. “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.” The authors call this phenomenon cosmic lockdown: a kind of lock that stabilises the field in whichever vacuum it has reached, preventing the slow quantum leakage that would otherwise, over vast timescales, allow transitions between minima.

The physical interpretation of cosmic lockdown connects it to a well-established quantum effect. The authors understand it as a manifestation of the quantum Zeno effect, the counterintuitive result that a quantum system which is continuously monitored can have much more difficulty moving from one state to another, because each measurement effectively resets its evolution. Of course, no one is literally observing the cosmological field in the model, and no conscious observer is required. “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.” In this sense, the rest of the Universe acts as an ever-present measuring apparatus, constantly extracting information about the field’s state and, as a byproduct of that information gathering, freezing it in place. The effect is a striking example of how openness to the environment, usually seen as a nuisance in quantum technology, can instead act as a stabilising force on cosmological scales.

The work remains a simplified model, and the authors are careful not to overclaim its reach. The study does not show that our current Higgs vacuum is protected by cosmic lockdown, and translating the result into a statement about the actual stability of the Higgs field would require more realistic treatments of the many interactions present in the early and late Universe. The result does, however, establish an interesting principle: interactions with the environment can make a false vacuum more stable by suppressing tunnelling towards another state, reversing the usual intuition that openness always undermines stability. “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, as some Standard Model calculations suggest may be the case, then cosmic lockdown points to at least one reassuring possibility: the ceaseless interaction of the field with its surroundings could help make a transition to a radically different state, one in which the structure of matter and the forces governing it would be altered, even more difficult than existing tunnelling estimates imply.

Subject of Research: Decoherence-induced suppression of quantum tunnelling between false and true vacua in an expanding Universe

Article Title: Cosmic lockdown: how the environment can isolate quantum fields

Article References: Cosmic lockdown: how the environment can isolate quantum fields. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: cosmology, quantum fields, decoherence, false vacuum, quantum tunnelling, Higgs field, quantum Zeno effect, vacuum stability, Hubble scale, Standard Model, early Universe, JCAP