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ON A clear night, watching the stars in an inky sky, one word comes to mind: calm. The starlight seems to speak of stability and permanence. And yet, hidden from the naked eye, the wider cosmos is a place of relentless upheaval. Every 10 seconds or so, somewhere in the universe, a star reaches the end of its life and caves in on itself before exploding with cataclysmic ferocity.

For all their ubiquity, we still don’t fully understand what triggers core-collapse supernovae. But we do suspect that elusive particles called neutrinos play a key role, and that observing them in the unimaginably extreme conditions inside a supernova could betray the exotic matter and forces that would lead us to a deeper theory of particle physics. “There’s just so much information in supernova neutrinos,” says , a theorist at Johannes Gutenberg University in Mainz, Germany.

The problem is that supernovae close enough to Earth to spill their secrets are rare. The most recent one came in 1987, and the 25 neutrinos we were able to capture from the blast continue to mystify researchers to this day – which explains why so many are so excited by the new generation of neutrino detectors currently being built around the world.

The idea is that when the next nearby star goes supernova, we will be ready. But it is a race against time because that is already long overdue. “You can imagine how exciting it will be if we actually observe one of these bursts,” says , a neutrino physicist at Duke University in North Carolina. “And how terrible it would be if we miss it.”

Neutrino detector

Neutrinos were predicted by calculation long before being found in experiments. In 1930, Wolfgang Pauli showed that a particular form of radioactive decay, known as beta decay, seemed to lose energy, a clear violation of known physics in which energy can be neither created nor destroyed. To restore normality to the universe, Pauli proposed that an as-yet-undetected particle must be at work to spirit the energy away. It was called the neutrino – Italian for “small neutral one” – as it was thought to be massless and to have no charge.

That is why people call neutrinos ghostly particles. Tens of billions of them pass through your fingertip every second unnoticed. They only reveal themselves when they occasionally interact with other particles via the weak nuclear force. This also means that they are difficult to investigate in particle physics experiments and, as a result, we know relatively little about them.

But these shy particles are arguably the key to whatever theory supersedes the standard model of particle physics, our current best description of the fundamental particles and forces of nature, which we know is incomplete. According to the standard model, neutrinos are massless particles that come in three distinct flavours: electron, muon and tau. But early neutrino experiments that monitored the versions of this particle produced by the sun, as well as by cosmic rays in the upper reaches of Earth’s atmosphere, deduced that neutrinos could transform between flavours. That threw up a problem, as this shape-shifting is only possible if they have mass, the origin of which can’t be explained by the standard model.

Physicists have concocted a scheme for these so-called neutrino oscillations to occur, as a kind of add-on to the standard model. It includes the idea of three discrete masses for the particles. Depending on the energy of the neutrino and the distance over which it has travelled, a probability can be calculated that it will change flavour. These probabilities depend on the masses, but, confusingly, the three possible masses don’t match the three flavours. Physicists don’t know the value of these masses, either. So the strange ways of neutrinos, and the clues they might contain about interactions with long-sought hypothetical particles, continue to elude us.

With this in mind, particle physicists are busy constructing several multi-billion-dollar neutrino experiments around the world. In the US, there is the (DUNE); in Japan, there is (Hyper-K); and in China, there is the (JUNO). When switched on within the next decade or so, these facilities will fire intense neutrino beams made in particle accelerators through warehouse-sized vats of liquid. Occasional flashes of light emanating from these vats will signal that a neutrino has interacted with the liquid. By varying the energy of the neutrino beam, and with each experiment having a different distance over which the beam has travelled, researchers hope to precisely grasp how neutrinos shape-shift.

These liquid vats are also sensitive to neutrinos arriving from outer space. The immense energy released from a collapsing star produces vastly more neutrinos than a terrestrial particle accelerator: 10⁴⁵, to be precise. What’s more, the energies imparted to these neutrinos are far larger than anything that can be made here, offering a fresh angle on how neutrinos mutate. “[Supernova neutrinos] are the kind of neutrinos that I’m most excited about detecting,” says Scholberg, who is a DUNE collaborator.

This isn’t just a numbers game, though. The ultra-dense conditions inside a core-collapse supernova could also reveal new particles and interactions that would leave their imprint on the signals from these events. Take the most recent nearby supernova, SN1987a, the light from which suddenly appeared in the night sky on 23 February 1987. At the time, neutrino detectors were just starting to mature, so detectors in Japan, the US and the then Soviet Union captured only a handful. “That was a tremendous detection, [giving] enormous information, but it was still only a couple of dozen neutrinos,” says Scholberg. They nevertheless allowed physicists to put new limits on the existence of all kinds of novel theoretical particles, including axions, dark photons and another type of neutrino known as a sterile neutrino – all of which are contenders to be the stuff that makes up dark matter, the mysterious source of gravity invoked to explain our observations of how galaxies rotate, clump together and form in the first place.

“Just imagine how much we’re going to learn from the tens of thousands of events that we’ll have when the next galactic supernova explodes,” says Kopp.

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But there is a problem. Compared with particle accelerators, which make carefully controlled neutrino beams, the cores of supernovae are chaotic and poorly understood. Only by simulating star explosions accurately will physicists be able to make sense of data from a nearby supernova – and there are already signs that these computer simulations are still missing a key component.

A supernova is imminent when a giant star runs out of nuclear fuel. In a matter of milliseconds, the once-furious stellar core, typically about the same diameter as Earth, stops generating energy and collapses into a ball just 50 kilometres across. In the process, electrons that circle each nucleus are forced into the centre of each atom, where they combine with protons to produce neutrons. Each one of these reactions creates a neutrino that escapes into space. “The neutrino is actually the dominant energy-loss mechanism for the supernova,” says , a particle physicist at Stanford University, California.

But when we model core-collapse supernovae in a supercomputer, the star often fails to explode. The problem with these simulations is that when the core collapses, the rest of the star begins to fall downwards onto it. As this material strikes the core, it creates a shock wave that starts to propagate outwards through the in-falling matter. If this shock wave reaches the surface, the star explodes and the supernova is born. The trouble is that the surrounding matter is falling so quickly that the outward-moving shock wave stalls. Hence, in current simulations, it never reaches the star’s original surface and the supernova never quite happens – which suggests there is something missing from our understanding of how these explosions proceed.

In recent years, theorists have had . By allowing a minuscule percentage of the escaping neutrinos to be absorbed by the shock wave, the wave gains enough energy to carry on and explode the star. Yet they still have trouble matching their simulations to real events.

For one thing, according to an , our best simulations aren’t consistent with supernova SN1987a. “We thought since all modern theories agree with each other more or less, in terms of [the number of neutrinos produced in the supernova], then surely they must agree with SN1987a data,” says at Fermi National Accelerator Laboratory in Illinois, who co-authored of the analysis. “To our surprise, it seems like just counting neutrino detections – the simplest thing you can do – showed some disagreements.”

Modern simulations predict that we should have detected more neutrinos in 1987. It is a counterintuitive twist that the more sophisticated our computer simulations of supernovae have become, the more they have diverged from the one and only actual supernova for which we have data. “It seems like the data has a lot to say about modern simulations,” says Li.

Hidden forces

The missing ingredient could be our imperfect understanding of neutrinos, suggests Li. For one, neutrino oscillations aren’t implemented in modern simulations. Some extended models of particle physics also include “secret” interactions between neutrinos that cause them to scatter from one another at the extremely high densities expected in a supernova. These interactions are introduced to try to . A found that if these interactions do take place, then this could be observed in a nearby supernova because such behaviour would lengthen the time over which the neutrinos leave the core. Instead of flying out directly, they would jostle each other. “They influence each other in highly non-trivial ways. This is one of the big puzzles of neutrino physics,” says Kopp.

The fact that modern supernova simulations predict too many neutrinos may already be a hint of new particles beyond the standard model, suggests Li. “The generation [of such particles] would take away some of the predicted neutrino luminosity and make it match better with the data,” says Li. She isn’t the only one thinking in this direction. Graham says that particles of dark matter escaping the explosion undetected “could easily be the dominant energy-loss mechanism of the supernova, even more than neutrinos”.

In effect, a supernova could be a dark matter factory. Depending on their mass, these particles would be made in abundance during the collapse of the core. They would then immediately diffuse out of the core. As a result, the number of neutrinos arriving at Earth would fall away more quickly than the current simulations suggest. This effect could also reveal the character of the dark matter in a way that traditional observations simply cannot. “Supernovae are really good laboratories for looking for particles [that only couple through the weak nuclear force],” says Graham – as many theorists suspect dark matter does.

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In pursuit of dark matter, researchers are doing everything from burying vats of xenon deep underground to sending a balloon floating above the Antarctic. When will their creativity pay off?

There is another tantalising possibility that could emerge from such a neutrino signal. If we see the neutrino blast stop suddenly, it would indicate that the star itself has collapsed into a black hole, as nothing can escape its interior. Observing this up close would be a first for astronomers, and if they could then figure out how it happens, it could help to unlock a host of puzzles about how galaxies work. It could even offer extreme tests of fundamental theories of nature. “My favourite astrophysical scenario would be if there’s a collapse to a black hole, so we can actually see that in real time,” says Scholberg.

The prospect of a nearby supernova is, without doubt, exciting. The neutrino data we collect could well be unique in terms of its quantity and quality, illuminating how a supernova explosion grows, as well as the enigmatic workings of neutrinos, black holes and dark matter. But will nature oblige?

Betelgeuse

There are signs that it might. In 2019, the red giant star Betelgeuse, which is around 700 light years from Earth, dimmed unexpectedly by a factor of three. This led to speculation about whether its energy generation was becoming unstable in the prelude to a supernova. By April 2023, however, the star had returned to normal and many astronomers have begun to favour an explanation that postulates a large dust cloud formed from matter released at the surface of the star, temporarily obscuring our view – rather than a sign that the star has run out of fuel and is on the verge of collapse. We just have to wait and see.

There are other red supergiant stars only hundreds or thousands of light years from Earth – Eta Carinae and Antares, for example – that astronomers are keeping a close eye on. But the truth is that the next big one could come from anywhere in the Milky Way, or a neighbouring galaxy, at any time.

Scholberg hopes that the next galactic supernova bides its time for just a few more years, at least until the next generation of neutrino detectors is ready. “My dream is to have a supernova with DUNE, JUNO and Hyper-K running all at the same time,” says Scholberg. Then again, at the other end of the scale, it could come too late because there is no guarantee that one will explode in the lifetime of these experiments. We have to get lucky, in other words.  If we do, says Kopp, capturing and interrogating supernovae neutrinos could end up being “by far the most important legacy of these experiments, even outshining all the fantastic accelerator-based neutrino physics that they are going to do”.

The message is clear: keep calm and carry on building the detectors.

Stuart Clark is a consultant to 91av. His latest book is Beneath the Night: How the stars have shaped the history of humankind