How neutrinos are beginning to reveal the secrets of our galaxy

The day before this year’s Nobel prize in physics was announced, I was quoted in The New York Times as saying the question of why neutrinos are non-trinary is one of the most pressing scientific issues facing my field. What a happy coincidence that the next day, the Nobel committee announced the prize had been given to Francis Halzen for his leadership on the IceCube Neutrino Observatory. I joked on social media that I was somehow responsible for the committee noticing this important experiment in Antarctica, which studies neutrinos that come from deep space, often looking for how they provide information that other signals – light, gravitational waves – cannot by themselves.

Regular readers of this column know that I love to talk about these subatomic particles because they have the distinct property of being non-trinary. There are three flavours of neutrino and, as they travel through space, they oscillate between these identities. Neutrinos refuse the notion of a fixed identity. We understand these oscillations to be related to their mass. But we actually still don’t understand how they get their mass.

Despite this mystery, we do know plenty about where neutrinos are made. One place is in stars, including our own sun. The nuclear fusion processes that make all the light we get from stars also make neutrinos as a by-product. Those same phenomena also play a role in stellar deaths, like the fantastical explosions we call supernovae. In fact, next year we will be celebrating the 40th anniversary of our first time connecting observed neutrinos with such an explosion.

The first supernova detected in 1987 – designated Supernova 1987A – was a brilliant collapse of a former blue supergiant star. It was the closest supernova to our solar system in centuries, and we witnessed it across the electromagnetic spectrum: optical, infrared, ultraviolet and even in the X-ray. But one of the most interesting things about 1987A’s discovery is that hours before the light became visible, three different neutrino detectors on Earth noticed an influx of these ghostly particles.

In other words, neutrinos were our first messenger from the stellar death that occurred 168,000 light years away. The reason the neutrinos arrived first is down to how deeply antisocial they are. Neutrinos pass by other matter like it’s not even there. So, while the light particles – made in stars alongside neutrinos – were trying to escape the plasmas and atoms they ran into, the neutrinos were streaming freely out of the collapsing star into the cosmos.

In the 39 years since 1987A’s neutrinos helped us place bounds on the mass of neutrinos, and also how many neutrino flavours there are, neutrino astrophysics has become a new vista on the universe.

In the years after 1987A, Halzen led the group that conceptualised IceCube to use those neutrinos as a lens on the cosmos. Over the past few decades, thousands of scientists have worked together to make this concept into a reality in Antarctica and used the instrument to do some truly fascinating science. For example, Naoko Kurahashi Neilson at Drexel University in Pennsylvania led the effort to make a complete map of the Milky Way using only neutrinos. It took almost a decade, but, in 2023, IceCube unveiled a groundbreaking new atlas of our home galaxy. The image is very different from what we get with light: it’s not symmetric and is full of neutrino blobs that are begging us to figure them out. The asymmetry is especially tantalising to me because that’s usually an indicator that something interesting is happening – but what?

These days, Neilson is leading a group within IceCube that is trying to understand what neutrinos can tell us about the galactic centre, especially any details about what is accelerating particles in the region to relativistic speeds. In September, the group shared new research about the challenges of matching neutrino sources with the most high-energy light sources we know, on the assumption that the neutrinos and the light are coming from the same place. The hope is that we can establish what exactly is sending these energetic and mysterious particles out into the void. From my point of view, they’re showing that our galaxy is one of the best naturally occurring particle physics experiments out there.