From the first cosmic neutrinos in 2013 to a flaring blazar, a hidden galactic core and our own Milky Way, here is what the Nobel-winning IceCube observatory has found.
For most of history, astronomy meant light. First visible light, then radio waves, X-rays and gamma rays, all of them forms of the same thing. In 2015, gravitational waves added a second kind of messenger. High-energy neutrinos from deep space are a third, and the IceCube Neutrino Observatory at the South Pole is the instrument that made them a working tool. That is why this year’s Nobel Prize in Physics, awarded to Francis Halzen, names not only the observatory but “the discovery of high-energy neutrinos of astrophysical origin”.
2013: the first cosmic neutrinos
IceCube was completed in December 2010. For its first two years, it recorded a steady stream of neutrinos made in Earth’s own atmosphere, when cosmic rays slam into air molecules. Those were expected. What astronomers wanted was something different: neutrinos with energies far beyond anything the atmosphere produces, arriving from all over the sky.
In 2013 the collaboration reported them. Among the first were two events with energies of about a thousand trillion electron volts, about a billion times more energetic than typical neutrinos from the Sun, which team members nicknamed Bert and Ernie. Further data built the case into a firm detection of a diffuse flux of astrophysical neutrinos, at a statistical significance far beyond the threshold physicists require. Physics World named the result its Breakthrough of the Year for 2013.
It was proof that the universe produces such particles in large numbers. It did not say where they came from.
2017: a blazar lights up
The first answer came four years later. In September 2017 IceCube detected a single high-energy neutrino and, within a minute, sent an automatic alert to observatories around the world. Telescopes turned to the patch of sky it pointed to and found a blazar, a galaxy whose central black hole fires a jet of matter almost directly at Earth. It was flaring brightly in gamma rays at the time. Archival IceCube data then revealed an earlier burst of neutrinos from the same object.
For the first time, a source of high-energy cosmic neutrinos had been identified, and it had been found through cooperation between a neutrino detector and conventional telescopes. That is what astronomers call multimessenger astronomy: watching one event through several kinds of signal at once.
Hidden engines and our own galaxy
Later results deepened the picture. In 2022 IceCube reported neutrinos from NGC 1068, a nearby active galaxy whose core is buried in thick dust and gas. Strikingly, the galaxy emits far fewer high-energy gamma rays than the neutrino signal would suggest. The likely explanation is that the neutrinos are produced in a region so dense that light cannot escape it, but neutrinos can. Here was the original promise of neutrino astronomy fulfilled: a view into a place that light cannot reach.
In 2023 the collaboration reported something closer to home: neutrinos from the plane of our own Milky Way. Our galaxy, which astronomers have mapped in every kind of light, could now be seen in neutrinos too.
The detector has also begun telling the three types, or flavours, of neutrino apart at these energies, including the elusive tau neutrino; the collaboration has reported seven astrophysical tau neutrino candidates. The mix of flavours arriving at Earth tests how neutrinos change type as they travel, over distances no laboratory experiment can match.
A third neutrino Nobel
Neutrinos have earned Nobel Prizes before. In 2002, the prize honoured pioneers who detected neutrinos from the Sun and from a nearby supernova. In 2015, it honoured the discovery that neutrinos oscillate between types, which proved they have mass. Those prizes were about neutrinos as particles. This year’s prize is different in kind: it is about neutrinos as messengers, used to study the cosmos rather than themselves.
IceCube contributes to particle physics as well. Its dense inner section measures how atmospheric neutrinos change type as they cross the Earth, and the detector is also used in indirect searches for dark matter. The prize, however, rewards the astronomy.
What remains unknown
For all its successes, neutrino astronomy is young. Most of the cosmic neutrinos IceCube has recorded have not been traced to any individual source. Whether blazars, dust-shrouded galactic cores, exploding stars or something else produce the bulk of them is still debated. Answering that needs more neutrinos, and that means bigger detectors.
Several are on the way or planned. In the Mediterranean, the KM3NeT project is building neutrino telescopes in deep seawater, and a large instrument is also operating in Russia’s Lake Baikal. IceCube’s own collaboration has proposed a much larger successor at the South Pole. Halzen said he hoped the Nobel would help win its approval.
Indian physicists take part in this effort, both as members of the IceCube collaboration and as independent researchers who search its publicly released data for neutrinos from objects such as pulsars. India’s own plan for a large underground neutrino laboratory, the India-based Neutrino Observatory, has faced years of delay, which makes those links more important.
A decade and a half after its completion, IceCube has done what its builders hoped: it has turned a ghost particle into a telescope. The sky, it turns out, looks different in neutrinos, and astronomers are only beginning to read it.
– Yadavalli VR Srinivas
