TheRoyal Swedish Academy of Sciencesannounced the awarding of the 2026 Nobel Prize in Physics to the BelgianFrancis Halzen, a professor ofUniversity of Wisconsin–Madison, in the United States.
The award recognizes Halzen for his decisive contributions to the creation ofthe IceCube neutrino observatory. It also awards the laurel for the discovery of high-energy astrophysical neutrinos – a fact that can be translated as opening a new window for the observation of the Universe.
But what are neutrinos?
Theneutrinosare the most mysterious particles in physics. Despite permeating all ofthe cosmosIn large numbers, with trillions of them passing through our bodies every second, they interact so little with the matter they traverse, traveling vast distances of the Universe without leaving a trace.
Therefore, we commonly refer to them as "phantom particles." These are subatomic particles without electric charge and with a very small mass, and they are produced, for example, inside the Sun and in our own atmosphere.
The high-energy neutrinos observed by IceCube, in particular, originate from the most extreme events in the cosmos, such as the processes that lead to the explosion of stars or that occur around black holes.
From the perspective of Astronomy, neutrinos are considered very special cosmic messengers. While light can be absorbed by gas and dust present in the cosmos, neutrinos easily pass through these obstacles, bringing information from regions of the Universe that would otherwise be inaccessible.
Another important aspect is that, since neutrinos have no electric charge, they propagate without their trajectories being deflected by interaction with magnetic fields.
In this way, unlike other particles, such as protons and electrons – which also carry information about some of these extreme astrophysical phenomena, but are deflected by the Galaxy's magnetic field, losing their directionality – neutrinos point directly to their regions of origin.
Halzen's contribution was fundamental in establishing a new field of astronomy, neutrino astronomy. This field is also called multi-messenger astronomy, as it introduces a new messenger to complement the studies of the cosmos that astronomers already made with electromagnetic radiation.
A new field – but full of challenges
However, the biggest challenge in neutrino astronomy is detecting them.
Just as a neutrino can bring us direct information from distant and deep places in the cosmos because it is not absorbed by the matter present there, it can easily pass through a detector without being detected. Therefore, to increase the chance of capturing a neutrino signal, it is necessary to build a giant detector.
Based on this challenge, in the late 1980s, Halzen helped develop a bold idea: to use thehighly transparent ice of Antarcticaas a giant telescope.
The idea was to install sensors in deep and dark areas of the Antarctic ice, capable of detecting the weak light signals resulting from the interaction of neutrinos with the ice, and wait for these rare interactions to occur.
The result was the observatoryIceCube, built in Antarctica. Unlike a traditional telescope, it consists of 5,160 light sensors distributed over 86 underground cables buried between 1.5 and 2.5 kilometers deep. Together, they occupy a volume of about one cubic kilometer of ice, and function as a hidden camera in the dark.
How IceCube sensors work
When a neutrino undergoes one of the rare interactions with the ice atoms, this collision produces a secondary charged particle, amuon.
Due to the large energy of the neutrino that originated it, this muon moves at a speed greater than that of light itself in the ice - not in a vacuum, which is the speed limit imposed by Einstein's theory of relativity for any object in the Universe - generating a flash calledCherenkov light, which the IceCube light detectors can detect.
Because there are many, and they are immersed in a perfectly transparent and dark environment, the IceCube sensors not only record the time and intensity of the Cherenkov light produced, but also the trace left by the muon's passage through the ice volume.
Thus, scientists can reconstruct the direction from which it came, and estimate the energy of the neutrino, allowing us to create a map of the sky, but with neutrinos.
The IceCube was built during the summers between 2004 and 2010. The construction was carried out through a careful process in which the ice was drilled with hot water and the sensors were inserted by cables to the deep regions where they would then be fixed when the ice refroze.
In just a few years of operation, in 2013, the IceCube Collaboration registered the detection of28 high-energy neutrinos, whose energy was incompatible with the processes capable of generating neutrinos in the atmosphere, meaning that their origin was necessarily astrophysical.
Thus, the astronomy with neutrinos began, which then became a new cosmic messenger.
This new approach to astronomy took a significant step forward in 2017, when the IceCube detector registered an extremely energetic neutrino originating from the same direction as the distant galaxy calledTXS 0506+056.
At the moment the neutrino was observed, the galaxy was emitting a strong burst of gamma rays.
The coincidence between the two events allowed, for the first time, to link one of the neutrinos observed by IceCube to its likely origin, revealing previously inaccessible information about its internal processes.
Once again, in 2022, the IceCube found evidence of neutrinos originating from another galaxy with high energy activity.NGC 1068In this, the center houses a supermassive black hole obscured by gas and dust. And in the following year, in 2023, it produced thefirst image of the Milky Way made with neutrinos.
All of these results consolidate Astronomy with Multi-Messengers as a new way to study high-energy astrophysical phenomena.
A way in which each of the messengers currently accessible to astronomical observation tells a part of the story and complements our vision of the cosmos.
A new area of study – and an important joint effort
It is true that astronomy with neutrinos is still a young field, and most of the cosmic neutrinos detected have not been associated with a specific source.
The signals are rare, the background signal from atmospheric neutrinos is large and imposes important limitations. Reconstructing the direction of these particles requires complex and sometimes still imprecise analyses.
At the time of such an important award as the Nobel, it is worth remembering that Halzen did not do this alone. Today, the IceCube brings together approximately450 scientists, engineers, and computer specialists from 58 institutions and 14 countries.
The IceCube observatoryhas just received new sensors, and the IceCube-Gen2, still under construction, will have an instrumented volume eight times larger than the original, increasing its ability to capture these rare cosmic messengers, and to discover and study the large cosmic particle accelerators.
And what about Brazil?
Despite not being a member country of the IceCube collaboration, Brazil has an important role to play in multi-messenger astrophysics. The projects ofCherenkov Telescope Array Observatory (CTAO)and ofSouthern Wide-Field Gamma-ray Observatory (SWGO), both with important national participation, will be gamma-ray observatories capable of working in close synergy with IceCube to reveal and study in detail the sources of high-energy astrophysical neutrinos.
The SWGO, in particular, is about to begin construction in the Chilean Andes, with important funding fromNational Fund for Scientific and Technological Development (FNDCT)through theFinep.
with international leadership ofBrazilian Center for Physics Research (CBPF), a research institute ofMinistry of Science, Technology and Innovation (MCTI)located in Rio de Janeiro, the SWGO will be the first observatory of this type to observe the sky of the Southern Hemisphere, opening another window for astronomy.
It will be a new opportunity to help experiments such as IceCube to unravel the long-standing mystery of the large particle accelerators of the Universe.
Ulisses Barres de Almeida receives funding from the Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ), the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), and the Financiadora de Estudos e Projetos (FINEP).