High-energy neutrinos help us understand where and how the universe’s most energetic processes occur, because they can travel undisturbed from their sources to Earth. This allows scientists to study extreme astrophysical environments, such as regions near black holes, as well as violent phenomena like supernovae.

– Neutrinos are sometimes called ghost particles because they pass us by almost completely unnoticed. They can travel straight through walls – yes, even through the entire globe – without being affected. Only in the very rare cases when a neutrino passes close enough to an atom and collides with it can it be detected, says Olga Botner, professor of experimental particle physics at Uppsala University. She has worked with IceCube since 1998, when the prototype detector AMANDA was developed, and has since held several leading roles in IceCube.

– Since detector material consists mostly of empty space, an enormous volume is required for such collisions to occur at all. Using around 5,000 sensors, IceCube monitors a cubic kilometre of glacial ice to capture these extremely rare events.

From ideas to international observatory

IceCube is an international research project and the result of several decades of work. The journey spans from a vision in the 1960s, through sketches and tests in the early 1990s, to the validation of the prototype detector AMANDA at the South Pole in 2001. In 2013, a flux of cosmic neutrinos was detected for the first time, and over the past decade, individual astronomical neutrino sources have also been identified.

– Swedish researchers from Stockholm and Uppsala have contributed to IceCube since 1993, including through the development of the detector concept and the construction of equipment, initially in collaboration with colleagues from three American universities. The project received early financial support in Sweden from the Knut and Alice Wallenberg Foundation and the Swedish Research Council. The Polar Research Secretariat has also contributed through logistical support and personnel during the installation campaigns at the South Pole, says Olga Botner.

IceCube Upgrade and Sweden’s continued role

Fifteen years after completion, IceCube is now being upgraded during the 2025/26 Antarctic summer. The upgrade includes more than 600 new sensors, installed on six new detector strings in the central part of the array. These have denser instrumentation and include special calibration units that will provide a more detailed understanding of the properties of the glacial ice.

A thorough understanding of glacial ice is crucial for reconstructing the energy and direction of arrival of the neutrinos. The denser instrumentation also enables the detection of neutrinos with lower energies than before, potentially opening the door to studying new types of astronomical objects and offering improved opportunities to investigate the fundamental properties of neutrinos.

– Sweden played a significant role in the construction of IceCube and is also contributing to this upgrade. Around 1,000 of IceCube’s total of 5,000 sensors have been built and validated in Stockholm and Uppsala. All 86 kilometres of deep cables that supply the sensors with electricity and communications were developed and manufactured in Sweden by Ericsson Cable (now Hexatronic). For the IceCube Upgrade, Sweden is once again contributing cables from Hexatronic, both at depth and on the glacier surface. Swedish researchers have also developed cameras that will enable linking structural variations in the ice to the calibration data, says Olga Botner.

The Swedish efforts in IceCube Upgrade are supported by the Swedish Research Council/RFI and by personnel from the Polar Research Secretariat who are participating in the installation of the equipment at the South Pole.

A new window to the universe

IceCube has opened a completely new observation window to the universe. With the new units in IceCube Upgrade, researchers hope to identify more neutrino sources at lower energies and produce more detailed computer simulations of the ice. This will, in turn, improve the reconstruction of neutrino direction and energy – not only for future observations, but also retroactively for data collected over the past 15 years.

The goal is to explore the universe’s most energetic processes with greater precision than ever before.

In addition to mapping neutrinos from extreme environments, IceCube is also part of what is often called multi-messenger astronomy. This means that scientists combine information from multiple “messengers” from space – such as neutrinos, electromagnetic radiation (light of different wavelengths), and sometimes even gravitational waves – to get a more complete picture of what happens in violent cosmic events. When IceCube records a particularly interesting neutrino event, the observatory can send real-time alerts to other facilities, which can quickly point their telescopes at the same part of the sky and try to capture simultaneous signals.