What happens near black holes? How does a supernova explode? And what is the origin of the inconceivably energetic particles that shower down on the Earth as cosmic radiation? Physicists hope to get answers to these questions from an unusual telescope in a spectacular location: buried deeply within the ice of the South Pole, the sensors of the IceCube detector are waiting for neutrinos from outer space – ghostly elementary particles produced by cataclysmic events in the far reaches of the universe. Physicists from the DESY location in Zeuthen are playing a major role in the construction and operation of the world's largest particle detector.
Neutrinos are extremely light particles that pass through everything in their path almost unhindered. This makes them ideal cosmic messengers. That’s because neutrinos reach the Earth on a direct path, coming for example from the centres of distant galaxies, millions or even billions of light-years away. Light beams or gamma radiation from these galaxies easily get caught in nebulae on their way to Earth. Charged particles are deflected by cosmic magnetic fields, so that their actual source can no longer be determined. Neutrinos are affected by neither nebulae nor magnetic fields – which is why they can provide us with information from regions of space from where almost no other signal reaches the Earth.
What makes neutrinos ideal cosmic messengers, is also their drawback: they fly through matter so easily that every second, billions of neutrinos pass through every square centimetre of the Earth without leaving a trace. Only very rarely does a neutrino collide with a matter particle. Therefore, gigantic detectors are needed to occasionally observe such a neutrino event.
Particle tracks in the eternal ice
The IceCube neutrino telescope is frozen into the eternal ice of Antarctica – with an enclosed volume of one cubic kilometre, it is the largest particle detector in the world. At a depth of up to 2.5 kilometres, 86 steel cables with more than 5000 light-sensitive detector modules are embedded in the ice. These optical modules observe the tiny blue flashes of light that occur when neutrinos react with the crystal clear ice and thereby produce showers of charged particles. Based on these flashes of light, the flight direction of the neutrinos can be determined. From this, the physicists can in turn reconstruct which regions of space the elusive particles are coming from and where they originated.
As the second largest partner after the University of Wisconsin-Madison (USA), DESY is one of the major players in the international IceCube consortium. A quarter of the IceCube detector modules was produced at DESY in Zeuthen, and the Zeuthen researchers are also heavily involved in the data analysis.
Insight into cataclysmic cosmic events
Most neutrinos that hit the Earth arise in the sun or the Earth's atmosphere. Far less common are neutrinos that come from outside our solar system. Such extraterrestrial neutrinos are of particular interest to physicists because they provide insight into the vast cosmic objects from which they originate: gamma-ray bursts, supernovae, pulsars or the giant particle jets emitted by the supermassive black holes at the heart of active galaxies.
One scenario of interest might involve signals of a supernova exploding somewhere in our own galaxy. A celestial fireworks display of this kind would release an extremely large number of neutrinos that would doubtlessly be discernible with IceCube. Experts presume that a star would not be able to explode at all were it not for the neutrinos forcing the shell of the star outward with tremendous pressure. With IceCube, they hope to verify this theory and understand in detail how a star explodes.
First neutrinos from outer space
The IceCube collaboration announced its first major breakthrough in 2012. In their measurement data, the researchers identified two events with a tremendous energy of 1000 tera-electronvolts (TeV) – which is significantly more than what is expected for atmospheric neutrinos. Several more high-energy events were spotted in 2013. All the tracks differed so markedly from the atmospheric variant that the implication was clear: IceCube had tracked the first high-energy neutrinos from the depths of space.
The gigantic telescope at the South Pole thus opens a whole new branch of astronomy. With increasing numbers of events, the scientists hope to identify spatial or temporal accumulations that will allow them to identify individual sources of high-energy neutrinos in the cosmos.
- Neutrino telescope in the South Pole ice, complemented by a detector field at the ice surface (IceTop)
- Completion: December 2010
- Volume: one cubic kilometre
- Depth in the ice: between 1450 and 2450 metres
- 86 strings with 60 optical modules each
- 5600 optical modules in total