To most people the idea of burying a telescope in the ice would not be the best of ways of looking at the Universe. But, contrary to optical telescopes that are built in high-elevation sites around the world, when it comes to neutrino telescopes the deeper they are buried underground, the better. And a recent study published earlier this month reports that the biggest of these facilities, the IceCube Neutrino Observatory in Antarctica, has detected its first extraterrestrial neutrinos coming from outside of the Solar System.
Neutrinos are subatomic, elementary particles like electrons. But unlike electrons that have a negative electric charge, neutrinos have none, which makes them imperviable to electromagnetic interactions with ordinary matter. Indeed, billions of neutrinos pass through every cubic centimeter of the Earth and our bodies every second, without leaving any trace. Yet, much like the hypothesized dark matter particles, neutrinos can in rare occasions collide with normal matter. When such rare collisions take place, they result in the emission of small flashes of blue light, better known as Cherenkov radiation. And it is this nature of neutrinos that makes them very important in astrophysics research in general and in the study of cosmic rays in particular.
But what do neutrinos have to do with cosmic rays?
Cosmic rays are high-energy, electrically charged particles (mainly high-energy protons and heavy atomic nuclei) that permeate the Universe, coming from every direction of the sky. They are believed to be the products of such high-energy events in the Universe like supernova explosions, colliding black holes, and gamma-ray bursts. Because of their nature, after being emitted cosmic ray particles are frequently scattered and deflected in interstellar space, from interstellar gas clouds, magnetic fields, and other causes in between, which makes it difficult for astrophysicists to pinpoint their exact point source. It is for this reason that the exact origin of cosmic rays has remained a big mystery for astrophysics since their discovery in the early 20th century.
Furthermore, the Earth’s atmosphere and geomagnetic field prevent most of the cosmic rays from reaching the surface. In order to better study high-energy sources in the Universe, the best way is to study the neutrinos that these sources emit. Since neutrinos rarely interact with matter, they will travel in a straight line from their source toward Earth, passing through the obstacles that would block or deflect cosmic rays and other electromagnetic radiation. As an effect, they will also have retained all of their initial properties while reaching Earth, giving invaluable insights to astrophysicists about the exact conditions and properties of their place of origin.
Since neutrinos are so elusive, the best way to observe them is to build detectors, usually around huge tanks filled with a liquid medium, that in most cases is pure water. The detectors around the tank are photomultiplier tubes that have the ability to detect the light that passes through them. When a neutrino collides with an atom of the liquid inside the tank, it will result in the emission of Cherenkov radiation that will be detected by the photomultiplier tubes around the tank. The higher the energy of the original point source, the higher the energy of the colliding neutrinos will be. For such very high-energy sources, neutrino detectors need to be sensitive to energies in the TeV range. (eV is the unit measuring the energy of subatomic particles. 1 eV is the energy needed to move an electron through an electric potential difference of 1 Volt.) And the best location for neutron detectors is underground, where they are shielded by other sources of natural background radiation.
Such is the case with the IceCube Neutrino Observatory in Antarctica. It is located at the exact point of the geographic South Pole, underground the U.S. Amundsen-Scott South Pole Station. Instead of being inside a huge tank of water, its neutrino detectors are distributed around a cubic kilometer of pure water ice, extending 2.5 km below the station. The ice in these depths is really transparent and made of pure water, which in turn makes the detection of Cherenkov radiation easier. The detector itself consists of 86 strings in total, which run all the way from the surface down to the bedrock, ranging in depth from 1.5 to 2.5 km. Each individual string has 60 optical sensors, which brings the total number of sensors to 5,160. This whole underground facility makes up the IceCube Detector. On top of this, at the surface, sits the IceTop Array, consisting of a series of optical sensors as well—2 for each string of the IceCube Detector. This IceTop Array is used for complementary cosmic ray observations that hit the Earth’s atmosphere, producing showers of secondary particles.
The construction of the IceCube Neutrino Observatory was over in late 2010. Even though the detector had commenced observations while the construction was still ongoing, it wasn’t until it was finished that observations took advantage of its full detection capabilities. During that time, it had recorded hundreds of thousands of neutrino collisions in the ice. But in early 2012, it detected two neutrinos with an energy higher than 1 PeV each (1 PeV is the equivalent energy of a 1 megaton atomic bomb explosion). Scientists had affectionately nicknamed these two neutrinos as “Bert” and “Ernie,” borrowing the names of the two beloved Sesame Street characters. And running through the data accumulated during the previous years, they discovered 28 more neutrino signatures with an energy of at least 50 TeV. Although lower in energy than Bert’s and Ernie’s, these 28 neutrino signatures were still of high enough energy for physicists to realise that they weren’t coming from the neighbourhood.
“We have some really compelling evidence that we have neutrinos from beyond Earth’s atmosphere and beyond the solar system,” says Nathan Whitehorn, associate researcher at the University of Wisconsin-Madison, Department of Physics.“This is the first indication of very high-energy neutrinos coming from outside our solar system, with energies more than one million times those observed in 1987, in connection with a supernova seen in the Large Magellanic Cloud,” adds Francis Halzen, principal investigator for the IceCube Observatory. Although the number of detected neutrinos was too small for scientists to be able to pinpoint their exact source, they hope that with more such detections coming in, they will be able to accurately point to the neutrinos’ sources in the sky. And that could be really big news for astrophysics, not only because it could help to solve the mystery of the cosmic rays’ origin, but would shed some light to the processes behind the most energetic and violent events in the Universe as well.
It turns out that having something coming from outer space isn’t necessarily a cause for panic and terror, as was depicted in many retro sci-fi movies, but a cause for joy at unraveling some of the Universe’s biggest mysteries.