Where do the universe’s highest-energy neutrinos come from?
Scientists have several theories, but they don’t know for sure. Scientists have discovered that the highest-energy neutrinos are not produced on Earth; they originate from beyond our solar system and have energies well beyond what we can produce in particle accelerators. What’s heating up these very energetic particles is one of those exciting science riddles.
The same property that makes neutrinos so difficult to catch—their aversion to interacting with matter—also makes them wonderful communicators from far-flung parts of the universe. Cosmic neutrinos, unlike light, can come to us from places that are opaque, revealing secrets of incredibly dense areas in our cosmos. And, unlike cosmic rays, which are charged nuclei that may have the same origins as cosmic neutrinos, neutrinos travel unaffected. Whereas cosmic rays are deflected by magnetic fields, cosmic neutrinos may be traced back to their origins and reveal information about the incredibly energetic processes that gave rise to them.
Supernova remnants, black holes, pulsars, explosions known as gamma ray bursts, and reactions in galaxies’ densely populated centers are also possible answers (called active galactic nuclei). However, it may possibly be something completely new that we haven’t seen before.
Scientists have recorded neutrinos so energetic that they must have originated outside our solar system using the IceCube experiment in Antarctica. Big Bird, Bert, and Ernie were given creative names by scientists based on Sesame Street characters. Other major projects (such as KM3NeT, which would use instruments strewn throughout a huge portion of the Mediterranean Sea) are also interested in catching these outlier particles. These cosmic emissaries heralded the birth of neutrino astronomy, a new means of learning about our vast, weird cosmos.
Several ground-based cosmic-ray studies throughout the world have detected extremely high-energy cosmic rays, but their sources are unknown. Because cosmic rays can be deflected in our galaxy’s magnetic field and no longer point back to their beginnings, locating their sources is particularly difficult. Scientists seek to learn more about high-energy cosmic rays by using cosmic neutrinos.
In our own galaxy and beyond, there are multiple probable cosmic sources of these ultrahigh-energy cosmic rays and neutrinos: galactic sources, such as supernova remnants, and extragalactic sources, such as active galactic nuclei and gamma ray bursts. Neutrino astronomy is a rapidly emerging science in which scientists use neutrinos instead of photons to scan the sky. IceCube, ANTARES, and KM3NeT are among the difficult projects that hope to shed light on the sources of these ultrahigh-energy neutrinos.
Despite the fact that the IceCube experiment has discovered numerous neutrinos with tremendous energy, no one has been able to pinpoint a specific cosmic source. So, while scientists have proof of their existence, they have no understanding how they are produced or where they originate. Scientists, on the other hand, thrive on problems like this.
As we approach the so-called Greisen–Zatsepin–Kuzmin (GZK) limit, high-energy neutrinos become very fascinating. The GZK limit is a theoretical upper limit on cosmic ray energies from distant sources, defined as anything more than 150 million light-years away from our pale blue dot. Because of the way cosmic rays scatter off cosmic microwave background photons, the low-energy light particles left behind from the Big Bang, they should have this limit, at least in principle.
This ultimate limit is around 1020 electronvolts, which is almost 70 million times higher than the highest energy we’ve been able to achieve in the lab. While cosmic rays must adhere to this limit, neutrinos do not. Neutrinos produced at these enormous energy are still capable of reaching us on Earth. The discovery of these tremendously energetic but very rare neutrinos could open up a whole new way of looking at and exploring the universe.
Scientists are coming up with new and fascinating ways to look for neutrinos. Let me give you an example. The entire Antarctic ice shield will be studied using radio technology in a high-altitude balloon.