Ever since their discovery in the 1960s, ultrahigh-energy cosmic rays have captivated scientists, who wonder where they come from. Like all cosmic rays, they are arguably misnamed: they are not “rays” of radiation but rather subatomic particles, such as protons or even entire nuclei, zipping through space. Such ultrahigh energies come from ultrahigh speeds, approaching that of light itself.
To be considered “ultrahigh,” a cosmic ray must carry on the order of a quintillion electron volts, or 1,000 peta-electron volts (PeV), of kinetic energy—about one hundredth of what would be required to tap out a single character on a keyboard. Squeezing so much energy into such a tiny object—a trillion times smaller than a speck of dust—far exceeds the capabilities of humankind’s accelerators, which, at their best, only manage to produce particles with about the energy of a flying gnat.
And as jaw-dropping as an average ultrahigh-energy cosmic ray may be, the very rare overachievers that researchers have managed to observe are truly astonishing, carrying energies up to 300 times greater—a whopping 300,000 PeV. For reference, that means an especially speedy subatomic projectile hurtling out of deep space can pack the wallop of a well-hit tennis ball.
Astrophysicists do not yet know what exactly accelerates these particles to such ludicrous speeds, but they desperately wish to find out. The only plausible culprits are truly cataclysmic events—such as the explosive deaths of massive stars or the voracious feeding of supermassive black holes beyond the Milky Way—meaning that these extraordinary particles must be messengers from the depths of extragalactic space, carrying secrets from some of the most extreme physics in the universe.
There is, however, one big problem. As charged particles, all cosmic rays are diverted on their travels by any electromagnetic fields they come into contact with, making it nearly impossible to trace them back to their true celestial origins. Fortunately, researchers have found that nature offers another way forward: studying neutrinos, electrically neutral particles thought to be produced in the same sources as the highest-energy cosmic rays themselves.
“I think of neutrinos as the perfect messenger particle,” says Abigail Vieregg, an astrophysicist at the University of Chicago. “They’re unique in that they travel from far away in the universe without interacting with anything or getting bent in magnetic fields on their way here.”
Probing the Universe with Neutrinos
An average neutrino has a 50–50 chance of passing through an entire light-year of lead—9.5 trillion kilometers of dense metal—entirely unscathed. That profound aloofness gives the particles an advantage over other messengers: because they rarely interact with matter, neutrinos point straight back to where they came from. But this is a double-edged sword. An unavoidable consequence of traversing the universe as if it were transparent is that neutrinos typically pass through detectors on Earth in the same way—without a trace.
To increase the odds of seeing a neutrino, scientists must build gigantic detectors such as the IceCube experiment at the South Pole, which consists of a cubic kilometer of Antarctic ice fitted with an array of optical sensors. As the world’s largest neutrino observatory, IceCube searches for flashes of light emitted by charged particle showers produced when neutrinos collide with molecules in the ice. In 2018 IceCube reported a neutrino from a giant flaring blazar. And as recently as February, it saw evidence of a neutrino from a star being ripped apart by a black hole.
But at the highest energies, “IceCube just runs out of steam,” Vieregg says, noting that it would take at least 100 cubic kilometers of ice to have a reasonable chance of observing the optical traces of ultrahigh-energy neutrinos because particles accelerated to such extreme speeds are exceedingly rare. The issue lies with the spacing between detection units: light can only travel some tens of meters in ice before scattering or being absorbed, so the optical array must be packed densely, strictly limiting achievable detector size.
Thus, the sources of ultrahigh-energy particles remain undiscovered because an IceCube-style observatory of 100 cubic kilometers far surpasses the boundaries of technical and financial feasibility. In their quest to observe the first ultrahigh-energy neutrino, astrophysicists have instead shifted focus to the more economical approach of radio detection. Radio waves can travel hundreds of meters further in ice than optical light, so a sparser array of detection units can be built to cover a much larger volume at a fraction of the cost.
“Radio is the future,” says Tonia Venters, an astrophysicist at NASA’s Goddard Space Flight Center. “I view it as a complementary probe with the potential to do what we’re finding very challenging with other detection techniques.”
Neutrino Radio Emission
The radio emission of charged particle showers in materials like ice is even more intense than optical signals at ultrahigh energies, making it an attractive probe into the extreme universe. This phenomenon is known as the Askaryan effect, after Russian-Armenian physicist Gurgen Askaryan, who first predicted it in 1962.
But early attempts to observe the Askaryan effect proved unsuccessful, leading to widespread skepticism that it could be used in in ultrahigh-energy particle detection. “There was a lot of doubt as to whether this was a real effect,” says Peter Gorham, an astrophysicist at the University of Hawaii at M?noa. “Not many high-energy particle physicists were taking this seriously.”
Nevertheless, a small but resilient team of physicists persevered, and the field reached a turning point in 2000, when they confirmed the Askaryan effect in the back of a trailer at the Stanford Linear Accelerator Center (SLAC).
Now, nearly 60 years after Askaryan’s prediction, neutrino detection in the radio regime is just taking off. “The new physics that may come out this is not even something we can dream of,” says Gorham, who was a member of the team at SLAC. “We’ll learn about the nature of cosmic accelerators and observe regions of energy space that we can’t access any other way.”
Next-Generation Radio Efforts
Led by Gorham at the University of Hawaii at M?noa, a pioneering effort in neutrino radio astronomy was ANITA (Antarctic Impulsive Transient Antenna), which began gathering data in 2006. Composed of a progressively updated set of antennas slung beneath a giant helium balloon, ANITA conducted four approximately month-long observing campaigns across a 10-year period, each time soaring several kilometers in the air to scan the Antarctic ice sheet below for signs of radio emission from ultrahigh-energy neutrino strikes.
In January NASA funded the Payload for Ultrahigh Energy Observations (PUEO), a next-generation experiment that will build from the heritage of ANITA. Their high-altitude perspective gives balloon-borne detectors such as ANITA and PUEO a unique edge over ground-based experiments because they can monitor more than a million square kilometers of ice in their neutrino searches. PUEO’s first flight is anticipated in 2024, and it will incorporate multiple technological advancements over ANITA for an increased sensitivity to more energies, as well as a higher neutrino event rate.
But the enlarged field of view boasted by balloon-borne searches is counterbalanced by the fact that, precisely because the antenna arrays fly so far above the ice, they may not be able to see radio emissions from fainter neutrino signals. Another downside is the reality of difficult weather: poor conditions are a regular disruption for any sort of balloon work over the Antarctic ice sheet. To address these problems, astrophysicists are adopting a “best of both worlds” approach, creating new radio arrays inside large volumes of ice that can then work in tandem with balloon-borne experiments for a broader energy coverage. Preceded by a slew of smaller efforts, researchers are gearing up for the installation of the Radio Neutrino Observatory in Greenland (RNO-G), an in-ice experiment led by the University of Chicago.
“RNO-G will be the largest radio detector ever built in ice, with 35 stations of antennas installed over the next three years,” says Stephanie Wissel, a Pennsylvania State University astrophysicist involved in the construction of the observatory. Many researchers are optimistic that RNO-G will soon permit an initial peek into the extreme universe with the first detection of an ultrahigh-energy neutrino.
But if not, the in-ice radio array concept will be scaled up for use in IceCube’s proposed successor, IceCube-Gen2, which will have 200 stations of antennas surrounding an enhanced optical system. “IceCube can see neutrinos up to about 10 peta-electron volts. But with the added radio component, this will go up to thousands or even hundreds of thousands,” says Vieregg, who is principal investigator of both PUEO and RNO-G. This expanded energetic reach comes in at only 10 percent of IceCube-Gen2’s total budget, an impressive nod to the cost-effectiveness of radio detection.
A more novel detection strategy will hunt for radio waves from charged particle showers in air rather than ice. The former result from neutrinos interacting underground, near the surface of our planet: with the right conditions, these Earth-skimming neutrinos can create high-energy particles that escape into the atmosphere and decay into extensive, radio-emitting air showers.
This is the strategy for the Giant Radio Array for Neutrino Detection, or GRAND—an apt name for an experiment of its enormous size. Organized and funded by institutions in France, China, the Netherlands and Brazil, the international GRAND collaboration hopes to discover the origins of ultrahigh-energy cosmic rays with an ambitious proposal for a 200,000-square-kilometer radio array (that is, an array about the size of Nebraska).
“The idea is to build not one monolithic array but 20 arrays of 10,000 antennas each,” says Mauricio Bustamante, an astrophysicist at the University of Copenhagen, who co-authored the proposal for GRAND. The locations of these arrays are important, he explains, because they need to be in “radio-quiet” areas—far from artificial sources of significant radio emission. To date, GRAND has identified a few remote sites in the Tian Shan Mountains of Central Asia, with plans to scout for additional locations all over the world.
With a variety of next-generation radio experiments on the way, the astrophysics community is buzzing with ideas about what the future may hold after one of nature’s most energetic and elusive messengers is finally found. “I greatly anticipate the discovery of the first ultrahigh-energy neutrino,” Wissel says. “I’m not sure which experiment will do it first, but it will open up a new window to the universe with lots of potential for discovery.”
And for scientists familiar with the history of the field, the exploration of new cosmic frontiers is an ode to the past: physics flourished in the 20th century by studying what particles came from the sky. “It’s a natural turn of events that we go back again to cosmic accelerators when we want to find out more than what our own machines can tell us,” Bustamante says. “That’s the whole purpose of studying the highest-energy particles of our universe.”