Ultrahigh-energy cosmic rays have captivated scientists since their observation in the 1960s, who are curious about where they originate from. Like all cosmic rays, they are arguably misnamed: they are subatomic particles zipping through space, not “rays” of radiation. These ultrahigh energies are the product of ultrafast speeds that exceed the speed of light.
A cosmic ray must have kinetic energy on the order of a quintillion electron volts, or 1,000 PETA-electron volts (PeV), to be called “ultrahigh”—roughly one-hundredth of what is needed to type a single character on a keyboard. Squeezing as much energy into such a small object—a trillion times smaller than a speck of dust—far exceeds the capability of human-made accelerators, which can only contain things with the energy of a flying gnat at most.
And as impressive as an ordinary ultrahigh-energy cosmic ray is, the very rare overachievers discovered by researchers are incredible, with energies up to 300 times more significant—a staggering 300,000 PeV. To put it another way, a high-speed subatomic projectile hurtling through space will pack the punch of a well-hit tennis ball.
Astrophysicists aren’t sure what accelerates these particles at such incredible speeds, so they’re eager to find out. Only genuinely cataclysmic phenomena, such as the explosive deaths of giant stars or the voracious feeding of supermassive black holes outside the Milky Way, are possible suspects. Consequently, these extraordinary particles must be messengers from the farthest reaches of extragalactic space, bearing secrets from the universe’s most intense mechanics.
However, there is one major issue. Since cosmic rays are charged particles, any electric fields they encounter redirect them on their journeys, making it almost difficult to track them back to their original celestial source. Fortunately, scientists have discovered that studying neutrinos, electrically neutral particles thought to be produced in the same sources as the highest-energy cosmic rays, provides an alternative path forward.
According to Abigail Vieregg, an astrophysicist at the University of Chicago, neutrinos are “the perfect messenger particle.” “On their way here, they travel from far away in the universe without colliding with anything or being bent by magnetic fields.”
NEUTRINO RESEARCH INTO THE UNIVERSE
A neutrino has a 50–50 probability of traveling completely unharmed across a light-year lead—9.5 trillion kilometers of dense metal. Since neutrinos seldom interfere with the matter, they have an advantage over other messengers in that they point back to where they come from. However, this is a two-edged sword. Since neutrinos travel across the cosmos as though they were invisible, they usually move through detectors on Earth in the same way—without leaving a trace.
Scientists would construct massive detectors like the IceCube experiment at the South Pole, which consists of a cubic kilometre of Antarctic ice equipped with an array of optical sensors to maximize their chances of seeing a neutrino. IceCube, the world’s largest neutrino observatory, looks for light bursts caused by charged particle showers generated as neutrinos interfere with ice molecules. IceCube discovered a neutrino from a massive flaring blazar in 2018. It even saw signs of a neutrino from a star being torn apart by a black hole as early as February.
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. The optical array must be packed densely, strictly limiting achievable detector size.
As a result, the origins of ultrahigh-energy particles remain unknown due to the physical and financial impossibility of building an IceCube-style observatory with a volume of 100 cubic kilometres. Astrophysicists have turned their attention to the more cost-effective method of radio discovery in their attempt to observe the first ultrahigh-energy neutrino. Since radio waves move hundreds of meters farther in ice than optical rays, a sparser array of detection units may be constructed to protect a much wider area for a fraction of the cost.
Tonia Venters, an astrophysicist at NASA’s Goddard Space Flight Center, believes that radio is the way of the future.
RADIO EMISSION OF NEUTRINO
At ultrahigh energies, the radio emission of charged particle showers in materials such as ice is much more intense than optical signals, rendering it an appealing probe into the outer cosmos. The Askaryan effect is named after Russian-Armenian scientist Gurgen Askaryan, who predicted the phenomenon in 1962.
However, early efforts to observe the Askaryan effect failed, leading to widespread doubt that it could be used in ultrahigh-energy particle detection. According to Peter Gorham, an astrophysicist at the University of Hawaii at Manoa, “there was a lot of concern about whether this was a real effect.” “It wasn’t taken seriously by many high-energy particle physicists.”
Despite this, a small but tenacious group of physicists persevered. The field achieved a watershed moment in 2000 when the Askaryan impact was confirmed in the back of a trailer at the Stanford Linear Accelerator Center (SLAC).
Neutrino’s discovery in the radio regime is just now taking off, almost 60 years after Askaryan’s estimate. “The new physics that can emerge from this is not yet something we can imagine,” says Gorham, a member of the SLAC team. “We’ll hear about cosmic accelerators and observe areas of energy space that we can’t reach any other way.”
EFFORTS IN NEXT-GENERATION RADIO
ANITA (Antarctic Impulsive Transient Antenna), which started collecting data in 2006 and was led by Gorham at the University of Hawaii at Manoa, was a seminal endeavor in neutrino radio astronomy. Over a ten-year duration, ANITA, which consisted of a continuously upgraded series of antennas slung under a gigantic helium balloon, performed four nearly month-long observing projects, Every time, flying several kilometers into the air to search for evidence of radio emission from ultrahigh-energy neutrino hits on the Antarctic ice sheet below.
In January, NASA announced funding for the Payload for Ultrahigh Energy Observations (PUEO), a next-generation experiment based on the ANITA legacy. Because of their high altitude, balloon-borne detectors like ANITA and PUEO have a distinct advantage over ground-based tests in that they can track over a million square kilometers of ice in their neutrino searches. The first flight of PUEO is scheduled for 2024, and it will feature many technical advances over ANITA, including improved exposure to a broader range of energies.
However, the increased field of view provided by balloon-borne searches is offset by the fact that, precisely because the antenna arrays fly so high above the ice, they may miss radio emissions from fainter neutrino signals. Another disadvantage is the reality of bad weather: any type of balloon work over the Antarctic ice sheet is frequently disrupted by bad weather. Astrophysicists are designing modern radio arrays within vast quantities of ice that can then operate in parallel with balloon-borne studies for broader energy coverage to solve these issues. Researchers are preparing for the installation of the Radio Neutrino Observatory in Greenland, which will be preceded by a slew of more minor attempts (RNO-G)
“With 35 stations of antennas constructed over the next three years, RNO-G will be the largest radio detector ever developed in ice,” says Stephanie Wissel, a Pennsylvania State University astrophysicist interested in the observatory’s construction. With the first observation of an ultrahigh-energy neutrino, several experts believe RNO-G would shortly have a first glimpse into the extreme universe.
If not, the in-ice radio array design will be scaled up for use in IceCube-Gen2, the planned successor to IceCube, which will have 200 antenna stations surrounding an improved optical structure. “IceCube can detect neutrinos with energies of up to 10 beta electron volts. However, with the addition of the radio component, this number would rise to tens of thousands, if not hundreds of thousands,” says Vieregg, who is also the principal investigator of both PUEO and RNO-G. This increased energetic scope costs just 10% of IceCube-overall Gen2’s budget, demonstrating the radio detection’s cost-effectiveness.
Rather than salt, a new monitoring technique would look for radio waves from charged particle showers in the air. The former is caused by neutrinos interacting underwater near our planet’s surface. Given the right circumstances, these Earth-skimming neutrinos will produce high-energy particles that escape into the atmosphere and decay into large, radio-emitting air showers.
This is the Giant Radio Array plan for Neutrino Detection, or GRAND—a fitting term for such a massive experiment. With an ambitious project for a 200,000-square-kilometer radio telescope, the international GRAND partnership, organized and financed by organizations in France, China, the Netherlands, and Brazil, aims to explore the source ultrahigh-energy cosmic rays (that is, an array about the size of Nebraska).
“The concept is to install 20 arrays of 10,000 antennas each, rather than one monolithic array,” says Mauricio Bustamante, an astrophysicist at the University of Copenhagen who co-authored the GRAND plan. He states why the position of these arrays is essential since they must be in “radio-quiet” regions, away from artificial sources of substantial radio emission.GRAND has found a few remote areas in Central Asia’s Tian Shan Mountains, with hopes to scout out more places around the globe.
The astrophysics world is bursting with ideas about what the future may bring after one of nature’s most energetic and enigmatic messengers is eventually discovered, with several next-generation radio experiments on the way. Wissel states, “I am very excited about the detection of the first ultrahigh-energy neutrino.” “I’m not sure which experiment would be the first to do so, but it will open up a new window to the world with plenty of space for discovery.”
For physicists familiar with the field’s roots, the quest for new cosmic frontiers is a nod to the past: astronomy succeeded in the twentieth century by exploring what particles come from space. “It’s an inevitable turn of events that we return to celestial accelerators because we want to learn more than our robots can,” Bustamante says. “That is the very point of observing our universe’s highest-energy particles.”