Three decades ago, we spotted the single most energetic particle ever seen, nicknamed the 'Oh-My-God particle'. Since then, we have seen many more ultra-high-energy cosmic rays – and now we are unravelling the mystery of what produces them.
The helicopter was flying high through the night sky with its door slightly ajar. Johannes Eser and Matthew Rodencal were in the back controlling a laser pointing out through the gap. They aimed towards a balloon 35 kilometres above them and fired.
It sounds like a scene from a spy movie, but Eser and Rodencal, then at the Colorado School of Mines, were actually testing a plan to spot ultra-high-energy cosmic rays, the most energetic particles ever discovered. They stream across the universe before slamming into our atmosphere and emitting a tiny flash of light. The laser was supposed to mimic that flash.
This twilight helicopter ride happened nearly a decade ago, but is part of a saga that goes back to at least 1991. In October that year, we detected the single most energetic particle ever seen. It had the kinetic energy of a bowling ball dropped from shoulder height, crammed into a subatomic-sized package. It quickly became known as the “Oh-My-God particle” and, naturally enough, scientists were desperate to know where it came from.
Since then, we have spotted many similar particles. Huge ground-based detectors have provided us with maps of where they might come from, together with a shortlist of the extreme cosmic objects that could produce them. But truth be told, we still don’t have all the answers. That is why scientists now want to take the cosmic ray hunt into the atmosphere – and ultimately into space – in an effort to solve the mystery once and for all.
This story really began with another balloon in 1911. At that time, physicist Victor Hess climbed into a hot air balloon, taking with him instruments to measure levels of radiation as he ascended. He found the readings increased as he went up – contrary to the prevailing belief that they would decline with altitude – and concluded that this radiation must be caused by something coming from space, not Earth. That something became known as cosmic rays, though we now know them to be particles, often protons or clusters of protons and neutrons.
Cosmic rays
When cosmic rays hit our atmosphere, they usually collide with molecules in the atmosphere, producing a shower of energetic particles that rain down. (These descendants of the original particle still contain a lot of energy and have been suspected of interfering with the electronics of aircraft.) It is this shower of secondary particles that we have learned to detect, allowing us to infer the energy of the cosmic ray that produced it. We now know that cosmic rays come in a range of energies. The least energetic are the most common, with each square centimetre of the outer atmosphere being hit once a minute by one of them. The most energetic are much rarer – they strike only once a century per square kilometre.
The rays that Hess detected were relatively modest in energy, it turns out, measuring less than 1 gigaelectronvolt (GeV). It wasn’t until the 1960s that more extreme versions were found, when physicist John Linsley used an array of ground detectors in New Mexico to spot the shower created by a cosmic ray with the vastly greater energy of 100 exaelectronvolts (EeV).
That was a staggering find. But the best was yet to come. In the 1980s, a larger project called the Fly’s Eye telescope array was built in Utah. It had more than 100 detectors, each equipped with a 1.5-metre-wide mirror to look for the flash of particles colliding in the atmosphere. Each of the telescope’s detectors were designed to point at a different part of the field of view, in a similar way to insects’ compound eyes. It was this that earned the telescope its name. “We were hoping we might pick up something really unusual,” says David Kieda at the University of Utah, who worked on the telescope at the time.
On the night of 15 October 1991, Fly’s Eye spotted the flash of a cosmic ray with a whopping 320 EeV of energy. The researchers sat on the result for more than a year, only announcing it in 1993 once they were convinced it was real. The discovery was initially called the “WTF particle”, but after going public, a new name was coined by a now-retired engineer called John Walker. That same year, a book about the then-hypothetical Higgs boson, called The God Particle, had been published. “Something with the energy of a brick landing on your toe seemed a lot more impressive [than the Higgs],” says Walker. “So I called it the ‘Oh-My-God particle’.” Walker wrote about it on his website and the name stuck.
The OMG particle remains the wildest cosmic ray ever seen. It arrived from the direction of the Perseus constellation in the northern hemisphere and hit our atmosphere at 99.99999999999999999999951 per cent of the speed of light. Its energy was millions of times higher than anything produced at the Large Hadron Collider, the world’s best particle accelerator (see “Powerful particles”, below).
Since 1991, hundreds more particles with an energy over 1 EeV have been found, and these are now known as ultra-high-energy cosmic rays (UHECRs). But what could be producing them? It must be an object of truly awesome power. “We just found this fascinating,” says Angela Olinto at the University of Chicago.
Back in the 1990s, scientists thought these zinger particles could be “remnants from the early universe”, says Rene Ong at the University of California, Los Angeles. The idea was that more-massive particles in the early universe may perhaps have decayed into cosmic rays. Some even wondered if studying them might provide a window back to a cosmic era when the fundamental forces may have been merged. “There was a huge amount of interest because of the implications for this model,” says Kieda.
But there was a problem. In 1964, astronomers had discovered cosmic microwave background radiation, the remnant heat of the big bang that pervades the universe. The following year, three scientists – Kenneth Greisen, Georgiy Zatsepin and Vadim Kuzmin – suggested this would place a limit on how far extremely energetic particles could travel. Any with an energy above 50 EeV coming from more than 300 million light years from Earth would be destroyed as they interacted with the background radiation. This became known as the GZK cutoff. “It’s essentially a speed limit for the universe,” says Eric Mayotte at the Colorado School of Mines.
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| The Fly’s Eye telescope array, Utah, pictured in 1983 Fly’s Eye Collaboration, University of Utah |
By 2007, an upgraded version of the Fly’s Eye telescope had observed dozens more UHECRs and seen that there was a pronounced drop in numbers that had energies beyond the GZK cutoff. Such rare, highly energetic particles were now sometimes referred to as Extreme-Energy Cosmic Rays (EECRs). This observation suggested that whatever was firing them at us must be relatively close in space and time.
One of the most powerful mechanisms we know of for accelerating particles is for them to ride an expanding shock wave from a cosmic explosion. The shock waves can be filled with a jumble of magnetic fields, and particles can criss-cross these fields, picking up more and more energy as they go.
These waves can be created in several ways. One involves what is known as a tidal disruption event, where a star is torn apart by a black hole, producing a powerful jet of plasma as the star’s material is swept up. “They’re fascinating systems,” says Glennys Farrar at New York University. But they last only a short time, often a matter of months, casting some doubt on their potential to be the source of OMG particles.
Active galactic nuclei
A more promising candidate might be active galactic nuclei, or AGNs. These are dense and highly luminous areas at the centre of some galaxies, assumed to house a supermassive black hole. They are relatively common too. “One in every 1000 galaxies has an AGN,” says Daniele Fargion at the Sapienza University of Rome in Italy. The closest to Earth, Centaurus A, is roughly 13 million light years away. Such AGNs can fire out jets of superheated plasma, and these constantly produce shock waves as they affect surrounding intergalactic space, says James Matthews at the University of Oxford. This means AGNs are a promising contender for producing OMG particles.
Other short-lived events – such as powerful explosions known as gamma ray bursts, the sources of which are unknown – have also been proposed as the source, as have starburst galaxies, where intense regions of star formation could form the conditions necessary to accelerate particles to the speed required.
To know for sure, we need a comprehensive map of where the highest-energy cosmic rays come from. Over the past decade, that map has been taking shape, thanks to two facilities. The Pierre Auger Observatory is an array of 1600 detectors in Argentina that observes incoming cosmic rays in the southern hemisphere sky. The prosaically named Telescope Array in Utah, with more than 500 detectors, does likewise in the northern hemisphere’s sky.
Processing their observations isn’t straightforward because the trajectories of EECRs are bent by magnetic fields on their way to us, meaning their true origin needs to be reconstructed using computer models. Still, by 2017, both observatories had released their own map of where cosmic rays come from. Each of these showed an intriguing hotspot in the sky where large numbers of the particles appear to come from (scroll down to see the combined map). One centres on Centaurus A, the other on a galaxy about 53 million light years from us called M87, famous for the black hole at its centre that was imaged in 2019. Both galaxies are known to contain an AGN, which bolstered the case for those being the source of EECRs.
Case closed? Not at all. While confidence in the Centaurus A hotspot has grown since 2019, with more cosmic rays found to be coming from its direction, the opposite is true for the M87 hotspot. “If anything, the significance is falling, which is pretty worrying,” says Alan Watson, who set up the Auger observatory but is now retired.
Complicating matters further is a fresh result from the Telescope Array, announced at a conference in October 2022. Scientists revealed the array had seen its most energetic cosmic ray yet. At 244 EeV, it is below the original OMG particle, but still a whopper. “It’s the highest energy we’ve seen,” says Jihyun Kim at the University of Utah, part of the array’s team. Yet its apparent origin is nowhere near any other possible sources, seemingly in an empty region of space. “We are trying to understand what happened,” says Kim.
All of which takes us back to that nocturnal helicopter flight. The test was part of the preparations for an international collaboration called EUSO-SPB2. At its heart is a cosmic ray detector on a balloon lofted into the stratosphere. From this vantage point, it will be capable of spotting many more of the flashes of light from incoming cosmic rays than a ground-based detector can. The balloon launched on 13 May, but crashed unexpectedly shortly afterwards. The team hasn’t yet confirmed its next steps.
Probe of Extreme Multi-Messenger Astrophysics
However, EUSO-SPB2 is predominantly a demonstrator for an even more ambitious project: a billion-dollar space telescope that can monitor even larger regions of Earth’s atmosphere for EECRs. In 2020, NASA decided to develop this idea, called the Probe of Extreme Multi-Messenger Astrophysics (POEMMA), and it could launch in the 2030s.
“By looking down from space, you can see a huge chunk of the atmosphere,” says Olinto, who leads POEMMA, providing at least 10 times more detections than current Earth-based arrays. POEMMA should also be able to discern the nature of incoming particles more easily than ground-based arrays, namely whether they are protons or heavier nuclei of iron, carbon, helium and other elements. Having this knowledge allows their path back through space to be more easily reconstructed.
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| Rate of incoming ultra-high- energy cosmic rays |
Meanwhile, arrays on Earth are still gathering observations of cosmic rays – which isn’t without challenges. “Sometimes the battery is dead. Sometimes cows and horses chew our cables. Or sometimes people, just normal Utah people, shoot our detectors on the Telescope Array,” says Kim. “I don’t understand why. Just for fun I guess.”
Scientists are also upgrading the Pierre Auger Observatory to an enhanced array called AugerPrime, which is due to be finished next year. There is also a plan to upgrade the Telescope Array to make it four times larger. Both projects should provide vital new data on the sources of UHECRs. “We are really on the cusp of getting to the core questions,” says Mayotte.
Beyond the standard model
There is also a left-field possibility. Maybe, just maybe, there is some exotic, as-yet-undiscovered physics that allows some EECRs particles to exceed the GZK distance limit and perhaps originate much further back in the universe’s history than we have assumed. “Maybe they don’t follow these rules,” says Kim. That is an exciting prospect. “If we can’t explain them using the forces and particles that we know, it opens up the possibility that we are exploring beyond the standard model of particle physics,” says Kieda.
Matthews has another alternative. In a paper published in March, he suggested that large groups of galaxies, such as the nearby Council of Giants, could deflect cosmic rays from more plausible sources like AGNs. It could be that the picture we see of cosmic rays coming at us from all over the place is a mirage; maybe they come from one source, like Centaurus A, and get deflected in strange ways. “You get this pattern where there are blobs from echoes,” says Matthews.
Discovering the origin of the highest-energy cosmic rays may have huge implications for our understanding of the universe. But there are broader implications too. These rays are dangerous because, without an atmosphere, they could reach us and skim through living tissue, leaving a wake of damaged cells and DNA. This means they could affect the habitability of some other worlds, where these high-energy bullets reach the surface. “It may be very important to know what elevated doses of ultra-high-energy cosmic radiation can do to a biosphere” before a world has developed a protective atmosphere, says Noémie Globus at the University of California, Santa Cruz. “It could destroy life or sterilise a planetary surface.”
For now, the mystery pervades, yet an answer may not be far away – albeit not a simple one. “I think there’s going to be lots of different kinds of OMG particles,” says Farrar, perhaps some from AGNs, some from tidal disruption events and some from gamma-ray bursts. Finally figuring this out would be a fitting reward for scientists who are willing to go to quite some lengths to solve the puzzle – even firing lasers from a helicopter.
Powerful Particles
