Ultraheavy nuclei lose energy more slowly, allowing them to survive cosmic distances
In the vast silence between galaxies, particles of almost incomprehensible energy arrive at Earth carrying messages from the universe's most violent moments. A Penn State-led international team now proposes that the most extreme of these cosmic rays — including the legendary Amaterasu particle detected in 2021 — may be composed of nuclei heavier than iron, a distinction that would explain both their survival across cosmic distances and point investigators toward cataclysmic sources like neutron star mergers and collapsing stars. The question of where these particles come from has endured for six decades; this research does not close that chapter, but it offers, for the first time, a credible way to turn the page.
- The 2021 detection of the Amaterasu particle — carrying 240 quintillion electron volts — reignited a decades-old crisis in astrophysics, because no known nearby source could plausibly explain it.
- The core tension is one of survival: most particles lose too much energy crossing intergalactic space to arrive at Earth with such extreme power, leaving physicists scrambling to explain what kind of particle could endure the journey.
- Penn State's Kohta Murase and an international team ran computational simulations revealing that ultraheavy nuclei — heavier than iron — shed energy far more slowly than protons or lighter nuclei, making them the most viable candidates for the highest-energy arrivals.
- The Amaterasu particle's apparent origin in a cosmic void — a region with no obvious powerful source — deepens the mystery and suggests either unknown sources or the limits of current detection technology.
- The next generation of observatories, including AugerPrime in Argentina, is being positioned to detect the compositional fingerprints that would confirm or refute this hypothesis, potentially identifying the universe's most extreme engines at last.
In May 2021, a detector array in Utah registered something that would earn a name from Japanese mythology: the Amaterasu particle, a cosmic ray carrying roughly 240 quintillion electron volts of energy — one of the most powerful ever recorded. For sixty years, physicists have struggled to explain how such particles exist at all, and a new study led by Penn State may have found a key piece of the answer.
Ultrahigh-energy cosmic rays are subatomic particles — protons and atomic nuclei — that travel at nearly the speed of light. The most extreme among them exceed energies seven orders of magnitude beyond anything produced at CERN. Astrophysicists have long suspected they originate from the universe's most violent events, but the precise mechanism and the nature of the particles themselves have remained elusive.
Professor Kohta Murase and an international team from institutions including Kyoto University and Virginia Tech pursued a deceptively simple hypothesis: what if the most energetic cosmic rays were composed of nuclei even heavier than iron? Their computational simulations showed that such ultraheavy nuclei lose energy far more slowly as they travel through intergalactic space than protons or lighter nuclei do. Over millions or billions of light-years, that subtle advantage becomes decisive — allowing them to arrive at Earth with energies that lighter particles simply could not retain.
Published in Physical Review Letters, the findings don't claim all ultrahigh-energy cosmic rays are ultraheavy, but suggest the most extreme events may be — and that this distinction could reshape how astronomers search for sources. The leading candidates remain the universe's most catastrophic phenomena: neutron star collisions, massive stellar collapses into black holes, and the formation of magnetars, all of which can also produce gamma-ray bursts.
The Amaterasu particle adds a further puzzle: its inferred trajectory points back toward a cosmic void, a region with no obvious powerful source. This may hint at sources yet undiscovered, or at the boundaries of current detection. Future observatories like AugerPrime in Argentina will have the sensitivity to detect compositional signatures that could confirm or challenge the hypothesis — and perhaps, at last, bring a sixty-year mystery closer to resolution.
In May 2021, the Telescope Array Project in Utah recorded something extraordinary: a cosmic ray so energetic it would take a name from Japanese mythology. The Amaterasu particle, as scientists came to call it, arrived at Earth carrying roughly 240 quintillion electron volts of energy—a figure so vast it sits alongside only a handful of other events in the entire history of cosmic-ray observation. For six decades, physicists have puzzled over how such particles come to exist at all, let alone how they travel across the universe to reach our planet. A new study led by researchers at Penn State may have found a crucial piece of the answer: these extreme particles might not be what scientists assumed they were.
Ultrahigh-energy cosmic rays are subatomic particles—mostly protons and atomic nuclei—that streak through space at nearly light speed. The ones that matter most to this mystery are those exceeding 100 quintillion electron volts, energies so far beyond what the CERN Large Hadron Collider can produce that the comparison feels almost absurd. The Amaterasu particle was seven orders of magnitude more energetic than anything humans have ever created in a laboratory. Yet despite their rarity and power, their origins have remained stubbornly obscure. Astrophysicists have long suspected they come from the universe's most violent events—neutron star collisions, supernovae, the collapse of massive stars into black holes. But the mechanism that accelerates them to such extremes, and the exact nature of the particles themselves, has eluded explanation.
Kohta Murase, an astronomy professor at Penn State, led an international team that included researchers from Kyoto University, Virginia Tech, and institutions across Japan and China. Their hypothesis was deceptively simple: what if the heaviest cosmic rays weren't made of iron or lighter elements, but of nuclei even heavier still? To test this, the team ran detailed computational simulations tracking how particles of different masses would behave as they traveled through the vast distances of intergalactic space. The results were striking. Ultraheavy nuclei—those heavier than iron—lose energy far more slowly during their cosmic journey than protons or intermediate-mass nuclei do. This means they can retain enough energy to reach Earth at the extreme levels observed, while lighter particles would have shed too much energy along the way.
The physics here hinges on a simple principle: mass matters. A heavier nucleus, traveling through the thin gas and radiation fields scattered across intergalactic space, experiences less relative energy loss than a lighter one. It's a subtle advantage, but over distances of millions or billions of light-years, it becomes decisive. "At energies comparable to that of the Amaterasu particle, ultraheavy nuclei lose energy more slowly than protons or intermediate-mass nuclei, making them better able to survive cosmic distances and reach Earth at extreme energies," Murase explained. The team's findings, published in Physical Review Letters on May 7th, don't claim that all ultrahigh-energy cosmic rays are ultraheavy. Rather, they suggest that some of the most extreme events might be, and that this distinction could fundamentally change how astronomers search for their sources.
If ultraheavy nuclei are indeed responsible for the most energetic cosmic rays, it narrows the field of possible origins considerably. The candidates are the universe's most catastrophic events: the collision of two neutron stars, the explosive collapse of a massive star into a black hole, or the formation of a magnetar—a neutron star with an extraordinarily powerful magnetic field. All three phenomena can also generate gamma-ray bursts, among the most luminous explosions known. The Amaterasu particle itself points in an intriguing direction: its inferred arrival direction traces back to a cosmic void, a region with no obvious source of ultrahigh-energy cosmic rays. This puzzle may hint at sources we haven't yet identified, or at the limitations of current detection methods.
The research also hints at a possible asymmetry in the cosmic-ray sky. Observations suggest subtle differences between ultrahigh-energy cosmic rays arriving from the northern and southern hemispheres. If ultraheavy nuclei contribute significantly to the highest-energy events, future data should reveal a composition heavier than iron, providing a way to test the hypothesis directly. The next generation of observatories—particularly the proposed AugerPrime facility in Argentina and the planned Global Cosmic Ray Observatory—will have the sensitivity to detect such compositional signatures. In the meantime, theoretical work on the extreme physics of black holes and magnetars may offer additional clues. The 60-year mystery remains unsolved, but for the first time, scientists have a concrete way to narrow the search.
Citações Notáveis
At energies comparable to that of the Amaterasu particle, ultraheavy nuclei lose energy more slowly than protons or intermediate-mass nuclei, making them better able to survive cosmic distances and reach Earth at extreme energies.— Kohta Murase, Penn State professor of astronomy and astrophysics
A Conversa do Hearth Outra perspectiva sobre a história
Why does it matter whether these particles are heavy or light? Aren't they cosmic rays either way?
Because composition tells you where they came from. A proton and an iron nucleus behave differently in space—they lose energy at different rates. If you can figure out what's actually arriving at Earth, you can work backward to identify the source.
So the Amaterasu particle—that's the one from 2021—you're saying it might be made of something heavier than iron?
Possibly. The team's simulations show that ultraheavy nuclei can reach Earth at the extreme energies we observe, while lighter particles would have lost too much energy on the journey. It's not proof, but it's a plausible explanation for a 60-year mystery.
What would be accelerating these particles to such extreme energies in the first place?
The most violent events in the universe. Neutron stars colliding, massive stars collapsing into black holes, magnetars with unimaginable magnetic fields. These are the only phenomena we know of that could do it.
And the Amaterasu particle itself—do they know where it came from?
That's the strange part. Its direction points to a cosmic void, a region with no obvious source. It's a puzzle within the puzzle. Future observatories might help solve it.
How will they test this hypothesis?
By looking for compositional signatures in the next generation of cosmic-ray detectors. If ultraheavy nuclei are responsible for the most extreme events, the data should show it. That's what AugerPrime and other new observatories are designed to do.