For decades, physicists assumed the universe’s most powerful particles stopped at iron. Anything heavier would shatter crossing the gulfs between galaxies. New work suggests they may be ultraheavy cosmic rays.
The most extreme rays reaching Earth could be built from atoms heavier than anything a dying star can forge. If so, a famous puzzle about where they originate starts to have an answer.
A particle from nowhere
In 2021, hundreds of detectors across the Utah desert lit up at once. The Telescope Array had caught a single cosmic ray hitting the upper atmosphere. It burst into a shower of lesser particles that rained down.
The energy was almost absurd, roughly 40 million times what the most powerful atom smasher can reach. Researchers named it the Amaterasu particle, after the sun goddess of Japanese mythology.
Its direction only deepened the mystery. Traced backward, the detection seemed to begin in the Local Void, a nearly empty pocket of space beside our galaxy. Nothing violent enough to launch it sits there.
Reading a cosmic ray
Cosmic rays are not rays at all. They are charged particles, protons and the bare cores of atoms, flung across space at nearly light speed. The most energetic have puzzled physicists for more than 50 years.
Because they carry electric charge, their paths bend in the magnetic fields threading the galaxy. By the time one reaches Earth, its heading is lost. That is why tracing a particle to its source is so hard.
What a particle is made of offers another clue. The debris shower it creates reveals whether a light proton or a heavier nucleus struck. Recent measurements suggest the mix turns heavier at higher energies.
Ultraheavy cosmic rays
B. Theodore Zhang, a researcher at Kyoto University‘s Yukawa Institute for Theoretical Physics, and colleagues pushed that trend to its limit. They asked whether the highest-energy cosmic rays might be ultraheavy nuclei, heavier than iron.
Such heavy nuclei are not built in ordinary stars. They form in the violent settings that forge nature’s heaviest elements, like gold, platinum and uranium. There, atoms swallow neutron after neutron in seconds.
That extra heft brings an advantage. A heavier nucleus carries far more electric charge. Charge is what lets a cosmic accelerator drive a particle past energies that lighter ones never reach.
Surviving the journey
There was good reason to doubt the idea. A heavy nucleus is fragile, and physicists assumed it would break apart before crossing the gulfs between galaxies. So the team tested it with a detailed simulation.
As a cosmic ray travels, it plows through the faint microwave glow left over from the Big Bang. Those gentle collisions chip away its energy and can crack a nucleus apart. Lighter particles wear down fastest.
No existing software could handle nuclei heavier than iron, so the team built their own to track how each breaks down. Then came a surprise.
Below a certain energy threshold, their simulations found, heavy nuclei lose energy far more slowly than protons. They could survive distances that once seemed out of reach.
That single result rewrites the picture. If the most energetic rays are ultraheavy, they could reach us intact from sources once thought too distant. The team could then bound their share of detector data.
Where they are born
What could forge such nuclei and fling them across the universe has two answers, and both are catastrophes. One is the collapse of a massive star into a black hole. The other is the merger of two neutron stars.
These are the same cataclysms thought to forge gold and the heaviest elements, natural cradles for heavy nuclei. When the team tallied the energy they release, it matched what the highest-energy cosmic rays demand.
Kohta Murase, a physicist at Pennsylvania State University who helped lead the work, names the deaths of massive stars and neutron star collisions as the likeliest factories. Both are rare and spectacularly violent.
A heavier particle also bends more sharply in magnetic fields, so the empty sky the Amaterasu particle came from need not be its true home. Its real source could lie off to one side, hidden by its curved path.
What comes next
None of this names the exact object that made the Amaterasu particle. What the work establishes is sturdier. For the first time, calculations show the heaviest cosmic rays can survive the crossing.
Their energy also fits the output of dying and colliding stars. That ties together two great puzzles, the origin of the most energetic particles and the origin of the heaviest elements.
A separate analysis draws the same link to colliding neutron stars. The idea also makes a clean, testable prediction.
If these particles are ultraheavy cosmic rays, the next generation of detectors should find the most energetic ones heavier than iron, where lighter models expect almost none.
The study is published in the journal Physical Review Letters.
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