In the 1930s, the French physicist Pierre Auger placed Geiger counters along a ridge in the Alps and observed that they would sometimes spontaneously click at the same time, even when they were up to 300 meters apart. He knew that the coincident clicks came from cosmic rays, charged particles from space that bang into air molecules in the sky, triggering particle showers that rain down to the ground. But Auger realized that for cosmic rays to trigger the kind of enormous showers he was seeing, they must carry fantastical amounts of energy—so much, he wrote in 1939, that “it is actually impossible to imagine a single process able to give to a particle such an energy.”
Upon constructing larger arrays of Geiger counters and other kinds of detectors, physicists learned that cosmic rays reach energies at least 100,000 times higher than Auger supposed.
A cosmic ray is just an atomic nucleus—a proton or a cluster of protons and neutrons. Yet the rare ones known as “ultrahigh-energy” cosmic rays have as much energy as professionally served tennis balls. They’re millions of times more energetic than the protons that hurtle around the circular tunnel of the Large Hadron Collider in Europe at 99.9999991 percent of the speed of light. In fact, the most energetic cosmic ray ever detected, nicknamed the “Oh-My-God particle,” struck the sky in 1991 going something like 99.99999999999999999999951 percent of the speed of light, giving it roughly the energy of a bowling ball dropped from shoulder height onto a toe. “You would have to build a collider as large as the orbit of the planet Mercury to accelerate protons to the energies we see,” said Ralph Engel, an astrophysicist at the Karlsruhe Institute of Technology in Germany and the coleader of the world’s largest cosmic-ray observatory, the Pierre Auger Observatory in Argentina.
The question is: What’s out there in space doing the accelerating?
Supernova explosions are now thought to be capable of producing the astonishingly energetic cosmic rays that Auger first observed 82 years ago. Supernovas can’t possibly yield the far more astonishing particles that have been seen since. The origins of these ultrahigh-energy cosmic rays remain uncertain. But a series of recent advances has significantly narrowed the search.
In 2017, the Auger Observatory announced a major discovery. With its 1,600 particle detectors and 27 telescopes dotting a patch of Argentinian prairie the size of Rhode Island, the observatory had recorded the air showers of hundreds of thousands of ultrahigh-energy cosmic rays over the previous 13 years. The team reported that 6 percent more of the rays come from one half of the sky than the other—the first pattern ever definitively detected in the arrival directions of cosmic rays.
Recently, three theorists at New York University offered an elegant explanation for the imbalance that experts see as highly convincing. The new paper, by Chen Ding, Noémie Globus, and Glennys Farrar, implies that ultra-powerful cosmic-ray accelerators are ubiquitous, cosmically speaking, rather than rare.
The Auger Observatory and the Telescope Array in Utah have also detected smaller, subtler cosmic ray “hot spots” in the sky—presumably the locations of nearby sources. Certain candidate objects sit at the right locations.
More clues have arrived in the form of super-energetic neutrinos, which are produced by ultrahigh-energy cosmic rays. Collectively, the recent discoveries have focused the search for the universe’s ultra-powerful accelerators on three main contenders. Now theorists are busy modeling these astrophysical objects to see whether they’re indeed capable of flinging fast-enough particles toward us, and if so, how.
These speculations are brand-new and unconstrained by any data. “If you go to high energies, things are really unexplored,” Engel said. “You really go somewhere where everything is blank.”
A Fine Imbalance
To know what’s making ultrahigh-energy cosmic rays, step one is to see where they’re coming from. The trouble is that, because the particles are electrically charged, they don’t travel here in straight lines; their paths bend as they pass through magnetic fields.
Moreover, the ultrahigh-energy particles are rare, striking each square kilometer of Earth’s sky only about once per year. Identifying any pattern in their arrival directions requires teasing out subtle statistical imbalances from a huge data set.
No one knew how much data would be needed before patterns would emerge. Physicists spent decades building ever larger arrays of detectors without seeing even a hint of a pattern. Then in the early 1990s, the Scottish astrophysicist Alan Watson and the American physicist Jim Cronin decided to go really big. They embarked on what would become the 3,000-square-kilometer Auger Observatory.
Finally, that was enough. When the Auger team reported in Science in 2017 that it had detected a 6 percent imbalance between two halves of the sky—where an excess of particles from one particular direction in the sky smoothly transitioned into a deficit centered in the opposite direction—“that was fantastically exciting,” said Watson. “I’ve worked in this field for a very, very long time”—since the 1960s—“and this is the first time we’ve had an anisotropy.”
But the data was also puzzling. The direction of the cosmic-ray excess was nowhere near the center of the Milky Way galaxy, supporting the long-standing hypothesis that ultrahigh-energy cosmic rays come from outside the galaxy. But it was nowhere near anything. It didn’t correspond to the location of some powerful astrophysical object like a supermassive black hole in a neighboring galaxy. It wasn’t the Virgo cluster, the dense nearby concentration of galaxies. It was just a dull, dark spot near the constellation Canis Major.
Noémie Globus, then a postdoc at the Hebrew University of Jerusalem, immediately saw a way to explain the pattern. She began by making a simplification: that every bit of matter in the universe has equal probability of producing some small number of ultrahigh-energy cosmic rays. She then mapped out how those cosmic rays would bend slightly as they emanate from nearby galaxies, galaxy groups, and clusters—collectively known as the large-scale structure of the cosmos—and travel here through the weak magnetic fields of intergalactic space. Naturally, her pretend map was just a blurry picture of the large-scale structure itself, with the highest concentration of cosmic rays coming from Virgo.
Her cosmic-ray excess wasn’t in the right spot to explain Auger’s data, but she thought she knew why: because she hadn’t adequately accounted for the magnetic field of the Milky Way. In 2019, Globus moved to NYU to work with the astrophysicist Glennys Farrar, whose 2012 model of the Milky Way’s magnetic field, developed with her then graduate student Ronnie Jansson, remains state of the art. Although no one yet understands why the galaxy’s magnetic field is shaped the way it is, Farrar and Jansson inferred its geometry from 40,000 measurements of polarized light. They ascertained that magnetic field lines arc both clockwise and counterclockwise along the spiral arms of the galaxy and emanate vertically from the galactic disk, twisting as they rise.
Farrar’s graduate student Chen Ding wrote code that refined Globus’ map of ultrahigh-energy cosmic rays coming from the large-scale structure, then passed this input through the distorting lens of the galactic magnetic field as modeled by Farrar and Jansson. “And lo and behold we get this remarkable agreement with the observations,” Farrar said.