The Pierre Auger Observatory in Argentina has spent almost ten years looking for the source of ultra-high-energy cosmic rays — but to no avail. Now the observatory faces an uncertain future.
The tank looks oddly out of place here on the windy Pampas of western Argentina. Surrounded by yellow grass and spiky thorn bushes, the chest-high plastic cylinder
could be some kind of storage container — were it not for the bird-spattered solar panels and antennas on top.
More tanks can be seen in the distance, illuminated by a crimson Sun dropping behind the far-off Andes. “Some locals think that the tanks influence the weather: they make it rain or snow, or make a dry season,” says Anselmo Francisco Jake, the farmer who owns this stretch of land. “But I know they don’t. I know they catch cosmic rays.”
Jake is right. There are 1,600 of these tanks, spaced over a 3,000-square-kilometre expanse that could fit all of Luxembourg with room
to spare. Together they comprise the Pierre Auger Observatory: a US$53-million experiment to reveal the mysterious origins of ultra-high-energy cosmic rays, the most energetic subatomic particles known to exist.
But for all its size, the array has fallen short. After almost ten years of hunting, it has observed dozens of ultra-high-energy cosmic rays but has not managed to solve the mystery of where they come from. As a detector, “the device worked twice as well as we expected”, says project co-founder James Cronin, a retired astrophysicist at the University of Chicago in Illinois. But the particles seem to be coming from all over the sky, with too little clustering for researchers to pinpoint the sources. “It’s up to nature with experiments like this one,” he says.
Now, the Auger team is putting its hopes on a proposed upgrade that might settle the question by improving Auger’s resolution considerably. Five designs are being evaluated internally by a committee of Auger physicists, who are expected to present their final selection to the array’s many funding agencies in November. The trouble is, there is a sixth option, too. “In the worst-case scenario, and I don’t want to think about it, we may get shut down,” says Auger’s deputy project manager, physicist Ingo Allekotte.
An upgrade would require an investment of roughly $15 million, and some argue that the money would be put to better use elsewhere. “Although it was worth building Auger, it was a gamble that unfortunately didn’t yield much new understanding,” says Eric Adelberger, a physicist at the University of Washington in Seattle. “Cosmic-ray physics has delivered very few surprises and progress is terribly slow. Maybe it is time to move on.”
That would be a blow to science — and to Argentina, say Auger’s supporters. These flagship projects do more than just conduct research, says Pablo Mininni, head of the physics department at the University of Buenos Aires. They also raise awareness of physics and draw young people into the field. “Such a big project deserves some continuity,” he says.
Physicists have known for more than a century that Earth is continually bombarded by charged particles from space — many of which have energies that are astonishing even by particle-physics standards. It is not uncommon for cosmic rays to have hundreds or thousands of times the 7 trillion electron volts (1012 eV) soon to be achieved by the most powerful human-made particle accelerator, the Large Hadron Collider (LHC) near Geneva in Switzerland.
Most of these particles are now thought to be protons and other light nuclei originating far outside the Solar System, probably in cataclysmic stellar explosions known as supernovas. But on very rare occasions, cosmic rays have hit Earth’s atmosphere at energies of 1018 eV or more. The most energetic example on record — the ‘Oh-My-God particle’ detected1 on 15 October 1991 in the skies above Utah — had 3×1020 eV, about 40 million times that of the LHC. And therein lies a mystery: calculations suggest that the expanding shock wave of a supernova detonation cannot accelerate charged particles beyond about 1017 eV. No one knows what physical process
could accelerate particles to higher energies — or even what those particles might be (see Nature 448, 8–9; 2007).
22 | NATURE | VOL 514 | 2 OCTOBER 2014
