Particle physics is a numbers game. Whether at terrestrial atom-smashers like the Large Hadron Collider in Switzerland or experiments looking further afield, like the Pierre Auger Observatory in Argentina, scientists set up their detectors and wait for particle collisions – or ‘events’ – to occur.
As the number of events builds up, patterns and probabilities begin to emerge. Eventually, with luck (and sophisticated statistical analysis), enough occur for scientists to be confident that the patterns are not coincidence, but rather evidence for a rare and special thing: a new fact about the world.
One such fact, sifted from subtle irregularities in 12 years’ worth of cosmic ray observations at Pierre Auger, is that Earth is continually showered with the cores of atoms from distant galaxies.
The research found that high-energy cosmic rays – atomic nuclei hurled from stellar cataclysms at enormous speed – are distributed in an uneven pattern across the sky, which confirms that they come from galaxies beyond our own. The findings are published in the journal Science. (As is often the way with results from large collaborative projects, the paper is authored by some 400 scientists from institutions around the globe.)
“We’re used to seeing light coming from outer space,” says Bruce Dawson of the University of Adelaide, whose team works on the analysis of the observatory data. “But this is the first conclusive evidence of real stuff from other galaxies falling on the Earth.”
Cosmic rays are fast-moving particles that hit Earth’s atmosphere and produce showers of secondary particles that can be detected at ground level.
These move at astonishing speeds. The fastest ever recorded, the so-called Oh-My-God particle observed in 1991, was travelling so close to the speed of light that it carried as much kinetic energy as a cricket ball moving at 100 kilometres per hour.
This extreme energy makes the origin of cosmic rays something of a mystery. The theory is that shockwaves in space produce intense magnetic fields that accelerate particles to high speed.
One place such shockwaves would be found is in supernovas, but exploding stars don’t pack enough punch to produce the higher energy cosmic rays. For that, you need something bigger: colliding galaxies, maybe, or the intensely bright jets, fired from the centres of some other galaxies, known as active galactic nuclei and believed to be powered by supermassive black holes.
Many cosmic rays are single protons, but some are heavier atomic nuclei. “That tells us something about the environment of the accelerator,” says Dawson, “but the details are still unclear.”
An important step in working out what produces the particles is determining where they come from. Pinning down the sources is no mean feat. For one thing, high-energy cosmic rays are rare (at energies above 1019 electronvolts, for instance, a square kilometre of Earth’s surface can expect to see just one particle per year). For another, unlike light, the particles are electrically charged, which means their course is deflected by magnetic fields in space.
Spotting these cosmic rays is what the Pierre Auger Observatory, a field of 1600 particle detectors laid out on a hexagonal grid covering 3000 square kilometres in the shadow of the Andes, was built to do.
When a high-energy cosmic ray collides with a molecule in the atmosphere, it produces a shower of billions of subatomic particles that cascade down in an expanding disk that can grow to several kilometres in diameter.
To catch these cascades, Pierre Auger uses two methods: ground-based detectors are complemented by telescopes that watch the air above the observatory for tell-tale flashes caused when a particle from the cascade hits a nitrogen molecule.
Each particle detector is a large tank of ultra-pure water that records bursts of Cherenkov radiation, the optical equivalent of a sonic boom that occurs when a particle travels through the tank faster than the speed of light in water.
With precise analysis of the timing of light flashes observed in the detectors and the telescopes, researchers can calculate the speed, mass and direction of the cosmic ray that started the cascade.
Pierre Auger started producing high-quality data in 2004, and the new study presents an analysis of more than 30,000 high-energy cosmic rays detected between 2004 and 2016. The researchers found that, while the rays come from all directions in the sky, they are unevenly distributed (or anisotropic).
The highest intensity of cosmic rays appears to come from a part of the sky away from the centre of the Milky Way that is close to a region in which there is a high density of other galaxies. When the researchers took into account an estimate of the effect of the galactic magnetic field on the cosmic rays, they found that the particles appeared to be coming from even closer to the high-density galactic region.
This confirms that the particles raining down on us are not coming from inside our galaxy, and it’s consistent with the idea that they are flung out by colliding galaxies or the supermassive black holes that power active galactic nuclei.
The result has a statistical significance of 5.2 sigma. (5 sigma is often considered the threshold value for a genuine finding.) This means that the probability the finding is a result of chance is around 1 in 5 million.
The next step, says Dawson, is to narrow down the origins of the cosmic rays even further. He thinks the broad excess on the sky indicates that the particles have a range of different masses, which in turn means they have different electric charges and are magnetically deflected by different amounts.
The Pierre Auger Observatory is about to have an upgrade that will extend its life and improve the detection of particle masses, among other things.
“The real hope,” says Dawson, “is that the upgrade can identify protons in the very high energy range. We expect those to travel in quite straight lines.” This would mean they point back more directly to their origins, and help identify the kinds of galaxies that spit them out.