Outer space is awash in neutrinos, subatomic particles that travel at the speed of light and have no mass — and that provide a cosmic “smoking gun.”
On Thursday, astronomers announced a breakthrough: An observatory in the South Pole called IceCube finally detected high-energy neutrinos from along the galactic plane, or the band of the Milky Way across the night sky.
Low-energy subatomic particles come from supernovas and the Sun. But high-energy neutrinos, like those detected in the new work published Thursday in the journal Science, arise from chaotic and perplexing events.
Investigating them requires creativity. Thanks to the deep, ultra-dense, and clear ice in Antarctica, researchers with the IceCube Neutrino Observatory observed the byproduct of high-energy neutrinos that beamed out from faraway cosmic rays, traveling in straight lines at nearly the speed of light, until they entered the kilometer-deep ice of this subzero continent.
What creates high-energy neutrinos?
Neutrinos were first detected during the nuclear revolution of the mid-twentieth century. Their release can pinpoint some of the wildest phenomena our universe can deliver, like the dizzying frenzy of particles swirling just outside the event horizon of a supermassive black hole.
In particular, IceCube is hunting for cosmic rays. These powerful slingshots are charged particles that have been accelerated by some underlying mechanics at play across the universe.
Neutrinos are a sign of cosmic rays. Plus, neutrinos can put out information about cosmic rays that haven’t reached Earth, like cosmic text messages. Neutrinos can thus help scientists understand what is out there capable of accelerating cosmic rays to the highest energies.
“The smoking gun for the cosmic rays is the neutrino,” Steve Sclafani, IceCube collaborator and postdoctoral researcher at the University of Maryland, tells Inverse.
When cosmic rays smack into something, like an interstellar cloud, they release gamma rays and neutrinos. While gamma rays are hard to work backward from, neutrinos are reliable. “If we’re looking at a faraway galaxy, and this is producing gamma rays and this is producing neutrinos, some of those gamma rays of the highest energies may not escape the galaxy. They may interact and cascade down to lower energies, or if they are high enough energy they will interact on the way to Earth,” Sclafani says.
But neutrinos mind their own business. “Neutrinos, because they are weakly interacting, can do this science of the very high energy,” he adds.
A game of Telephone
The signatures IceCube detected with its cubic kilometer of pristine ice were short-lived, nano-second-long flashes of light that the observatory's sensors are optimized to see.
Like a game of telephone, cosmic rays hint at the wild events that accelerate them, neutrinos reveal cosmic rays, and high-energy neutrinos also need a messenger.
“We don’t have the ability to see the neutrinos directly. What we see is the byproducts of the neutrino interaction,” Sclafani says. When a neutrino strikes atoms in the ice, it knocks out protons or neutrons and causes a nuclear reaction. This produces speedy secondary particles. They are charged, and give off a bluish light called Cherenkov light. “That is what our light sensors are optimized to see,” he adds.
A multi-messenger approach
“I’ve always been interested in the marriage of particles and astronomy,” Sclafani says. “IceCube is this really unique detector that kind of sits in the middle of this.”
Neutrino astronomy supplements the more traditional space observations made in visible light and the rest of the electromagnetic spectrum. The goal is to have a multi-messenger approach to cosmic investigations. “Light gets blocked by dust and gas, but the neutrinos will go straight through,” Sclafani says.
“We now hope to have established the multi-messenger techniques that will allow us to pinpoint the cosmic ray sources in the galaxy which, arguably, represents one of the oldest problems in astronomy,” Francis Halzen, IceCube principal investigator and physicist at the University of Wisconsin–Madison, tells Inverse.
The new work confirms theoretical predictions suggesting a flux of neutrinos beam out from the inner regions of our galaxy, Luigi Fusco, peer-reviewer of the paper and particle physics lecturer at the University of Salerno, Italy, tells Inverse.
It also “paves the way for a lot of exciting studies,” Fusco says, about cosmic rays near and far.
