New Dawn of Particle Astronomy

High-Energy Neutrinos Herald a New Dawn of Particle Astronomy

Neutrinos from deep space are opening up a new kind of astronomy

By Michael Moyer

IceCube laboratory in AntarcticaImage: COURTESY OF SVEN LIDSTROMNational Science Foundation

The starry glow of the night sky brings news from the distant edges of the cosmos, as light fills astronomers' telescopes with the bizarre and wondrous processes in the universe. But light cannot tell the whole story—often it reveals only an object's superficial glow. To better understand the cores of powerful astrophysical objects, scientists are studying individual particles that can tell a firsthand tale of the extreme events that launch them outward at tremendous speed. A promising new frontier has just opened up that should bolster those investigations.

For more than a century now scientists have trapped particles known as cosmic rays to gather clues about the universe. Cosmic rays are charged particles (mostly protons) ejected by cosmic outbursts. Some have as much energy as a tennis ball served up at 90 miles per hour. Unfortunately, it is impossible to track a cosmic ray back to its source in the sky; magnetic fields twist the paths of charged particles into knots before those particles reach Earth.

The lightweight, neutral particles known as neutrinos do not have that problem. Neutrinos are famous for their ghostly behavior—they can emerge unmolested from the center of a violent outburst, zip straight across the universe and pass cleanly through Earth's atmosphere. Those qualities make neutrinos exquisite carriers of astronomical information. The trick is catching them once they arrive.

Scientists have constructed a giant neutrino detector, known as IceCube, a mile under Antarctic ice in the hopes of netting these astronomical neutrinos. And earlier this year the IceCube project announced that it had found 28 neutrinos so energetic that they must have come from outside the solar system. Two of the neutrinos, highlighted in a July study in Physical Review Letters, carry so much energy—hundreds of times that of the particles in the Large Hadron Collider—that affectionate astronomers have singled them out with names: Ernie and Bert.

As to what birthed these high-energy neutrinos, speculation abounds. They could have emerged from gamma-ray bursts, mysterious and short-lived cataclysms that briefly rank as the brightest objects in the universe; shock waves from exploding stars; or so-called blazars, jets of energy powered by supermassive black holes. Or Ernie and Bert may be the particle spawn of dark matter, the unidentified stuff that provides much of the universe's mass—or perhaps even a sign of more exotic phenomena.

In truth, scientists cannot glean much from a mere 28 particles. So far the high-energy neutrinos do not seem to point back to a specific source, which would give scientists more to go on. “Everybody's reading the tea leaves,” says Francis Halzen, director of the IceCube Particle Astrophysics Center at the University of Wisconsin–Madison. But with IceCube expected to run for at least another decade, the era of particle astronomy is just beginning.

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