News Feature: Seeing the ghostly universe

Imagine if astronomers had only ever detected a few dozen photons of light from the cosmos. There would be no star-strewn sky to gaze up at in wonder, no Hubble Space Telescope images to adorn your computer desktop: just a smattering of points on a plot showing the photons’ energies and trajectories. Not such a cheery screensaver.

The IceCube laboratory at the South Pole waits patiently to capture fleeting neutrinos from deep space. Photo courtesy of Sven Lidstrom, IceCube, National Science Foundation.

Ernie makes his entrance. With an estimated energy of 1.14 PeV, this signal shows one of the most energetic neutrinos ever spotted. Image courtesy of the IceCube collaboration.

Substitute photons with neutrinos, though, and the sparse diagram drives physicists giddy with delight. Unveiled in November 2013, this plot records the first high-energy neutrinos that have traveled from beyond our Solar System. The elusive subatomic particles were generated by some of the most powerful events in the Universe, and have been detected over the past few years by the IceCube observatory at the South Pole.

By pinpointing the origins of high-energy neutrinos, astrophysicists hope to learn more about the processes that make them: the tumult around the supernova explosion of a star, for example, or the maelstroms around supermassive black holes. “There’s a whole ghostly universe we don’t see,” says John Beacom, a neutrino theorist at Ohio State University in Columbus. “Neutrinos can reveal it.”

Neutrinos could also help to solve one of the biggest mysteries in astrophysics. The conditions that create high-energy neutrinos may also produce ultrahigh-energy cosmic rays, protons or atomic nuclei that have been accelerated close to the speed of light. Although facilities such as the Pierre Auger Observatory in Argentina have detected some 150,000 of these cosmic rays, the electrically charged particles are buffeted by magnetic fields during their journey, making it difficult to pin down where they came from.

In contrast, neutrinos fly straight and true. They are electrically neutral and almost massless, unperturbed by magnetic fields and oblivious to dust clouds that block photons of light. That trait potentially enables scientists to draw a line from IceCube right back to the particles’ birthplace. However, the neutrinos’ “powerful magic comes at a terrible price,” says Beacom. “It’s almost impossible to detect them.”

Introducing Bert, Ernie, and Big Bird

The Sun’s fusion reactions pour out vast numbers of neutrinos, with billions of them passing unnoticed through your eyes every second. Solar neutrinos were first detected in the late 1960s, using a 380-cubic-meter tank of perchloroethylene deep underground at the Homestake Gold Mine in Lead, South Dakota. However, it was not until 1987 that detectors, such as Kamiokande II in Japan, saw about two dozen neutrinos coming from outside the Solar System, born in a supernova in a nearby galaxy.

Those neutrinos arrived with energies measured in tens of millions of electron volts (MeV). Higher-energy neutrinos are much rarer, however, so larger detectors were built to find them, including one submerged in the Mediterranean Sea and another in Russia's Lake Baikal. IceCube, completed in 2010 after seven years of construction, is by far the most sensitive. Made of 86 detector-studded strings buried in a cubic kilometer of ice, the observatory hunts for flashes of blue light created by the particle fallout when a neutrino occasionally hits a proton or a neutron in the ice.

IceCube typically sees about 275 neutrinos a day. Almost all of these are produced in Earth’s atmosphere when cosmic rays slam into molecules of air, and they typically have energies below 100 trillion electronvolts (100 TeV). Therefore, it came as a shock in 2012 when IceCube team members spotted signs of two neutrinos boasting 10-times that energy, each clocking in at about 1 peta electronvolt (PeV, or 10¹⁵ eV) (1). “As soon as you see the event on the online display on your computer screen, you know you have not seen something like this before,” recalls IceCube’s principal investigator Francis Halzen of the University of Wisconsin-Madison. The energetic arrivals were dubbed Bert and Ernie.

The IceCube team began looking for signs of other high rollers in the detector’s data, and identified 26 more neutrinos with energies down to about 30 TeV that had probably come from outside the Solar System (2).

Bert and Ernie certainly packed a punch, but the scientists were surprised that they did not find any neutrinos with higher energies. As cosmic rays are accelerated—by being bounced across the shock front created by an expanding supernova remnant, for example—their collisions should create a spectrum of neutrinos whose energies tail off smoothly at higher energies. Models of this process predicted that IceCube’s data should harbor eight or nine neutrinos with more energy than Bert and Ernie.

The apparently sharp cutoff at 1 PeV prompted a slew of possible explanations. Some argued that the high-energy neutrinos might instead come from the decay of dark matter, the invisible material whose gravitational pull keeps swirling galaxies from flinging themselves apart. If dark matter particles had a mass limit of a few PeV, any neutrinos they produced would carry less energy (3). However, dark matter particles that massive “would be very exotic,” Halzen says doubtfully.

Alternatively, supernova remnants in our Galaxy slamming into nearby gas clouds could be releasing the neutrinos (4). However, if most of the neutrinos came from inside the Galaxy, they should line up along the disk where most of its stars reside, and that’s not the case, says Beacom. If the neutrinos are indeed extragalactic, that might favor sources such as the tempestuous regions near giant, feeding black holes called active galactic nuclei.

In the past few months, the IceCube team has analyzed another year of observations, bringing their tally of high-energy neutrinos to 37. The new data include the most energetic neutrino yet, with about twice the energy of Bert or Ernie. Naturally, the team named it Big Bird.

The discovery puts pressure on some of the models but does not rule any of them out. Disappointingly, none of the neutrinos have so far aligned with any known objects beyond our Galaxy. “They could be anything,” says Halzen. “It’s clear we have to find more events.”

Ghost Hunters

To catch more high-energy neutrinos, the team hopes to expand IceCube to about five times its current size. The project—still under discussion, and not yet funded—would cost about half as much as IceCube’s initial $270 million budget, and take about five years to complete.

Meanwhile, other observatories are about to join the hunt. A proposed neutrino facility called KM3NeT would put a detector about five times as big as IceCube at the bottom of the Mediterranean Sea, giving it some key advantages over its polar rival. Light scatters less in liquid water than it does in ice—possibly because of the small bubbles frozen in ice—so researchers could reconstruct the incoming direction of a neutrino more accurately.

KM3NeT would also offer a much clearer view of neutrinos arriving from the galactic center, which lies above the southern hemisphere. At IceCube’s South Pole location, these neutrinos stream down from the same direction as neutrinos produced in the atmosphere by cosmic rays. However, at KM3NeT’s northern hemisphere site, the two types of neutrinos would arrive from different directions, making a true galactic-center neutrino easier to spot. “We may be lucky to find the first source,” says Maarten de Jong of the National Institute for Subatomic Physics in Amsterdam, The Netherlands, and part of the KM3NeT team. The first phase of the €220 million ($305 million) project, which includes laying seafloor power cables and deploying some detector strings, may be completed in the next two years.

However, the highest-energy neutrinos—those above 100 PeV—might be too rare for even these detectors to catch. Therefore, two other experiments are testing an alternative approach that might be more sensitive, and also much cheaper. The Askaryan Radio Array and Antarctic Ross Ice Shelf Antenna Neutrino Array (ARIANNA) will both put what are essentially television antennas in shallow holes in the Antarctic ice, to look for radio pulses produced in the shower of particles left by monster neutrinos. The radio pulses produced by most incoming neutrinos are far too weak to detect this way, but should be strong enough when a 100-PeV neutrino is responsible. “If things go well, we should see a signal in roughly a year of data,” says ARIANNA team member Spencer Klein of Lawrence Berkeley National Laboratory and the University of California, Berkeley.

Teaming Up

Simply racking up neutrinos may not be enough to pinpoint their origin, however. “Five years ago, most people thought the way to do things was to look for point sources,” says Klein, an IceCube team member. However, the detector’s hits are spread right across the sky, with no clear hotspots churning out the particles. That suggests IceCube may be picking up the odd neutrino from many different sources scattered throughout space, rather than being able to catch multiple neutrinos from a few relatively nearby sources (5). “I’m not that optimistic about being able to see a point source,” says Klein.

The solution might be to combine neutrino astronomy with cosmic-ray and gamma-ray observations, a burgeoning field called multimessenger astronomy. Gamma rays can be produced alongside cosmic rays and neutrinos, and gamma-ray observatories, such as the High Energy Stereoscopic System in Namibia and the Energetic Radiation Imaging Telescope Array in Arizona, are now looking for such overlaps with IceCube’s neutrinos, although preliminary searches have turned up no signals. Seeing one or more neutrinos coinciding with a flurry of photons could offer much stronger evidence of their origin, says Eli Waxman of the Weizmann Institute of Science in Rehovot, Israel (6).

There are limits to this approach: gamma rays are produced by many other processes in the Universe, and they tend to be absorbed by intervening matter over distances greater than 5 billion light years. However, once a coincident source of gamma rays and neutrinos is found, gamma rays are much easier to detect than their ghostly partners, which should make it easier to build up a more detailed picture of their source, says Teresa Montaruli of the University of Geneva in Switzerland. Montaruli is a member of a planned €200 million observatory called the Cherenkov Telescope Array that could probe gamma rays at higher energies than ever before, beginning around 2020. Spotting cosmic rays with extreme energies—say, 50 exa electronvolts (5 × 10¹⁹ eV) or more—might also help. Although extremely rare, these cosmic rays tend to be deflected less by magnetic fields, and give a truer sense of their origin.

For now, though, IceCube must continue to watch and wait for the occasional flash from its ghostly quarries, building up a portrait of the universe’s particle accelerators one hard-won pixel at a time. It will likely take years, if not decades, for a picture to emerge. Says Beacom, “Neutrino astronomy is for the patient.”