Naoko Kurahashi Neilson was on a Zoom call when she saw it for the first time.
She and two PhD students—Mirco Hünnefeld of TU Dortmund University in Germany and Steve Sclafani of Drexel University in the United States—had received permission to review the results of their analysis. Using 10 years of data and 60,000 detections from the IceCube Neutrino Observatory, they were trying to map the emission of tiny, ghostly particles called neutrinos from the band of the Milky Way.
Kurahashi Neilson remembers the three of them staring at the image together. Slowly, they realized that they were, indeed, looking at the first-ever neutrino image of our galaxy.
Ever careful, Hünnefeld and Sclafani immediately wanted to start performing checks of their analysis. “I remember telling them, ‘Hey, slow down, don’t let the moment slip away from you,’” says Kurahashi Neilson, a professor of physics at Drexel University and Sclafani’s advisor. “’Because in the history of humankind, we three—we’re the first ones to see our own galaxy in something other than light.’”
“Multimessenger astrophysics seems to be a field that is growing in its ability to respond to our desires to understand the universe.”
The IceCube collaboration published their Milky Way neutrino map on June 29. By complete coincidence, that was the same day that the North American Nanohertz Observatory for Gravitational Waves, or NANOGrav, announced they had also seen space in a new way, by finding evidence of a low-frequency gravitational-wave hum throughout the cosmos.
Each result is an exciting and important step forward in their respective fields. But together, they reinforce the capability of multimessenger astrophysics, in which astronomers use different cosmic “messengers”—including light, neutrinos and gravitational waves—to learn about astronomical objects.
For thousands of years, humankind has observed the night sky with our eyes. It wasn’t until the past century that astronomers developed methods for seeing the universe with something other than visible light, detecting other wavelengths of the electromagnetic spectrum such as radio waves, infrared, X-rays and beyond. Just over 50 years ago, our sun was the first astronomical object to be observed in multiple messengers: light and neutrinos.
Today, experiments like IceCube and NANOGrav—which both receive funding from the National Science Foundation—use their respective messengers of choice to investigate the farthest reaches of the universe and discover things that are impossible to learn any other way. Their latest results represent only a small part of the field’s potential.
“Multimessenger astrophysics seems to be a field that is growing in its ability to respond to our desires to understand the universe,” says Marcelle Soares-Santos, an associate professor at the University of Michigan who was not involved with either result. “I think there is more and more a drive to look at the cosmic frontier for some of these answers [to questions about our universe], and I think these results are showing us that there’s a lot to explore, a lot of excitement and a lot to discover.”
Our galaxy seen in neutrinos
The IceCube Neutrino Observatory is an array of over 5,000 light sensors frozen in a cubic kilometer of ice at the South Pole. The sensors detect Cherenkov radiation: blue light that gets emitted when a charged particle moves faster than light can through ice. These charged particles are produced when neutrinos collide with particles in the ice.
After photons, neutrinos are the most abundant particles in the universe. They are generated in a variety of terrestrial sources, but neutrino astronomers are interested in neutrinos that are made in the cosmos.
Astrophysicists have yet to determine with absolute certainty what astronomical objects create neutrinos, but IceCube has found strong evidence of neutrinos coming from a blazar, an active galaxy and, now, the plane of the Milky Way.
When a neutrino passes through the ice, IceCube’s in-ice sensors light up in patterns that are categorized as either line-shaped “tracks” or spherical “cascades.” When Kurahashi Neilson joined the IceCube collaboration over a decade ago, she tried doing an analysis of neutrinos from the Milky Way, called a galactic plane analysis, using tracks, but she was unsuccessful. It later occurred to her to try using cascades instead.
Cascades turned out to be ideal for this analysis for a few reasons. For one, the Milky Way is an enormous feature in the sky, so identifying neutrinos coming from the galactic plane did not require the fine pointing toward the source that tracks provide.
Additionally, the IceCube detector sees neutrinos coming from the sky above the South Pole—including lots of tracks from neutrinos created in Earth’s atmosphere. Researchers weren’t interested in those kinds of tracks. By just using cascades from the southern sky, they were able to greatly reduce this background signal.
Cascades also tend to be lower-energy events, which fit the criteria for this analysis; IceCube researchers believe that neutrinos coming from sources within our galaxy are lower energy than those coming from extragalactic sources.
Now that IceCube has published the neutrino image, Kurahashi Neilson says the next step for astronomers is to compare it to other views of the Milky Way in a galactic-scale game of “spot the difference.”
“We’re going to learn what’s the same, what’s different,” she says. “Is this feature real or is it not real? Is it going to go away? Is it just statistical fluctuation? Are we really seeing something here in this part of the galactic plane that we see in other messengers and other wavelengths?”
Kurahashi Neilson also says the result brings neutrino astronomy back to our cosmic neighborhood. “I don’t think we’ve talked about galactic multimessenger astronomy a lot in the last couple of decades,” she says. “I think that’s what this [result] gives birth to.”
A background hum of gravitational waves
Gravitational waves are ripples in the fabric of spacetime caused by the acceleration of massive objects in space. In multimessenger astronomy, gravitational waves are useful because they can transmit information about objects in the universe that don’t shine in light—like black holes.
Predicted by Albert Einstein in his theory of general relativity, gravitational waves became veritably famous in 2016. That’s when scientists working on the Laser Interferometer Gravitational-Wave Observatory, or LIGO, announced that, the previous year, they had directly observed gravitational waves caused by merging black holes.
Since then, LIGO and its European counterpart, Virgo, have made dozens of gravitational-wave detections. But these are only one particular kind of gravitational wave.
“We’re opening a new window on the gravitational-wave spectrum,” says Sarah Vigeland, assistant professor of physics at the University of Wisconsin, Milwaukee and chair of NANOGrav’s Gravitational Wave Detection Working Group.
For 15 years, NANOGrav physicists searched for low-frequency gravitational waves by studying the signals of 68 beacon-like astronomical objects called pulsars. These neutron stars emit light from their poles; they spin rapidly, so astronomers on Earth see the light as flashing at incredibly precise intervals, leading to pulsars’ nickname of cosmic “lighthouses.”
To make their observations of the pulsars, NANOGrav used the Green Bank Observatory in West Virginia, the Very Large Array in New Mexico and—before its recent collapse—the Arecibo Observatory in Puerto Rico. At the same time, similar projects around the world used other telescopes to study other sets of pulsars, known as “pulsar timing arrays.”
If low-frequency gravitational waves indeed permeate the entire universe, as theorized by Einstein, the pulsars’ timing should shift in a predictable way, buffeted by the gravitational-wave background. And that is what NANOGrav and other collaborations saw. They published their evidence in a series of papers.
“You have a bunch of gravitational-wave sources throughout the universe, and they’re all emitting a single note,” says Vigeland. “But then the combination of all of them creates this hum of gravitational waves.”
What makes NANOGrav’s gravitational waves different from those detected by LIGO is their frequency. Where LIGO is attuned to see high-frequency gravitational waves from short-lived events like black hole mergers that happen on timescales of seconds, NANOGrav observes low-frequency gravitational waves produced on timescales of years to decades.
The difference is akin to observing with different wavelengths of light in astronomy: LIGO is like a gamma-ray telescope and NANOGrav is like a radio telescope. “They’re really complementary methods,” says Vigeland.
This NANOGrav finding shows great promise for the future of gravitational-wave astronomy. “Progress is happening at a much faster pace,” says Soares-Santos. “We went from making our first detection in 2015 [with LIGO] to being able to now make this detection with a totally different technique in a totally different wavelength and totally different sources—within a decade.”
The gravitational waves that NANOGrav sees may be produced by pairs of supermassive black holes that orbit each other; as their enormous masses pull them spiraling together, the black holes’ slow movements generate gravitational waves that propagate outward and across the universe like ripples of water in the ocean. (When the supermassive black holes eventually merge, they emit gravitational waves in a frequency that NANOGrav cannot observe.)
Vigeland notes that they have not yet confirmed any sources of the gravitational-wave background that NANOGrav and others observed. In addition to supermassive black hole binaries, there are exotic possibilities, like cosmic strings or theorized particles called axions. Astrophysicists will need to make more observations with more sensitive pulsar timing arrays to determine the exact sources. Once that’s done, making electromagnetic observations, possibly over the course of many years, will reveal even more information about the sources of this hum.
In the meantime, NANOGrav and other gravitational-wave experiments will continue to explore the capabilities of this messenger. “The most exciting thing to me is the idea that we will find something that we didn’t know even existed. I think that that is very likely, the more we observe and the more we look at different parts of the gravitational spectrum,” says Vigeland. “It certainly was true with electromagnetic astronomy—there have been so many surprises when people looked at different wavelengths. I think it’s likely that we’ll have the same kind of surprises with gravitational waves.”
Ultimately, seeing an astronomical object in light, gravitational waves and neutrinos would be the holy grail of multimessenger astrophysics. Unfortunately, seeing such an event is highly unlikely in a human lifetime; the most frequently occurring event that would produce observable versions of all three would be a supernova in our galaxy, which happens approximately once per century.
Still, using multiple messengers to see something new would be exciting and give astrophysicists more information about our mysterious universe.
Even though IceCube and NANOGrav have not yet made a multimessenger observation together, their recent results bode well for the burgeoning field’s future. “In both cases, we’re using non-traditional methods for observing the universe, and we’re really pushing forward what we’re able to do with things that are not light,” says Vigeland. “That’s a big step forward for astronomy. We used just light for thousands of years. I think this progress is pretty incredible.”