American Scientist

Every new way of looking at the universe opens previously unknown areas of science. The first optical telescopes allowed astronomers to chart the movement of planets and moons, which helped Isaac Newton formulate his law of universal gravitation. Since then, other kinds of telescopes have enabled us to see invisible forms of light—including infrared, x-rays, and radio waves—that have revealed newborn stars, black holes, and even the faint afterglow of the Big Bang.

Over the past decade, astronomers have begun to see the universe in an entirely new way. This universe is illuminated by tiny subatomic particles called neutrinos. Researchers are using these neutrinos to test speculative theories of quantum gravity and to probe the unseen dark matter that makes up around 85 percent of the matter in the universe.

Neutrinos are very lightweight particles, having less than one-millionth the mass of an electron. They are also electrically neutral and nearly inert, interacting with other particles only via the weak nuclear force. On the other hand, neutrinos are extremely abundant. The universe is flooded with neutrinos created in the early universe; many more are constantly being created by nuclear processes or in the collisions of high-energy particles. The closest wellspring is our own Sun, where fusion reactions create a prodigious flood of neutrinos. Looking farther into space, the hot gas swirling around black holes also emits neutrinos, and supernovas create intense bursts of the ghostly particles. Some of these cosmic sources are able to emit supercharged neutrinos that carry more than 100 trillion times as much energy as a photon of visible light.

Because they rarely interact with matter, neutrinos are bold explorers of the universe. Neutrinos produced in a faraway galaxy can traverse millions of light-years unscathed; neutrinos born in the core of a supernova fly right through the remains of the star as if it wasn’t even there. These particles fly through you, too. Trillions of them stream through your body every second without having any effect.

This indifference to matter also makes neutrinos exceedingly difficult to study, because most of them leave no footprint as they pass through our instruments. To capture these fleeting particles, we need an absolutely enormous array of detectors, such as those at the IceCube Neutrino Observatory at the South Pole. Then comes a painstaking process of data collection, analysis, and interpretation to discern the traces they leave as they pass. The end result is spectacular, however: My colleagues and I are finally bringing the mysterious neutrino universe into focus.

Across the Universe

IceCube, the world’s largest operating neutrino telescope, has been gathering data for more than a decade. Buried about 1.5 kilometers deep in the antarctic ice, it consists of an array of more than 5,000 sensitive light detectors called digital optical modules (DOMs) that are strung along cables that have been buried in drill holes. These DOMs are arranged in a roughly hexagonal grid that occupies a volume of approximately 1 cubic kilometer, looking out into approximately 1 billion tons of ultraclear glacial ice.

IceCube’s detectors cannot sense neutrinos directly, so they have to look for indirect signs. When a high-energy neutrino occasionally (very occasionally) interacts with the ice, it produces charged particles that move faster than the speed of light in that medium. The result is generated light known as Cherenkov radiation. Akin to the sonic boom that denotes an airplane traveling faster than the speed of sound, Cherenkov radiation is the visible signature of a passing neutrino. Most of the Cherenkov light is lost as it travels through the antarctic ice, but some of it hits the DOMs, which record precisely when the light arrives. Scientists then use data about the amount of light detected across the array, and the light’s exact time of arrival at each DOM, to infer the direction and energy of the initial neutrino (see figure below).

These observations can also reveal what kind of neutrino triggered each flash of light. Neutrinos come in three flavors—electron-, muon-, and tau-neutrinos—named after the types of particles they produce when they interact with matter. Thanks to the differences between these three particles, each neutrino flavor leaves a characteristic footprint in our detectors.

For example, energetic electrons can generate a shower of other electrons that each throw off Cherenkov radiation in a revealing cascade. Muons—the heavier cousins of electrons—travel long distances and then disintegrate, leaving extensive tracks in our detectors that are easily spotted. Taus are even heavier but extremely short-lived, producing only small trails between production and disintegration that are very difficult to detect. Only about a dozen individual tau-neutrinos have ever been identified, and it is the least studied of all neutrino flavors.

In 2013, two years after completing the detector array, the IceCube Collaboration announced that it had glimpsed very high-energy neutrinos arriving from distant, unknown sources. It was the first time that such high-energy neutrinos had even been seen coming from outside our Solar System. Unfortunately, we could not track these neutrinos back to a specific point of origin. Although IceCube’s detectors can roughly work out which part of the sky a neutrino has come from, the observatory does not have enough resolution to identify an individual source. It’s hardly surprising—if you put your thumb up to the sky, some 50 million galaxies may lie behind it. A neutrino arriving on Earth from that general direction could have originated in any one of those galaxies.

Over the past decade, astronomers have begun to see the universe in an entirely new way. This universe is illuminated by tiny subatomic particles called neutrinos.

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But we can draw on other data to help us deduce a neutrino’s origins. For example, a neutrino that came from a particular part of the sky could be linked to an event that was observed in some form of light using other telescopes. In 2017, for example, IceCube detected a high-energy neutrino that coincided with a flare of high-energy gamma rays from a source called TXS 0506+056. That object is a type of highly active galaxy known as a blazar. It was the first time that a neutrino detection had been pinned to a cosmic source beyond our galaxy.

Recently, researchers have improved the calibration of IceCube’s DOM detectors to provide better directional resolution. In 2022, this upgrade enabled the team to trace a high-energy neutrino back to NGC 1068, a galaxy more than 45 million light-years away from us. Like TXS 0506+056, NGC 1068 is an active galaxy.

Although we are still not able to pinpoint the specific objects within those galaxies that generated these neutrinos, other telescopes have shown that both TXS 0506+056 and NGC 1068 contain a supermassive black hole at their center. Those black holes are violently consuming matter, their intense gravitational field drawing a dense and swirling cloud of gas and dust around them. Within these clouds, protons are accelerated to near–light speed. Researchers think that particle collisions in the clouds disgorge the high-energy neutrinos that IceCube has spotted. Thus those neutrinos appear to be messengers arriving right from the precipice of distant black holes.

Flavor Balance

Neutrinos not only carry information about the sources that spawned them; they also bring with them a record of the physical laws that shaped their journey across the cosmos. Researchers working with IceCube are now using the flavor of detected neutrinos to test and improve theories that describe the fundamental principles of the cosmos.

Particle physics experiments on Earth predict that distant cosmic sources should generate about twice as many muon-neutrinos as electron-neutrinos, and very few tau-neutrinos. After the neutrinos are created, another physical process occurs, however. As a neutrino travels through space, it can change from one flavor to another due to a phenomenon known as neutrino oscillation. By the time a bunch of neutrinos reach Earth, we expect that oscillations should have produced an equal mix of flavors.

Any small deviations from an equal ratio of neutrino flavors would suggest that the particles were affected by unknown processes during their journey across the cosmos. By studying the flavor ratios of neutrinos that arrive at IceCube, we can potentially see the effects of these processes and test theories that go beyond our current understanding of physics. In particular, I have collaborated with Teppei Katori of King’s College London and Jordi SalvadÓ of the University of Barcelona to show that the ratio of neutrino flavors offers one of the most sensitive tests of quantum gravity.

Quantum gravity seeks to merge two of the most successful theories in physics: quantum mechanics and Albert Einstein’s general relativity. Quantum mechanics describes the dynamics of very small objects, such as a photon or an electron orbiting the nucleus of an atom, whereas general relativity is a theory of gravity that explains the large-scale evolution of the universe. Unfortunately, these theories cannot both be completely true. In some circumstances, they give wildly inconsistent or nonsensical answers.

Quantum gravity aims to overcome these problems by forming a bridge between the two theories. The effects of quantum gravity will be incredibly subtle in most situations, and only become significant at extremely small distances or at extremely high energies. One potential signature of quantum gravity involves the breakdown of a fundamental symmetry in the universe. This symmetry rests on the idea that there is no preferred direction in the universe—in other words, the cosmos has no label showing an arrow and the word “up.”

If this symmetry is broken, it means that there are discernible elements in space-time—glitches in the very fabric of the universe, you might say—that do indeed point in a particular direction. And that’s where neutrinos come in. As they travel from their faraway sources to Earth, neutrinos can interact with these space-time elements in ways that imprint distinct signatures on the neutrino flavor states. For example, some quantum gravity theories suggest that the interactions can switch off neutrino oscillations, forcing the neutrino to maintain the same flavor from source to detection. In some scenarios, this phenomenon could make all of the neutrinos from a source arrive on Earth as a single flavor, rather than the three-way mix we would otherwise expect.

Researchers working with IceCube are now using the flavor of detected neutrinos to test and improve theories about the fundamental principles of the cosmos.

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To test these far-reaching ideas, we first needed to prove that IceCube can detect all flavors of neutrinos—even the super-obscure tau-neutrinos. That effort pushed our observatory to its limits. Last year, I was part of the team that announced the first detection of a tau-neutrino from a cosmic source. Because the neutrino almost certainly did not start out with its tau flavor, our detection confirmed that it must have undergone neutrino oscillation during its vast journey.

We were able to identify this tau-neutrino by its unique light signal picked up by our IceCube detectors. When a tau-neutrino hits the ice, it produces a very short-lived tau particle and an accompanying flash of light. In about 100 billionths of a second, the tau disintegrates into other particles, releasing more light. So the signature of a tau-neutrino is a distinctive double flash: one signaling the production of the tau particle, followed by a second one that shows its disintegration. For very high-energy neutrinos, these two emissions might be only about 50 meters apart in the ice surrounding IceCube, the average distance a tau with a million times the energy of a proton can travel during its fleeting existence.

To look for these signatures, I worked with an international team to meticulously reanalyze IceCube data collected between 2010 and 2017. This tau-neutrino hunt was led by Juliana Stachurska, currently a postdoctoral researcher at the Massachusetts Institute of Technology.

While our team was at an IceCube Collaboration meeting in Atlanta, we went over our initial checks and found some confusing results. We all decamped to a coffee shop to figure out what was going on. Picture the scene: A group of young scientists huddling in the coffee shop, checking many lines of their computer code and poring over figures, desperately searching for a rare subatomic particle from deep space. Eventually, we looked at the raw data itself. We immediately saw that a pair of light sensors had captured two consecutive light emissions just 17 meters apart, a clear sign of a tau-neutrino.

When people discover a new neutrino, they usually get to name it—that is how rare these detections are. We called our precious tau-neutrino Double Double, partly because of its characteristic light signature, but also in honor of the classic double-cream-and-sugar coffee sold by the Canadian restaurant chain Tim Hortons. We were drinking a lot of coffee back then.

Our observation of the Double Double tau-neutrino is already helping to test quantum gravity theories. For now, the data show no sign of the space-time defects predicted by quantum gravity theories. That’s disappointing, but instructive. Every result will help physicists to refute and refine their ideas about how to unite quantum mechanics and general relativity.

A Mass of Data

Although IceCube has confirmed only two specific neutrino sources—the active galaxies TXS 0506+056 and NGC 1068—it has spotted hundreds of other high-energy neutrinos. By combing through these data, we should be able to confirm other sources that will give researchers more ways to study neutrino physics. An identified source provides a precise distance between where the neutrino was produced and where it was detected. Because neutrino oscillations depend on this distance, combining a known source location with information about the flavors and energy distribution of the neutrinos will fill in crucial pieces of the neutrino puzzle.

For example, researchers led by Kiara Carloni, a graduate student in my research group at Harvard University, recently proposed a way to use distant neutrino sources to uncover the nature of a neutrino’s mass, which is one of the biggest mysteries of contemporary physics. For a long time, physicists wondered if neutrinos had any mass at all. Previous experiments found that neutrinos have a mass that is nonzero, but one that is very, very small—so small that it cannot be measured in laboratories. The origin of this mass is unclear. Other particles, such as electrons or protons, gain their mass through interactions with the so-called Higgs field, embodied in the Higgs boson that was discovered in 2012 by the Large Hadron Collider at European Council for Nuclear Research (CERN), near Geneva. But the extreme smallness of a neutrino’s mass suggests that some other mechanism is at play.

José Valle at the University of Valencia in Spain posits that neutrinos gain their mass by partnering with so-called “sterile” neutrinos, which never directly interact with any other particles. These sterile neutrinos could be produced by the same kind of neutrino oscillations that enable the particles to change flavor. But even by the standards of our work at IceCube, finding a sterile neutrino would be quite a feat. In theory, an average neutrino would have to travel truly enormous distances to stand a chance of becoming sterile. Although we have seen hints of similar particles in our particle accelerators, Earth-bound experiments are simply not big enough to capture sterile neutrinos.

The good news is that neutrinos from distant galaxies and other cosmic sources have the right energy, and travel for sufficiently long distances, to trigger their conversion to a sterile form. Carloni’s work suggests that when this conversion happens, we should see an absence of neutrinos at a particular energy, because they no longer interact with our detectors once they have become sterile. In other words, we can’t detect the presence of sterile neutrinos, but we might be able to detect the absence of the neutrinos that sired them.

It is astonishing that these kilometer-scale telescopes can turn rock, water, and ice into sophisticated eyes that look deep into the invisible neutrino universe.

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Before we can perform these tests, we will need to identify many more neutrino sources. Fortunately, other observatories are poised to join IceCube in exploring the neutrino universe. The Cubic Kilometre Neutrino Telescope (KM3NeT) is being built at the bottom of the Mediterranean Sea, while the Baikal Deep Underwater Neutrino Telescope (Baikal-GVD) is taking shape in Lake Baikal in Russia. The first phase of Baikal-GVD was completed in 2021; KM3NeT is scheduled to become operational in 2025. Another venture called the Pacific Ocean Neutrino Experiment (P-ONE) could be constructed toward the end of the decade off the coast of Vancouver, Canada.

All three of these new neutrino observatories operate on similar principles as IceCube, but they use water instead of ice as the detector medium. Physicists expect that these water-based neutrino telescopes will have a better directional resolution than IceCube, boosting our chances of pinpointing specific objects as the sources of cosmic neutrinos.

We may also gain more insights from proposed neutrino observatories that would use large mountains as their detector medium, relying on solid rock to trigger the neutrino interaction. For example, there are plans afoot to build the Tau Air Shower Mountain-Based Observatory (TAMBO) in one of the deepest high-altitude canyons in the world, up in the Peruvian Andes. TAMBO would nicely complement IceCube and the other neutrino telescopes, because it would be a specialized tau-neutrino detector.

It is astonishing that these kilometer-scale telescopes can turn rock, water, and ice into sophisticated eyes that look deep into the invisible neutrino universe. After a decade of remarkable progress at IceCube, it’s clear that the era of neutrino astronomy is only just beginning.