Scientists Use AI to Discover 'Ghost Particles' From Our Own Galaxy In First

For the first time in history, astronomers have detected “ghost particles” called neutrinos that came from right here inside the Milky Way, as opposed to faraway galaxies, using a gargantuan detector buried under the ice at the South Pole, reports a new study. 

The discovery presents an entirely novel view of our galaxy that will help scientists identify the mysterious sources of high-energy particles, such as neutrinos and cosmic rays, which is a major goal in astronomy. 

Neutrinos are elusive particles that barely interact with matter as they rush across space, earning them the nickname “ghost particles.” They can pass straight through planets without slowing down or switching direction; indeed, trillions of neutrinos zoom through your body every second. The spectral nature of neutrinos makes them difficult to detect, even though they are among the most abundant particles in the cosmos.

Enter: The IceCube Neutrino Observatory, a detector that extends for more than a mile under the ice at the Amundsen–Scott South Pole Station in Antarctica. IceCube is a network of buried sensors that has captured thousands of high-energy neutrinos since it was completed in 2010, revolutionizing the emerging field of neutrino astronomy. 

But while scientists in the IceCube collaboration have identified many neutrinos that hail from distant galaxies, homegrown neutrinos from the Milky Way have remained tantalizingly out of reach until now. 

By applying machine learning to a 10-year dataset, the collaboration has at last achieved its long-sought goal of observing “neutrinos from the Galactic plane” that “provide strong evidence that the Milky Way is a source of high-energy neutrinos,” according to a study published on Thursday in the journal Science.

“This is the first time that we have seen our own galaxy in neutrinos,” said Ignacio Taboada, a professor of physics at the Georgia Institute of Technology and a member of the IceCube collaboration, in a call with Motherboard. “We have made a map of the galaxy in neutrinos.”

“We're starting to really be able to perform neutrino astronomy, and this is another milestone,” noted Mirco Hünnefeld, an IceCube member and PhD student at Technical University Dortmund, in a separate call with Motherboard. “We now have this new lens on our galaxy.”

IceCube is designed to capture extremely energetic neutrinos that were made by cataclysmic events, like the explosions of stars or the eruptions of black holes as they feed on matter. These pyrotechnic processes are also thought to be a source of cosmic rays, a class of energetic charged particles that are much heavier than neutrinos and contain valuable clues about the high-energy universe. 

Neutrinos and cosmic rays are produced by the same explosive events, but their journeys through the universe are very different. Whereas neutrinos sail through space essentially in a straight line, cosmic rays get entangled in magnetic fields that carry them in unpredictable trajectories, which makes it difficult to figure out where these particles originated. For this reason, scientists use neutrinos as a kind of tracer for cosmic rays, allowing them to spot sources of the particles that could otherwise not be located.

In this way, IceCube has helped to track down exotic sources of neutrinos that are located billions of light years away, in galaxies that tend to be far more eruptive and active than the Milky Way. However, identifying the lower-energy neutrinos that are presumed to emerge from our own galaxy has proved to be much more challenging.

“We’re inside our galaxy, so you would expect that our galaxy would have been the very first thing that we observed,” said Taboada. “Actually what we observed first, in 2013, were neutrinos coming from all directions, consistent with coming from distant galaxies.”

“It's really weird that it is the outside universe that is bright, not so much the galaxy’s neutrinos,” he added. “It took us an additional 10 years to actually see our own galaxy.”

The team was able to make this breakthrough by training a machine learning algorithm to analyze a vast dataset of IceCube observations that covers the past decade. In particular, the program compared the properties of so-called “cascade events” in the detector, which refer to the showers of particles that are created when neutrinos slam into the sensors. 

After sifting through more than 60,000 of these cascades, the algorithm identified a few hundred detections that originated within our own galaxy. Now that the team has captured this unique perspective on the Milky Way, they plan to probe the observations for insights about the sources of neutrinos and cosmic rays in our galaxy. 

“There is already some understanding of how cosmic rays propagate in the galaxy, so we have to put the pieces together and try to get to that answer,” Taboada said. “It will not be easy, but it will be very fun.”

The new results contribute to the emerging field of “multi-messenger” astronomy that compares observations from a range of sources, including light, neutrinos, cosmic rays, and gravitational waves.

Indeed, IceCube’s new discovery follows the breakthrough discovery of what is likely a gravitational wave “background” created by supermassive black holes in the early universe, which was announced on Wednesday. The coincidental timing of these two major advances underscores the rapid development of multi-messenger astronomy, which is revealing sides of the universe we have never seen before.

“It's exciting because we have opened a new door to the universe with neutrinos and gravitational waves,” Hünnefeld concluded. “We can learn so much more.”