‘Ghost particles’ paint a whole new picture of the galaxy we live in

From visible light to radio waves, our galaxy, the Milky Way, can be observed in many different ways. In a new image, we see our galaxy through a completely different lens: neutrinos.

The universe is full of elusive neutrinos. These uncharged subatomic elementary particles hardly interact with other matter. For this reason, they are also called “ghost particles”. The fact that the universe is full of them prompted scientists from IceCube Neutrino Observatory – which hunts neutrinos or “ghost particles” – an ambitious idea. Because in a new study, they have created an image of the Milky Way for the first time using neutrinos.

Learn more about neutrinos
We know that neutrinos occur in large quantities and normally pass through the Earth undetected. In fact, they are currently traveling through your body by the billions without you realizing it. This is because they are fundamental particles, but they don’t really care about other particles. The strange particles were first observed in 1956. In the meantime, we know that the enigmatic neutrino is the most abundant particle in the cosmos and plays an important role in the process that makes stars shine. As far as we know now, these neutrinos come in three “flavors”: you have muon, electron, and tau neutrinos. We know that neutrinos can change flavor during their journey. For example, in 2013 it was shown for the first time that a muon neutrino transforms into an electron neutrino.

From visible light to radio waves, the Milky Way galaxy can be observed in different ways due to the different frequencies of electromagnetic radiation it emits. But now scientists are going even further. For the first time, they have created a completely unique image of our galaxy. By determining the galactic origin of thousands of neutrinos. The neutrino-based image of the Milky Way is the first of its kind: a galactic portrait made with matter particles instead of electromagnetic energy.

The image
Below you can admire the new image of the Milky Way. The top image shows the Milky Way as you’ve probably seen it before, in visible light. The bottom one is the first ever recorded with neutrinos. “I remember saying, at this point in human history, we are the first to see our galaxy other than through light,” said physicist Naoko Kurahashi Neilson.

Two images of the Milky Way galaxy. The top one was made in visible light and the bottom one is the first ever captured in neutrinos. Image: IceCube/US National Science Foundation (Lily Le & Shawn Johnson)/ESO (S. Brunier) Collaboration

High-energy neutrinos – think of energies millions to billions of times greater than those produced by the fusion reactions that power stars – have been detected by the IceCube Neutrino Observatory, a neutrino detector at the South Pole, near the Amundsen-Scott South Pole Station. The massive observatory detects subtle signs of high-energy neutrinos using thousands of sensors buried deep in a cubic mile of clear, pristine ice. IceCube is looking for signs of high-energy neutrinos emanating from our galaxy and beyond, to the edge of the universe.

New view of our galaxy
Thanks to this detector, researchers have now painted a whole new picture of the galaxy we live in. “As so often, major scientific breakthroughs are made possible by advances in technology,” said Denise Caldwell, director of the Physics Division of the national science foundation. “The highly sensitive IceCube detector, combined with new data analysis tools, has now given us a whole new perspective on our galaxy. In the future, this image will be further refined and improved in a higher resolution. It could reveal hidden features of our galaxy never before seen by humanity.

Composite image of the Milky Way. This image combines the visible-light photo of the Milky Way and the first-ever neutrino-based image of the Milky Way into a single image. The neutrinos detected are represented here in blue. Image: IceCube/US National Science Foundation (Lily Le & Shawn Johnson)/ESO (S. Brunier) Collaboration

In addition to the scientists’ already impressive ability to detect elusive ghost particles, they’ve also succeeded in the even more ambitious goal of figuring out where they came from. When neutrinos interact with the ice under IceCube, these rare encounters produce fuzzy light patterns, which IceCube can detect. And some light patterns clearly point to a particular part of the sky, helping researchers pinpoint the source of the neutrinos. Other interactions, however, are much less clear. And that’s why the researchers developed a machine learning algorithm. It took them more than two years to carefully test and verify the algorithm using artificial data simulating neutrino detections. When they finally fed the actual data provided by IceCube to the algorithm, an image emerged with bright spots that correspond to locations in the Milky Way galaxy suspected of emitting neutrinos.

For many decades, scientists have made countless astronomical discoveries, thanks to new methods of observing the universe. For example, radio astronomy and infrared astronomy were once revolutionary developments. These were later joined by new phenomena, such as gravitational waves and now neutrinos. According to Neilson, the new neutrino-based image of the Milky Way is another important and groundbreaking advance, which will eventually lead to new discoveries of previously unknown aspects of the universe. “It’s a huge milestone to observe our own galaxy for the first time using particles instead of light,” she says. “As neutrino astronomy evolves, we will be able to observe the universe through new glasses. That’s why we do what we do: see something no one has ever seen and understand things we haven’t understood until now.

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