The Tremendous Challenge of Detecting the Cosmic Neutrino Background
When I started my career in cosmology four decades ago, I was intrigued by an inevitable consequence of the Big Bang model. In addition to the relic radiation of the cosmic microwave background, discovered in 1964 — two years after my birth, there should also be a cosmic neutrino background filling up the Universe. The neutrinos decoupled from the cosmic soup of matter and radiation when the Universe was a second old at a temperature of ten billion degrees Kelvin, corresponding to an energy scale that is twice the rest mass of the electron.
Detecting the neutrino background is of utmost importance for verifying that the Universe started with a hot dense phase. If detected, the relic neutrinos would constitute the earliest known messengers of the observable Universe as they originated from the farthest horizon. They are so because they couple to ordinary matter only through the weak interaction, which is much more feeble than electromagnetic interactions. Given that gravity is even weaker, primordial gravitational waves can probe the Universe at earlier times, but they were not detected so far.
Measuring the mass density of the cosmic neutrino background is also of great significance for fundamental physics as it can be used to constrain the neutrino masses and number of neutrino species, including the possibility of hypothetical sterile neutrinos that do not couple to ordinary matter through the known weak interactions. Such ghost particles could potentially make a significant contribution to dark matter, the unknown substance which makes up 84% of the matter density in the Universe.
Thinking through these fundamental implications, I dedicated three months of my youth to a brainstorming session with my innovative colleague at Princeton’s Institute for Advanced Study, Glenn Starkman. After scanning a huge swath of possible detection methods, we came out empty-handed with just a speculative detector concept summarized in a paper published here.
Fast forward 35 years, when a brilliant postdoc named Gonzalo Herrera entered my Harvard office a few months ago. Gonzalo told me about his work on constraining the cosmic neutrino background with data from the IceCube Neutrino Observatory. This observatory employs thousands of sensors under the Antarctic ice, distributed over a cubic kilometer. IceCube consists of spherical optical sensors, each with a photomultiplier tube and a data acquisition computer which sends digital data to the counting house on the surface above the array. In 2019, the U.S. National Science Foundation, in collaboration with US institutional and international partners, approved funding of the “IceCube Upgrade project”. Now, seven years later, the IceCube upgrade has been successfully deployed, as reported a few days ago here. IceCube is sensitive to Cherenkov radiation emitted by charged leptons (electrons, muons and taus) which travel faster than light in the ice after being generated through interactions of very energetic neutrinos with the ice. Thus far, IceCube has discovered astrophysical neutrinos, identified two galaxies as neutrino sources, and observed neutrinos from our own Milky Way galaxy.
In his past work, Gonzalo used IceCube to set an upper limit on the mass density of the cosmic neutrino background, which inevitably interacts with the highest-energy cosmic-rays to produce a background of energetic neutrinos to which IceCube is sensitive. As he was describing his work in my office, I asked Gonzalo: “can we set new limits on the cosmic neutrino background from the high-energy photons produced through its interaction with the highest-energy cosmic-rays?”
And so we did, in a new paper accessible here. Cosmic-ray scatterings with the cosmic neutrino background induce a flux of gamma-rays and X-rays from boosted meson decays and charged lepton processes. Confronting expectations with observational data on the diffuse gamma-ray background sets a new limit on the mass density of the cosmic neutrino background that is within a factor of 22,000 of its possible value. Our new limit is orders of magnitude stronger than state-of-the-art laboratory experiments, and comparable in sensitivity to searches of boosted relic neutrinos with IceCube. Additionally, we have found that directional anisotropies arising from the clustering of relic neutrinos in gravitational potential wells and the inhomogeneous distribution of cosmic-ray sources, can enhance the sensitivity by a factor of about 4.
Direct detection of the cosmic neutrino background is a challenging task, but very rewarding for improving our understanding of the early Universe. The new paper offers a novel detection channel through the gamma-rays and X-ray fluxes produced by cosmic-ray scatterings with the cosmic neutrino background on cosmological scales. This opens a new multi-messenger avenue for the detection of the cosmic neutrino background.
Science is work in progress, and I have better ideas now than I had in my youth. This underlines my optimism that the future will be better than the past thanks to advances pioneered not by AI agents but by in-person conversations with the next generation of scientists like Gonzalo.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
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