Signals from the deep
Deep beneath the surface of Siberia's Lake Baikal, the world's largest freshwater lake, an array of sensitive detectors is searching for some of the universe's most elusive particles. The Baikal-GVD (Gigaton Volume Detector) is a neutrino telescope, but it is not a traditional telescope. Instead of mirrors, it uses hundreds of spherical glass and steel optical modules suspended on strings, reaching depths between 750 and 1,300 meters. These modules watch for the faint blue glow of Cherenkov radiation. This light is produced when a high-energy neutrino, a particle with almost no mass that rarely interacts with other matter, happens to strike a water molecule. The lake's immense depth and clear freshwater shield the detectors from cosmic rays and other interference, making it a good location for this type of research.
The telescope's primary mission is to identify the sources of high-energy astrophysical neutrinos, which travel across the cosmos undeflected by magnetic fields. In recent years, between 2018 and 2023, the Baikal-GVD has detected a series of unusual events. Scientists observed a flux of neutrinos with energies exceeding 200 teraelectronvolts (TeV) coming from the direction of the Galactic plane. This was unexpected. The number of high-energy neutrinos coming from the Milky Way's plane was significantly greater than predicted by existing models of cosmic ray origin and propagation. This anomaly suggests that our understanding of particle generation within our own galaxy might be incomplete.
An intergalactic mystery
The Baikal-GVD is one of three major neutrino observatories in the world, forming a Global Neutrino Network along with IceCube at the South Pole and KM3NeT in the Mediterranean Sea. Data from Baikal-GVD has been compared with results from the IceCube experiment, and the findings correspond, strengthening the evidence for the anomalous neutrino flux. The joint analysis reduces the probability of a random error to a very low value.
The source of these excess high-energy neutrinos remains a puzzle. These particles are cosmic messengers, pointing back to their violent origins. The discrepancy between the observed signals and theoretical models challenges established concepts about how cosmic particles are accelerated to such high energies. Potential sources could be anything from the areas around supermassive black holes to the explosive deaths of massive stars. Other more exotic possibilities are also being explored, including the decay of superheavy dark matter particles or the existence of other unknown phenomena. The Baikal-GVD continues to expand; by 2021, its effective volume reached 0.4 cubic kilometers, and plans are in place to increase it further. Each new piece of data gathered from the depths of the ancient lake shows scientists the powerful cosmic engines that generate these ghostly signals.