Blazars, which are extremely active galaxies energizing supermassive black holes, emit powerful jets of particles and radiation directed towards Earth. These jets produce high levels of gamma-ray emission, reaching several teraelectronvolts (TeV). While these gamma rays theoretically create cascades of electron-positron pairs as they interact with the faint background light from stars, researchers have long struggled to identify the lower-energy gamma rays (in the GeV range) expected from these interactions. Despite extensive efforts using space telescopes such as Fermi, these GeV gamma rays have remained elusive, leaving scientists puzzled.
A research team from the University of Oxford in collaboration with the Science and Technology Facilities Council’s Central Laser Facility has embarked on a groundbreaking experiment at CERN’s HiRadMat facility. Their approach involved generating electron-positron pairs using the Super Proton Synchrotron and sending them through a controlled plasma environment, creating a laboratory analogy of a blazar-driven pair cascade traversing intergalactic space. This setup allowed researchers to investigate whether the instabilities in the beam-plasma interactions could account for the absence of GeV gamma rays.
The team’s results revealed surprising findings: the electron-positron beam remained narrow and largely undisturbed, with minimal impact from magnetic fields associated with the plasma. This observation suggests that beam-plasma instabilities are likely insufficient to explain the missing GeV gamma rays, supporting an intriguing hypothesis that intergalactic space contains a magnetic field formed in the early Universe.
Lead researcher Professor Gianluca Gregori noted that their study demonstrates how experimental work can significantly enhance the theoretical understanding of astrophysical phenomena observed through various telescopes. He emphasized the critical nature of global collaboration in advancing research within increasingly extreme physical contexts.
While the findings provide significant insight, they also raise additional questions. The early Universe is thought to have been uniformly structured, leading to uncertainty about how magnetic fields might have originated during that period. Researchers speculate that new physics beyond the established Standard Model may be involved. Future endeavors, including observatories like the Cherenkov Telescope Array, are anticipated to yield high-resolution data that could further explore these complex questions.
Co-investigator Professor Subir Sarkar described the experiment as an exhilarating venture that enriches research efforts at CERN, expressing hope that the results would spark greater interest in the field of plasma astrophysics. Dr. Pablo Bilbao also highlighted the importance of merging experimental results with computational models to deepen understanding of cosmic phenomena.
This collaborative research included contributions from various prestigious institutions, such as the University of Rochester, the Max Planck Institute for Nuclear Physics, and others. The findings of this study titled “Suppression of pair beam instabilities in a laboratory analogue of blazar pair cascades” have been published in the Proceedings of the National Academy of Sciences. The ongoing exploration in this area represents an exciting frontier in the quest to understand the universe’s fundamental processes.