A team of researchers has used laboratory experiments to shed light on a longstanding mystery about the gamma-ray emissions from blazars—active galaxies with supermassive black holes that send high-speed jets of particles toward Earth. These jets emit intense gamma rays, some with energies reaching several teraelectronvolts (TeV), which are observed by ground-based telescopes.
As these TeV gamma rays travel through space, they interact with background starlight and produce cascades of electron–positron pairs. According to current theories, these pairs should then interact with the cosmic microwave background and generate lower-energy gamma rays in the GeV range. However, such emissions have not been detected by instruments like the Fermi satellite.
Researchers have proposed two main explanations for this absence. One suggests that weak magnetic fields in intergalactic space deflect the pairs, causing the resulting GeV gamma rays to miss Earth. Another theory posits that plasma instabilities disrupt the pair beams as they move through sparse intergalactic matter, dissipating their energy before they can produce observable radiation.
To test these ideas, a collaborative team from the University of Oxford and STFC’s Central Laser Facility conducted experiments at CERN’s HiRadMat facility. They generated electron–positron pairs using CERN's Super Proton Synchrotron and passed them through an ambient plasma to create a scaled-down analogue of what happens in space near blazars. By examining how these beams behaved and whether magnetic fields formed around them, scientists aimed to determine if plasma instabilities could explain the missing GeV signals.
The results indicated that the pair beam stayed narrow and parallel as it moved through plasma, with little disruption or self-generated magnetic fields observed. This suggests that beam-plasma instabilities are unlikely to be responsible for the lack of GeV gamma-ray detections. Instead, it supports the idea that intergalactic magnetic fields—possibly relics from the early Universe—are redirecting or dispersing these emissions before they reach Earth.
Lead researcher Professor Gianluca Gregori from Oxford's Department of Physics stated: “Our study demonstrates how laboratory experiments can help bridge the gap between theory and observation, enhancing our understanding of astrophysical objects from satellite and ground-based telescopes. It also highlights the importance of collaboration between experimental facilities around the world, especially in breaking new ground in accessing increasingly extreme physical regimes.”
The findings raise further questions about how such cosmic magnetic fields originated since models suggest that conditions in the early Universe were highly uniform and not conducive to seeding such fields. Researchers believe future observations using advanced facilities like the Cherenkov Telescope Array Observatory may provide additional insights.
Co-investigator Professor Subir Sarkar said: “It was a lot of fun to be part of an innovative experiment like this that adds a novel dimension to the frontier research being done at CERN – hopefully our striking result will arouse interest in the plasma (astro)physics community to the possibilities for probing fundamental cosmic questions in a terrestrial high energy physics laboratory.”
Dr Pablo Bilbao added: “For a theorist, it’s extraordinary to see experiments now confirming and extending ideas that, until recently, existed only in simulations. These results show the power of combining large-scale computation with world-leading experimental facilities like CERN to probe the physics of cosmic plasmas.”
The international collaboration included scientists from institutions such as CERN; University of Rochester’s Laboratory for Laser Energetics; AWE Aldermaston; Lawrence Livermore National Laboratory; Max Planck Institute for Nuclear Physics; University of Iceland; Instituto Superior Técnico Lisbon; along with UK partners.
The study is published in Proceedings of the National Academy of Sciences (PNAS).