Researchers at CERN have generated plasma 'fireballs' to simulate jets from distant blazars, shedding light on the mystery of missing gamma rays. The experiment suggests that ancient intergalactic magnetic fields, rather than plasma instabilities, may explain the phenomenon. Published in PNAS on November 3, the findings highlight potential remnants from the early Universe.
An international team led by the University of Oxford used CERN's Super Proton Synchrotron accelerator in Geneva to produce plasma 'fireballs,' mimicking the behavior of particle jets from blazars. Blazars are active galaxies powered by supermassive black holes that emit narrow jets of particles and radiation at nearly the speed of light, including gamma rays up to several teraelectronvolts (TeV). These high-energy gamma rays travel through intergalactic space and interact with background light from stars, creating cascades of electron-positron pairs. These pairs are expected to collide with the cosmic microwave background, producing lower-energy gamma rays around 10^9 eV (GeV), yet NASA's Fermi satellite has not detected this signal.
Two main theories have been proposed for the discrepancy: weak intergalactic magnetic fields deflecting the pairs away from Earth, or instabilities in the thin intergalactic plasma draining energy from the beam. To test this, the researchers employed CERN's High-Radiation to Materials (HiRadMat) facility, sending beams of electron-positron pairs through a one-meter-long plasma to replicate cosmic conditions.
The results, published on November 3 in PNAS, showed the beam remained tightly focused with minimal disturbance or magnetic activity, indicating plasma instabilities are insufficient to explain the missing gamma rays. This supports the presence of primordial magnetic fields from the Universe's earliest moments, possibly involving physics beyond the Standard Model.
Lead researcher Professor Gianluca Gregori from the University of Oxford 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." Co-investigator Professor Bob Bingham from STFC Central Laser Facility and the University of Strathclyde added: "These experiments demonstrate how laboratory astrophysics can test theories of the high-energy Universe." Professor Subir Sarkar from Oxford noted: "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."
The collaboration included institutions such as STFC's Central Laser Facility, the University of Rochester, Lawrence Livermore National Laboratory, and the Max Planck Institute for Nuclear Physics. Future observatories like the Cherenkov Telescope Array may provide further insights into these magnetic fields.