CERN Recreates Mini ‘Cosmic Fireballs’ to Probe Missing Gamma‑Rays and Intergalactic Magnetism

Scientists recreate blazar-like 'cosmic fireballs' at CERN

In a first-of-its-kind laboratory experiment, researchers have reproduced miniature versions of the extreme plasma jets launched by feeding supermassive black holes — so-called blazars — to investigate why expected low-energy gamma-rays are missing from astronomical observations.

A team from the University of Oxford and the Science and Technology Facilities Council’s (STFC) Central Laser Facility (CLF) used CERN’s Super Proton Synchrotron (SPS) and the HiRadMat (High-Radiation to Materials) target area to generate electron–positron pairs. They then fired those matter–antimatter beams through about 1 meter (3.3 feet) of plasma, mimicking the relativistic conditions believed to exist in narrow blazar jets.

"These experiments demonstrate how laboratory astrophysics can test theories of the high-energy universe," said Bob Bingham of the University of Strathclyde. "By reproducing relativistic plasma conditions in the lab, we can measure processes that shape the evolution of cosmic jets and better understand the origin of magnetic fields in intergalactic space."

Why the experiment matters: Gamma-rays emitted by blazars travel across intergalactic space and interact with ambient background light, producing electron–positron pairs. Those pairs should then upscatter low-energy photons from the cosmic microwave background (CMB) via inverse Compton scattering, creating a detectable flux of lower-energy gamma-rays that telescopes such as NASA's Fermi spacecraft can observe. Yet the anticipated secondary gamma-ray signal has largely not been seen.

Several hypotheses have been proposed to explain the missing signal: weak intergalactic magnetic fields could deflect the pairs and redirect the secondary emission away from our line of sight; beam-driven plasma instabilities in the ultra-dilute intergalactic medium could dissipate the pair beam's energy before it produces secondary gamma-rays; or a primordial (relic) intergalactic magnetic field could perturb the pairs early in cosmic history.

To test the first two possibilities, the laboratory experiment examined whether the replicated electron–positron beams would become unstable, spread out, or generate magnetic fields as they cross the plasma. Contrary to expectations, the beams remained narrowly collimated, showed little spreading, and produced no strong instability-driven magnetic fields in the plasma wake.

These results suggest that beam-driven plasma instabilities are likely too weak, under the tested conditions, to explain the absence of expected secondary gamma-rays. That outcome strengthens the case for an intergalactic relic magnetic field as a more plausible explanation — a conclusion that raises new cosmological questions about how such a field could have been seeded in the near-uniform early universe.

The experiment's lead and co-authors note that resolving the origin of any relic field may require new physics beyond the Standard Model and observations from next-generation instruments such as the Cherenkov Telescope Array Observatory (CTAO).

"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," said Subir Sarkar of the University of Oxford. "Hopefully our striking result will arouse interest in the plasma astrophysics community to the possibilities for probing fundamental cosmic questions in a terrestrial high-energy physics laboratory."

The team's findings were published on Nov. 3 in the journal Proceedings of the National Academy of Sciences (PNAS).