Scientists Recreate Cosmic Fireballs To Solve Gamma Ray Mystery

Inside a cavernous hall at CERN, a beam of protons struck a graphite target and ignited a plasma that looked, in miniature, like the heart of a blazar. These fleeting fireballs, created by physicists from the University of Oxford and collaborators worldwide, may finally explain why high-energy gamma rays from distant galaxies vanish before reaching Earth.

The team used the Super Proton Synchrotron at CERN to generate dense streams of electrons and positrons, mimicking the charged particle jets emitted by supermassive black holes. By firing these pair beams through a plasma target, they recreated the conditions of intergalactic space in the laboratory, scaling down cosmic processes by many orders of magnitude.

Testing Theories Of The Missing Gamma Rays

Blazars are among the most luminous objects in the Universe, producing gamma rays billions of times more energetic than visible light. As these gamma rays travel through cosmic voids, they should generate cascades of lower-energy radiation. Yet, despite decades of telescope observations, the expected secondary gamma rays have never been detected. One long-standing theory blames hidden magnetic fields stretching across intergalactic space. Another suggests that the beams themselves dissipate energy through plasma instabilities.

To put these ideas to the test, Oxford physicist Gianluca Gregori and his colleagues directed their artificial fireball through a meter-long plasma chamber, measuring how the beam evolved and whether self-generated magnetic fields disrupted it. The experiment revealed that, contrary to some predictions, the plasma beam remained remarkably stable and nearly parallel, showing only minimal magnetic activity.

“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,” said Professor Gianluca Gregori. “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 finding suggests that beam-plasma instabilities are too weak to account for the missing gamma rays, pointing instead to the presence of faint, ancient magnetic fields threading the vast spaces between galaxies. These fields may be relics from the early Universe, formed before the first stars ignited.

Recreating The Cosmos In The Lab

Using a technique called the Fireball experiment, the researchers produced electron-positron pairs by striking a dense target with ultra-relativistic protons, then tracked their behavior as they moved through plasma. The setup, known as HiRadMat (High Radiation to Materials), allowed the team to visualize processes that occur on scales billions of times larger in the cosmos.

Even in such a controlled setting, the results echoed the deep puzzles of astrophysics. If the intergalactic medium does contain weak magnetic fields, they must have been seeded in the earliest moments after the Big Bang, long before galaxies formed. This could imply that new physics, beyond the Standard Model, shaped the primordial plasma of the Universe.

“These experiments demonstrate how laboratory astrophysics can test theories of the high-energy Universe,” said Professor Bob Bingham of the STFC Central Laser Facility. “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.”

The work represents a milestone for laboratory astrophysics, showing that particle accelerators can reproduce key phenomena of the early Universe. The team hopes future observations with the Cherenkov Telescope Array will test their conclusions under cosmic conditions, potentially revealing the elusive web of magnetism that permeates the Universe.

Proceedings of the National Academy of Sciences: 10.1073/pnas.2513365122