Researchers at the University of Oxford, working with international collaborators at CERN, have recreated a scaled version of cosmic plasma jets in the laboratory, offering new evidence that may explain why part of the Universe’s predicted gamma-ray light has never been observed. Led by Professor Gianluca Gregori, the team used CERN’s Super Proton Synchrotron to study how beams of electrons and positrons behave as they travel through plasma, mimicking conditions thought to exist around distant active galaxies known as blazars.
Arrowsmith, C. D., Miniati, F., Bilbao, P. J., Simon, P., Bott, A. F. A., Burger, S., Chen, H., Cruz, F. D., Davenne, T., Dyson, A., Efthymiopoulos, I., Froula, D. H., Goillot, A., Gudmundsson, J. T., Haberberger, D., Halliday, J. W. D., Hodge, T., Huffman, B. T., Iaquinta, S., … Gregori, G. (2025). Suppression of pair beam instabilities in a laboratory analogue of blazar pair cascades. Proceedings of the National Academy of Sciences, 122(45). https://doi.org/10.1073/pnas.2513365122
Blazars are powered by supermassive black holes that launch narrow, highly energetic jets of particles and radiation into intergalactic space. When these jets are aligned with Earth, they are observed as intense sources of gamma rays, sometimes reaching energies of several teraelectronvolts. Astrophysical models predict that as these gamma rays travel across the Universe, they should interact with background starlight and produce cascades of lower-energy gamma radiation. However, space-based observatories such as the Fermi Gamma-ray Space Telescope have consistently failed to detect this secondary signal, creating a long-standing discrepancy between theory and observation.
Professor Gianluca Gregori from University of Oxford stated,
“These experiments demonstrate how laboratory astrophysics can test theories of the high-energy Universe. 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.”
Two main explanations have been proposed. One suggests that weak magnetic fields spread across intergalactic space deflect the charged particles produced in these cascades, redirecting the resulting radiation away from Earth. The other points to plasma physics, proposing that instabilities within the particle beams themselves drain energy and suppress the expected gamma-ray emission before it can be detected.
To test these ideas, the research team carried out the Fireball experiment at CERN’s HiRadMat facility. Using the Super Proton Synchrotron, they generated intense beams of electrons and positrons and passed them through a meter-long region of plasma. This setup served as a laboratory analogue of a blazar-generated particle cascade moving through the extremely sparse plasma that fills intergalactic space, but compressed into a scale that could be measured directly.
High-resolution diagnostics allowed the scientists to track how the beam evolved as it traveled through the plasma, including changes in its shape and the magnetic fields it produced. If plasma instabilities were strong, the beam would be expected to spread out, fragment, or generate significant self-induced magnetic fields. Instead, the beam remained tightly collimated and showed minimal magnetic activity, even under conditions designed to amplify instability effects.
These observations suggest that beam-plasma instabilities are far weaker than previously thought and unlikely to be responsible for the missing gamma-ray signal. When extrapolated to astrophysical distances, the results strongly favor the presence of intergalactic magnetic fields that deflect charged particles over vast scales. Such fields would scatter the electron-positron pairs produced in gamma-ray cascades, preventing their radiation from reaching Earth-based detectors.
The findings have broader implications beyond high-energy astrophysics. Intergalactic magnetic fields are believed to be relics of the early Universe, potentially formed shortly after the Big Bang. Yet current cosmological models struggle to explain how such widespread fields could have emerged in an initially uniform cosmos. The CERN results therefore sharpen an existing tension between observation and theory, suggesting that new physics or revised early-Universe models may be required.
The study also highlights the growing role of laboratory astrophysics, where particle accelerators and plasma facilities are used to recreate extreme cosmic environments under controlled conditions. By bridging experimental plasma physics with astronomical observation, researchers can directly test mechanisms that would otherwise remain purely theoretical.
Future observations from next-generation instruments such as the Cherenkov Telescope Array Observatory are expected to provide more sensitive measurements of gamma rays from distant blazars. Combined with laboratory results like those from CERN, these data may help refine estimates of intergalactic magnetic field strength and origin.
For engineers and physicists, the work underscores how accelerator technology, plasma diagnostics, and computational modeling can converge to address questions traditionally confined to astronomy. In this case, recreating a small-scale version of a cosmic process has helped clarify how matter and fields behave across the largest distances known, and why some of the Universe’s light may never reach us at all.
Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).
