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In a world first, plasma “fireballs” have been created in the lab setting using the Super Proton Synchrotron accelerator at CERN, Geneva. The Oxford-led international science team experimented to crack the long-running puzzle of the Universe’s hidden magnetic fields and missing gamma rays. “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, lead researcher from the Department of Physics, University of Oxford. Testing two theories in the lab The research was specifically designed to investigate a key mystery surrounding blazars, which are active galaxies with supermassive black holes launching particle jets. These jets produce extremely high-energy gamma rays (TeV), which, as they travel across space, create a cascade of electron-positron pairs. These pairs are expected to scatter off the cosmic microwave background and then to produce lower-energy gamma rays in the GeV range. However, these GeV rays are consistently missing from observations by telescopes like Fermi. Scientists had proposed two main hypotheses for the missing radiation. Weak intergalactic magnetic fields are deflecting the pairs and steering the resulting GeV rays away from Earth. Or the electron-positron pair beams become unstable as they propagate, generating internal magnetic fields that dissipate the beam’s energy before the GeV rays can be produced. The researchers utilized CERN’s HiRadMat facility and the Super Proton Synchrotron to generate electron-positron pairs and accelerate them through a meter of plasma, thereby testing the theories. The experiment successfully created a scaled laboratory model of a blazar jet traveling through space. To test the disruption theory, the researchers directly examined the effects of beam-plasma instabilities by measuring the jet’s beam profile and associated magnetic field signatures. Unexpected stability points to primordial magnetism Surprisingly, the outcome defied expectations. The pair beam stayed narrow and almost parallel, showing little disruption or evidence of self-generated magnetic fields. Scaling the experimental findings up to cosmic distances suggests that beam-plasma instabilities are not strong enough to explain the absence of the GeV gamma rays. This outcome directly supports the competing hypothesis: that a relic intergalactic magnetic field is indeed present throughout the cosmos, consistently deflecting the particle pairs and causing the GeV emission to miss Earth. “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,” said Professor Bob Bingham, co-investigator from the STFC Central Laser Facility and the University of Strathclyde. Despite resolving one mystery, the findings introduce a new puzzle: if the intergalactic medium has a magnetic field, how could it have been generated in the extremely uniform early Universe? The researchers suggest that the answer to the origin of the magnetic field may involve new physics beyond the Standard Model. The facilities like the upcoming Cherenkov Telescope Array Observatory (CTAO) will deliver higher-resolution data to test these new ideas and further unravel the secrets of the magnetic cosmos.
