Scientists Unveil Cosmic Fireballs to Uncover Gamma Ray Secrets
An international team of scientists, led by the University of Oxford, has made a groundbreaking discovery by simulating cosmic fireballs in a laboratory setting to investigate the stability of plasma jets from blazars. Their findings, published in PNAS, offer new insights into the long-standing mystery of the universe's hidden magnetic fields and the elusive gamma rays.
Blazars, active galaxies fueled by supermassive black holes, emit narrow, high-speed beams of particles and radiation towards Earth. These beams produce intense gamma-ray emissions, detectable by ground-based telescopes, reaching up to several teraelectronvolts (TeV). As these TeV gamma rays travel through intergalactic space, they interact with background starlight, creating electron-positron pairs. These pairs should then scatter against the cosmic microwave background, generating lower-energy GeV gamma rays. However, these GeV gamma rays have eluded detection by space telescopes like the Fermi satellite, leaving scientists perplexed.
Two theories attempt to explain this phenomenon. Firstly, weak intergalactic magnetic fields could deflect the electron-positron pairs, diverting the lower-energy gamma rays from our view. Alternatively, plasma physics suggests that the pair beams may become unstable as they traverse the sparse matter between galaxies. In this scenario, small fluctuations in the beam trigger currents that generate magnetic fields, exacerbating the instability and potentially dissipating the beam's energy.
To test these theories, the research team, comprising experts from the University of Oxford and the Science and Technology Facilities Council's Central Laser Facility, utilized CERN's HiRadMat facility to generate electron-positron pairs with the Super Proton Synchrotron and guide them through a meter-long ambient plasma. This experiment created a miniature laboratory model of a blazar-driven pair cascade propagating through intergalactic plasma.
The results were astonishing. Contrary to expectations, the pair beam retained its narrow, nearly parallel shape, with minimal disruption or self-generated magnetic fields. When scaled to astrophysical dimensions, this finding implies that beam-plasma instabilities are insufficient to account for the missing GeV gamma rays. Instead, it supports the hypothesis that the intergalactic medium harbors a magnetic field, possibly a remnant from the early universe.
Lead researcher Professor Gianluca Gregori emphasized the significance of laboratory experiments in bridging the gap between theory and observation, enhancing our understanding of astrophysical objects as observed by satellites and ground-based telescopes. He also highlighted the importance of international collaboration in pushing the boundaries of experimental physics.
However, these findings raise new questions. The early universe is believed to have been highly uniform, and it remains unclear how a magnetic field could have emerged during this primordial phase. According to the researchers, the answer may lie in new physics beyond the Standard Model. They express hope that upcoming facilities like the Cherenkov Telescope Array Observatory will provide higher-resolution data to further test these theories.
Co-investigator Professor Bob Bingham underscored the value of laboratory astrophysics in testing theories about the high-energy universe. By replicating relativistic plasma conditions in the lab, scientists can measure processes that influence the evolution of cosmic jets and gain a deeper understanding of magnetic field origins in intergalactic space.
Co-investigator Professor Subir Sarkar expressed enthusiasm for this innovative experiment, which adds a novel dimension to frontier research at CERN. He hopes that their striking result will spark interest in the plasma astrophysics community, encouraging further exploration of fundamental cosmic questions in terrestrial high-energy physics laboratories.
