Unraveling the Mystery of Missing Gamma Rays: A Cosmic 'Fireball' Experiment
The universe has been keeping secrets, and scientists are determined to uncover them.
An international scientific endeavor, led by the University of Oxford, has embarked on a groundbreaking journey to recreate cosmic 'fireballs' and delve into the enigma of missing gamma rays. This world-first achievement, utilizing the Super Proton Synchrotron at CERN in Geneva, aims to unravel the mysteries of the universe's hidden magnetic fields and the absence of gamma rays.
Blazars, active galaxies powered by supermassive black holes, shoot beams of particles and radiation towards Earth at nearly the speed of light. These beams emit intense gamma rays, which ground-based telescopes detect. However, as these gamma rays travel through intergalactic space, they interact with the faint light from stars, creating a cascade of electron-positron pairs. According to theory, these pairs should then interact with the cosmic microwave background, producing lower-energy gamma rays. But here's where it gets controversial: these expected gamma rays have not been observed by space telescopes like the Fermi satellite.
One theory suggests that these electron-positron pairs are deflected by weak intergalactic magnetic fields, causing the lower-energy gamma rays to miss our telescopes' line of sight. Another hypothesis, rooted in plasma physics, proposes that the beams themselves become unstable as they traverse the sparse matter between galaxies. In this scenario, small fluctuations in the beam generate currents and magnetic fields, further destabilizing the beam and potentially dissipating its energy.
To test these theories, a collaborative team from the University of Oxford and the Science and Technology Facilities Council's Central Laser Facility employed CERN's HiRadMat facility. They generated electron-positron pairs and sent them through a meter-long ambient plasma, creating a laboratory-scale model of a blazar-driven pair cascade propagating through intergalactic plasma. By measuring the beam's profile and associated magnetic field signatures, the researchers directly examined whether beam-plasma instabilities could disrupt the jet.
The results were astonishing. Contrary to expectations, the pair beam remained remarkably stable, with minimal disruption and self-generated magnetic fields. When scaled up to astrophysical dimensions, these findings suggest that beam-plasma instabilities are not strong enough to account for the missing GeV gamma rays. This supports the hypothesis that the intergalactic medium contains a magnetic field, likely a relic from the early universe.
Professor Gianluca Gregori, the lead researcher, emphasized the significance of laboratory experiments in bridging the gap between theory and observation. "Our study highlights the power of collaboration between experimental facilities worldwide, especially in exploring extreme physical conditions." However, this discovery raises further questions. The early universe is believed to have been incredibly uniform, leaving scientists puzzled as to how a magnetic field could have been seeded during this primordial phase. The researchers suggest that the answer may lie in new physics beyond the Standard Model.
Professor Bob Bingham, a co-investigator, noted the importance of laboratory astrophysics in testing high-energy universe theories. "By recreating relativistic plasma conditions in the lab, we can measure processes that shape cosmic jets and better understand the origin of magnetic fields in intergalactic space."
Co-investigator Professor Subir Sarkar added, "This innovative experiment adds a new dimension to CERN's frontier research. Our striking result should spark interest in the plasma astrophysics community, showcasing the potential for probing fundamental cosmic questions in a terrestrial high-energy physics laboratory."
The findings, published in PNAS, open new avenues for exploration. Upcoming facilities like the Cherenkov Telescope Array Observatory are expected to provide higher-resolution data, allowing scientists to delve deeper into these cosmic mysteries. As we continue to unravel the universe's secrets, one thing is clear: the journey towards understanding is just as fascinating as the destination.
