Within a spacious auditorium at CERN, a proton beam impacted a graphite target, sparking a plasma that appeared, in miniature, akin to the core of a blazar. These brief fireballs, produced by physicists from the University of Oxford along with global collaborators, may ultimately clarify why high-energy gamma rays from far-off galaxies seem to disappear before reaching Earth.
The team utilized the Super Proton Synchrotron at CERN to produce dense streams of electrons and positrons, simulating the charged particle jets released by supermassive black holes. By directing these pair beams through a plasma target, they mimicked the conditions found in intergalactic space in the laboratory, reducing cosmic phenomena by many orders of magnitude.
Evaluating Theories Of The Absent Gamma Rays
Blazars rank among the Universe’s most brilliant entities, emitting gamma rays billions of times more potent than visible light. As these gamma rays traverse cosmic voids, they ought to generate cascades of lower-energy radiation. However, despite years of telescope studies, the anticipated secondary gamma rays have never been observed. One enduring theory attributes this to concealed magnetic fields extending across intergalactic space. Another posits that the beams themselves lose energy through plasma instabilities.
To investigate these hypotheses, Oxford physicist Gianluca Gregori and his team directed their artificial fireball through a meter-long plasma chamber, assessing how the beam changed and whether self-induced magnetic fields interfered with it. The experiment unveiled that, against some forecasts, the plasma beam remained astonishingly stable and nearly parallel, exhibiting only slight magnetic activity.
“Our study illustrates how laboratory experiments can help bridge the divide between theory and observation, enhancing our comprehension of astrophysical objects observed by satellite and ground-based telescopes,” remarked Professor Gianluca Gregori. “It also emphasizes the significance of collaboration among experimental facilities worldwide, particularly in pioneering efforts to access increasingly extreme physical conditions.”
The findings indicate that beam-plasma instabilities are too feeble to explain the missing gamma rays, instead suggesting the existence of subtle, ancient magnetic fields threading the immense expanses between galaxies. These fields could be remnants from the early Universe, formed before the first stars ignited.
Reproducing The Universe In The Laboratory
Employing a method known as the Fireball experiment, the researchers generated electron-positron pairs by striking a dense target with ultra-relativistic protons and then monitored their behavior as they traversed plasma. The configuration, referred to as HiRadMat (High Radiation to Materials), enabled the team to visualize phenomena that take place on scales billions of times larger in the cosmos.
Even within such a controlled environment, the outcomes resonated with the profound enigmas of astrophysics. If the intergalactic medium contains weak magnetic fields, they must have been initiated in the earliest moments post-Big Bang, well before galaxies emerged. This could suggest that new physics beyond the Standard Model influenced the primordial plasma of the Universe.
“These experiments underscore how laboratory astrophysics can evaluate theories of the high-energy Universe,” stated Professor Bob Bingham from the STFC Central Laser Facility. “By recreating relativistic plasma conditions in the laboratory, we can quantify processes that shape the evolution of cosmic jets and deepen our understanding of the origin of magnetic fields in intergalactic space.”
This research marks a significant achievement for laboratory astrophysics, demonstrating that particle accelerators can recreate vital phenomena from the early Universe. The team anticipates that future observations with the Cherenkov Telescope Array will validate their findings under cosmic circumstances, potentially uncovering the elusive magnetic web that pervades the Universe.
Proceedings of the National Academy of Sciences: 10.1073/pnas.2513365122
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