An international team of scientists, led by the University of Oxford, has achieved a world-first by recreating relativistic plasma "fireballs" in a particle accelerator at CERN (European Organization for Nuclear Research). The goal of this innovative experiment was to investigate a long-standing astrophysical mystery concerning the non-detection of expected lower-energy gamma-rays originating from distant, powerful galaxies called blazars.
The Blazar Gamma-Ray Mystery
Blazars are a type of active galactic nucleus powered by supermassive black holes that launch extremely fast, narrow jets of particles and radiation toward Earth. These jets produce very high-energy gamma-rays (TeV, or teraelectronvolts), which are successfully detected by ground-based telescopes.
The puzzle arises from the subsequent journey of these gamma-rays:
As TeV gamma-rays travel through intergalactic space, they should interact and scatter off the Extragalactic Background Light (EBL)—the collective dim light from all stars throughout the Universe.
This scattering process should create electron–positron pairs.
These electron-positron pairs should then scatter on the Cosmic Microwave Background (CMB) to generate a cascade of secondary, lower-energy gamma-rays (GeV, or gigaelectronvolts).
Space telescopes, like the Fermi satellite, are designed to detect these GeV gamma-rays, but they have largely eluded detection.
Two main hypotheses have been proposed to explain these missing GeV gamma-rays:
Intergalactic Magnetic Fields: Weak magnetic fields permeating the vast intergalactic medium could deflect the electron-positron pairs, steering the resulting GeV gamma-rays away from our line of sight.
Beam-Plasma Instabilities: As the particle beams travel through the sparse intergalactic plasma, they might become unstable. This instability could generate magnetic fields, dissipate the beam's energy, and essentially destroy the cascade before the GeV rays can be produced.
🔬 The CERN Experiment: HiRadMat
To test the second hypothesis of beam instability, the research team used CERN's Super Proton Synchrotron (SPS) accelerator, specifically the HiRadMat (High-Radiation to Materials) facility, to replicate the extreme conditions of a blazar jet.
Recreation: Scientists generated a high-energy beam of electron–positron pairs (a matter-antimatter pair) using the SPS.
Simulation: This beam was then blasted through a target containing plasma (a hot, ionized gas) over a distance of about one meter. This setup effectively created a laboratory-scale model of a blazar-driven particle beam traveling through the intergalactic plasma.
Observation: The team monitored the behavior of the beam within the plasma, specifically looking for signs of instability or self-generated magnetic fields that would cause the beam to widen and dissipate.
💡 Striking Results and Implications
The published results were contrary to the beam-instability hypothesis:
The electron-positron beam remained remarkably narrow and stable, with minimal disruption.
There was a noticeable absence of significant self-generated magnetic fields.
The conclusion drawn from this laboratory experiment is that beam-plasma instabilities are too weak to explain the mystery of the missing GeV gamma-rays. This strongly supports the alternative explanation:
The non-detection of the secondary gamma-rays is likely due to the deflection of the electron-positron pairs by a weak intergalactic magnetic field.
This implication is highly significant, suggesting the presence of a relic magnetic field leftover from the very early Universe. Future observatories, such as the Cherenkov Telescope Array (CTA), are expected to have the sensitivity to confirm this finding by potentially detecting subtle "pair halos" (circular rings of emission) around blazars, which would be a tell-tale sign of the magnetic field's influence. The experiment demonstrates the power of using terrestrial high-energy physics facilities to directly probe fundamental astrophysical questions.