An international consortium of scientists has successfully simulated the titanic jets of plasma ejected by supermassive black holes, creating miniature “fireballs” in a particle accelerator at CERN. This landmark experiment was designed to address a persistent cosmic puzzle: why a predicted cascade of lower-energy gamma rays from deep space has never been detected. The results cast significant doubt on one of two leading theories and provide the strongest evidence yet that the universe is threaded with ancient, primordial magnetic fields, remnants from the cosmos’s earliest moments.
For decades, astronomers have been mystified by the apparent absence of these gamma rays. Blazars, which are the ferociously bright cores of distant galaxies powered by black holes, blast streams of particles toward Earth at nearly the speed of light, producing extremely high-energy gamma rays. Theory predicts that as these powerful rays travel across billions of light-years, they should interact with faint background light, creating a shower of electron-positron pairs. These pairs ought to then generate a secondary, lower-energy gamma-ray signal, but space telescopes have failed to observe it. By recreating the essential physics of these jets in a controlled laboratory setting, researchers have now been able to test the mechanics of this process directly for the first time.
A Long-Standing Astronomical Puzzle
The universe is filled with extreme and powerful objects, but few can match the sheer luminosity of blazars. These are a specific type of active galactic nucleus where a supermassive black hole at the galaxy’s center actively feeds on surrounding matter. This process launches narrow, focused jets of magnetized plasma that happen to be aimed directly at Earth. These jets are cosmic particle accelerators, producing radiation across the entire electromagnetic spectrum, including the most energetic photons ever observed: teraelectronvolt (TeV) gamma rays, which carry a trillion times the energy of visible light.
Astrophysical models are clear on what should happen next. As these TeV gamma rays traverse the vast, near-empty voids of intergalactic space, they inevitably collide with the extragalactic background light—a faint, diffuse glow from all the stars that have ever existed. This interaction is governed by fundamental physics and should result in the creation of matter and antimatter in the form of electron-positron pairs. This cascade effect doesn’t stop there. These newly created pairs are still moving at relativistic speeds and should interact with the Cosmic Microwave Background (CMB), the ubiquitous thermal afterglow of the Big Bang. This second interaction is predicted to produce a large flux of new gamma rays at lower giga-electronvolt (GeV) energies. However, dedicated observatories, most notably NASA’s Fermi Gamma-ray Space Telescope, have consistently found that this expected GeV signal is missing.
Two Competing Scenarios
To explain the missing gamma rays, scientists have focused on two primary hypotheses that describe what could be happening to the electron-positron beams during their journey across the cosmos. The answer would have profound implications for our understanding of the universe’s structure and history.
The Magnetic Field Hypothesis
The first theory posits the existence of a weak but pervasive intergalactic magnetic field (IGMF). According to this model, as the charged electron-positron pairs are created, their paths would be bent and deflected by this magnetic field. Instead of continuing along the original line of sight from the blazar, the pairs would be scattered. Consequently, the lower-energy gamma rays they eventually produce would be dispersed over a wide area of the sky rather than forming a detectable beam aimed at Earth. This would make the signal so faint and diffuse that it would effectively vanish into the background noise, explaining why the Fermi telescope cannot see it.
The Plasma Instability Hypothesis
The second theory comes from the world of plasma physics. It suggests that the electron-positron beam itself becomes unstable as it travels through the tenuous plasma of the intergalactic medium. In this scenario, tiny fluctuations within the beam could generate powerful currents, creating localized magnetic fields. These fields would, in turn, reinforce the instability, causing the beam to rapidly lose its energy by heating the surrounding medium. If this happens, the beam would dissipate its energy long before it could interact with the CMB to produce the expected lower-energy gamma rays. The energy would be lost, and the secondary signal would never be created in the first place.
Simulating the Cosmos at CERN
To distinguish between these two possibilities, a team of researchers led by the University of Oxford, in collaboration with the UK’s Central Laser Facility and other international partners, devised a novel experiment. Their goal was to scale down a cosmic process occurring over millions of light-years into a controlled laboratory environment. They used the Super Proton Synchrotron (SPS), a 7-kilometer-circumference particle accelerator at CERN in Geneva, to create a scaled-down version of a blazar jet.
The Fireball Experiment
The experiment, conducted at CERN’s High-Radiation to Materials (HiRadMat) facility, involved firing a high-energy proton beam from the SPS into a graphite target. This collision generated a dense, focused beam of electron-positron pairs—a miniature, artificial cosmic fireball. This pair beam was then directed through a one-meter-long chamber filled with plasma, simulating the conditions of intergalactic space. By tracking the behavior of the beam as it propagated through the plasma, the scientists could directly measure whether it remained stable or dissipated its energy as predicted by the plasma instability hypothesis.
A Stable Beam Challenges Theory
The results of the experiment were striking. Contrary to the predictions of the beam instability model, the electron-positron fireball remained remarkably stable and parallel as it traversed the plasma chamber. It showed only minimal signs of magnetic disruption or energy loss. This laboratory evidence demonstrates that such plasma instabilities are not efficient enough to be the primary cause of the missing GeV gamma rays. The findings strongly favor the alternative explanation: that a faint but significant magnetic field permeates the voids between galaxies.
Vindication for Primordial Magnetism
By effectively ruling out the plasma instability theory, the CERN experiment provides compelling indirect evidence for the existence of the Intergalactic Magnetic Field. This conclusion, published in the journal PNAS, has profound consequences for cosmology. The uniformity of the early universe makes it difficult to explain how such large-scale magnetic fields could have been seeded. Their existence suggests they are likely primordial, formed in the very first moments after the Big Bang, and may even point toward new physics beyond the Standard Model of particle physics. These ancient fields would have played a crucial role in the subsequent formation of large-scale structures like galaxies and galaxy clusters.
Professor Gianluca Gregori of the University of Oxford stated that this research showcases how laboratory experiments can bridge the gap between theoretical ideas and astrophysical observations from telescopes. It validates the use of terrestrial accelerators to probe the physics of the cosmos on scales that are otherwise impossible to access. The results pave the way for a new era of experimental astrophysics, where the universe’s most extreme environments can be studied up close.
The Next Generation of Observatories
While the fireball experiment provides a powerful piece of the puzzle, the final confirmation will come from the sky. The next generation of gamma-ray observatories is poised to provide the high-resolution data needed to test these ideas further. The Cherenkov Telescope Array Observatory (CTAO), currently under construction, will be the world’s largest and most sensitive gamma-ray detector. Its unprecedented capabilities will allow astronomers to hunt for the faint, scattered signatures of the deflected gamma rays predicted by the magnetic field hypothesis. If CTAO can detect this faint halo of radiation around distant blazars, it would provide direct proof of the intergalactic magnetic fields and begin the process of mapping their strength and structure across the cosmos, opening a new window into the universe’s magnetic past.
