This plasma fireball feat nearly answers one of the oldest riddles in space

The glow was sparked by plasma fireballs - and an audacious attempt to recreate a blazar-like jet on Earth.

Using CERN’s Super Proton Synchrotron and the HiRadMat experimental hall, researchers built a controlled stand-in for part of a blazar environment and then watched how the particles behaved. Their target was a long-running astrophysics problem: models predict that some galactic cores should send us extremely hard gamma rays, yet many of those photons never seem to show up in our detectors.

Why gamma rays go missing

Blazars are found at the hearts of active galaxies, where supermassive black holes draw in surrounding material and launch two narrow particle jets at relativistic speeds. When one of those jets is pointed almost directly towards Earth, Doppler boosting makes the source appear dramatically brighter. In that alignment, observatories can detect emission spanning radio waves all the way up to gamma rays. Even so, theory often indicates that certain blazars ought to produce more very high-energy gamma photons than we actually measure.

So what happens to the missing photons? A leading explanation involves pair cascades. In this picture, an ultra-energetic gamma ray strikes diffuse background light spread through intergalactic space, converting into an electron–positron pair. Those particles can then upscatter ambient photons back into the gamma-ray band, creating a secondary glow that should, in principle, be visible to our telescopes.

In practice, the secondary signal is frequently weaker than expected. Two broad ideas have dominated the debate:

• A widespread intergalactic magnetic field could deflect the electron–positron pairs, spreading the cascade out and diluting what we see.
• Alternatively, the pair beam might shred itself via beam–plasma instabilities, bleeding energy into the plasma before it can re-radiate efficiently.

Building a jet on a laboratory bench: the HiRadMat blazar-jet analogue

To move the argument beyond inference from distant sources, an Oxford-led team translated the problem into a controlled laboratory test. At HiRadMat, a facility designed to deliver intense beams safely onto targets, they injected a beam intended to mimic an electron–positron stream into a plasma column about 1 metre long. They then measured how the beam evolved - its shape, its propagation, and the magnetic fields it generated within the plasma.

Key elements of the set-up included:

• Facility: CERN Super Proton Synchrotron feeding the HiRadMat irradiation area
  
• Analogue: a relativistic, pair-like beam entering a plasma column roughly 1 metre in length
  
• Goal: search for beam–plasma instabilities that would scatter particles and drain energy
  
• Diagnostics: tracking the jet profile and the self-generated magnetic fields inside the plasma
  

In the laboratory, the pair-like beam remained strikingly stable - showing no strong turbulence, no obvious energy loss, and no fragmentation.

That stability is the crux. If instabilities were truly the main mechanism at work, the beam should have developed visible structure, amplified internal fields substantially, and dissipated energy quickly. Instead, the experiment produced a largely straight, well-collimated flow through the plasma section, directly weakening the “instabilities first” explanation for the missing gamma rays.

Two competing explanations - and one now looks stronger

With rapid, disruptive instabilities failing to appear under these controlled, astrophysically motivated conditions, the balance of evidence shifts towards intergalactic magnetism. Even a very faint intergalactic magnetic field can nudge electron–positron pairs over millions of light-years, redirecting the cascade away from the original line of sight. The result is a smeared-out signal: emission can arrive more spread across the sky and delayed in time, so a telescope pointed at the blazar may register less of it - or detect it as a broad halo rather than a tight point source.

The most straightforward interpretation is that weak, pervasive magnetism across intergalactic space gently steers the cascade away from our view.

This reading is consistent with constraints accumulated over the last decade from both space-based and ground-based gamma-ray instruments. It does not require speculative new particle physics, but it does raise a profound implication: large-scale magnetism may have roots that pre-date galaxies and galaxy clusters.

What a primordial intergalactic magnetic field would imply

If an intergalactic magnetic field is responsible, the next question becomes origin. One school of thought argues that tiny magnetic “seed” fields were generated in the earliest moments after the Big Bang and later amplified as cosmic structure formed, influencing how gas flows, cools and forms the first stars. Another view suggests magnetism could have been generated later through shocks and turbulence as matter assembled into filaments, gradually building faint fields over time.

Both possibilities remain viable. What the new laboratory evidence does is narrow the menu: it indicates that, at least for the key regimes reproduced in the experiment, beam-driven instabilities do not easily destroy the cascade. That nudges the theoretical burden towards a magnetic field that already threads the space between galaxies, rather than one produced or “self-erasing” within the cascade process.

A further consequence is methodological. If intergalactic magnetism is the main driver of the missing-gamma-ray effect, then gamma-ray astronomy becomes a tool for mapping otherwise invisible magnetic structure in the cosmic web - including differences between voids and denser filaments.

Intergalactic magnetism would also influence other messengers. The same deflections that broaden gamma-ray halos can affect how cosmic rays propagate, complicating attempts to trace high-energy particles back to their sources and altering expectations for secondary emission produced along their paths.

Inside the experiment’s playbook

Laboratory astrophysics works by matching the right ratios rather than the raw scales found in nature. Although intergalactic space is unimaginably larger and thinner than any laboratory plasma, the team adjusted time, density and field strengths so the relevant dimensionless parameters aligned with those expected in the astrophysical setting. That approach allows the microphysics to be tested directly, rather than waiting for rare sky events or relying solely on modelling.

Question	Laboratory handle	What the team checked
Do pair-like beams trigger strong instabilities?	Inject a relativistic beam into plasma	Search for turbulence growth and magnetic-field amplification
Does the beam shed energy quickly?	Track the jet profile and spectral signatures	Look for broadening and clear energy dissipation
Could stability persist over long distances?	Use scaling arguments to extrapolate	Compare the laboratory regime to intergalactic parameters

At the bench scale, the beam remained orderly. With the scaling applied, the implication is that similar stability should hold over astrophysical propagation, leaving magnetic deflection as the leading explanation for why gamma rays go missing.

What telescopes will test next

Upcoming observatories should be able to discriminate between a sharply beamed signal and a magnetically smeared one. The Cherenkov Telescope Array Observatory (CTAO) is expected to improve sensitivity and angular resolution at very high energies. If some blazars are surrounded by a faint, extended glow, that would bolster the case for an intergalactic magnetic field. Likewise, a measurable delayed “echo” component would also fit the deflection-and-time-stretch scenario. Combining time profiles, energy spectra and halo angular sizes can constrain both the field strength and its coherence length.

Meanwhile, space-based instruments will continue long-term monitoring of bright gamma-ray sources, including extended flares. Coordinated campaigns spanning radio, X-ray and gamma-ray bands will help separate competing models. If magnetism differs between voids and filaments, multi-wavelength timing and morphology may reveal those environmental contrasts.

Terms that help

• Blazar: an active galactic nucleus with a jet aimed close to our line of sight, making it appear brighter through relativistic boosting.
  
• Electron–positron pair: two particles with equal mass and opposite charge, created when a sufficiently energetic photon converts energy into matter.
  
• Pair cascade: a chain reaction in which pairs upscatter ambient light back into gamma rays after an initial high-energy interaction.
  
• Intergalactic magnetic field: a very weak, large-scale magnetic field permeating the space between galaxies.
  

Why a laboratory can speak for deep space

Astrophysical plasmas cover extreme ranges of temperature and density, but their behaviour is often governed by dimensionless ratios. When a laboratory system reproduces those ratios, the underlying microphysics can be meaningfully compared. That is the foundation of this work, reported in the Proceedings of the National Academy of Sciences, and it is intended to complement - not replace - what telescopes observe.

There are, inevitably, caveats. A 1-metre plasma cell cannot reproduce billions of years of propagation, and diagnostics could miss rare modes that grow slowly. The authors note these limitations while emphasising that the dominant, fast-growing instabilities expected to matter most did not manifest. Future experimental runs can broaden coverage by varying plasma densities and beam energies, and by testing additional regimes where subtler effects might emerge.

Smart ways to widen the search

• High-resolution numerical simulations can bridge the gap between laboratory conditions and full intergalactic distances, probing whether any hidden instability channels remain.
  
• Stacking studies across many blazars can uncover faint halos that are too weak to identify confidently around individual sources.
  
• Joint interpretation alongside neutrino and gravitational-wave alerts can link magnetic-field environments to high-energy transients.
  
• Laboratory benchmarks can tighten priors used in statistical fits to gamma-ray observations, reducing model degeneracies.
  

For those interested in the hardware: HiRadMat is engineered to withstand intense beam delivery while supporting diagnostics that can survive harsh conditions, and the Super Proton Synchrotron provides the beam power that makes such tests feasible. Together, they enable experiments that previously existed largely on the pages of theory papers.

The headline may sound understated - a jet analogue that refused to wobble - but the implication is sharp. By undercutting a popular instability-based escape route, the result leaves subtle, ancient intergalactic magnetism as a leading reason gamma rays go missing. With the next generation of sky surveys and further carefully designed runs in Geneva, that inference could evolve into a quantitative magnetic map of intergalactic space.