Jets of high-speed particles stab out from a supermassive black hole at the heart of an active galaxy. The jets carve out giant, magnetised cocoons of plasma that can boost cosmic rays to improbably high speeds (Illustration: NASA)

Vast magnetic cocoons associated with galaxies whose black holes have stopped eating may be responsible for accelerating charged particles called cosmic rays to within a whisker of the speed of light.

It could explain one of the great mysteries of astrophysics – how enormously energetic cosmic rays make it to Earth, when common sense says they should long ago have run out of steam.

Cosmic rays are high-speed atomic nuclei, most commonly of hydrogen. Most come from objects within our galaxy, such as supernova remnants and pulsars.

But ultra-high-energy cosmic rays (UHECRs) – each packing the punch of a baseball – are an outstanding mystery. Although it is conceivable that they are produced near the Milky Way by the decay of super-heavy dark matter particles or by defects in space-time, the most likely sources are the most powerful objects in the universe – ‘active’ galaxies whose colossal black holes are devouring nearby matter, and gamma-ray bursts. These are far beyond our galaxy – and herein lies a very serious problem.


The problem arises because every cubic centimetre of space contains about 400 relic photons from the big bang fireball. They have little energy, but seen from the point of view of a speeding cosmic ray, they are enormously boosted in energy, becoming high-energy gamma rays. The interaction of cosmic rays with these gamma rays continually saps the very highest energy cosmic rays of energy. Therefore, we should detect none of them on Earth – but we do.

Gregory Benford of the University of California in Irvine, US, and Raymond Protheroe of the University of Adelaide in Australia have a solution.

Magnetised cocoons

They point out that a few per cent of the volume of the universe is filled by giant magnetised cocoons of plasma, or charged particles. These were carved out by “jets” stabbing out from the supermassive black holes at the heart of active galaxies.

“These colossal magnetic structures persist even when the galaxies have ceased their activity,” says Protheroe. “And they store truly vast amounts of energy in their fields.”

Benford and Protheroe point out that over billions of years, the slow decay of the magnetic fields inside such fossil cocoons induces an electric field. “These electric fields are strong enough to accelerate cosmic rays to ultra-high energies,” says Protheroe.

UHECRs on their journey to Earth pass through many such cocoons. “And the electric fields can repeatedly boost the energy, counteracting the effect of big bang photons,” says Benford. “Contrary to expectations, ultra-high-energy cosmic rays can make it to Earth.”

Others think the mechanism is intriguing. “Given what we know now, I think [decaying magnetic fields] are the least implausible explanation of ultra-high energy cosmic rays,” says Roger Blandford of Stanford University in California, US.

Journal reference: Monthly Notices of the Royal Astronomical Society (upcoming)