What speeds cosmic rays close to the velocity of light? Huge magnetic cocoons linked to galaxies whose black holes are inactive could solve the puzzle: the way the energy rich cosmic rays reach Earth, when normally they should have long finished their resources.

Cosmic rays represent high-speed atomic nuclei, mainly hydrogen, coming mostly from objects within our galaxy, like supernova remains and pulsars. Still, the ultra-high energy cosmic rays (UHECRs) form a big puzzle.

They are generated close to the Milky Way by the decomposition of super-heavy dark matter particles or by impairments in space-time, they could be rather generated by the most powerful space bodies, 'active' galaxies whose huge black holes are swallowing nearby matter, and gamma-ray bursts, all located far beyond Milky Way and this generates the puzzle.

Each cubic centimeter of space harbors about 400 relic photons from the big bang outburst, but they carry little energy, but compared to speeding cosmic ray, they are energy rich, turning into high-energy gamma rays. Cosmic rays interacting with these gamma rays are continually discharged, thus they should be undetectable on Earth. Still, they are.

The new study says that a few percentages of the universe's volume are represented by vast magnetized plasma cocoons (charged particles). These were shaped by "jets" coming from the supermassive black holes from the core of active galaxies.
"These colossal magnetic structures persist even when the galaxies have ceased their activity. And they store truly vast amounts of energy in their fields." said co-author Raymond Protheroe of the University of Adelaide in Australia.

Over billions of years, the gradual decay of the magnetic fields inside such fossil cocoons produces an electric field.

"These electric fields are strong enough to accelerate cosmic rays to ultra-high energies," said Protheroe.

UHECRs traveling towards Earth could pass through a lot of magnetized cocoons.

"And the electric fields can repeatedly boost the energy, counteracting the effect of big bang photons. Contrary to expectations, ultra-high-energy cosmic rays can make it to Earth." said co-author Gregory Benford of the University of California in Irvine, US.

Not everybody agrees.

"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.