Forget Iran and North Korea. Now there's another uranium source

Yeah, try slapping some sanctions on black holes

Astronomers have proposed that heavy elements in the universe may have been forged when small, primordial black holes swallowed neutron stars.

Boffins widely believe that elements lighter than iron are formed during nuclear fusion reactions in the cores of stars, or during supernova bursts. But to create elements heavier than iron, more violent reactions are needed – such as massive supernova explosions or the merger of binary neutron stars. Now scientists from the University of California, San Diego and the University of California, Los Angeles have suggested that there may have been another way.

In a paper to be published in Physical Review Letters next week (here’s the arXiv pre-print), the scientists explain that small black holes can spit out matter heavy with neutrons – another type of particle needed to make elements.

“A different kind of furnace was needed to forge gold, platinum, uranium and most other elements heavier than iron,” said George Fuller, co‑author of the paper and a professor of physics at UC San Diego’s Center for Astrophysics and Space Sciences. “These elements most likely formed in an environment rich with neutrons.”

Fuller and his team have speculated that heavy neutron stars can draw in small black holes. Neutron stars are the densest stars in space – a large amount of matter is compressed so tightly that it is estimated that a teaspoon of their surface has an equivalent mass of three billion tons (or 900 times the mass of the Great Pyramid of Giza).

The gravitational interaction between the pair causes the black hole to devour the neutron star inside out. As this happens, “the idea is that nuclear matter is flung off from the neutron star as [it] spins up as it shrinks. The 'shrinking' is caused by the black hole at its center growing by accretion. The centrifugally ejected matter will decompress, heat up in the process, and retain enough of its neutron excess to make the r‑process [rapid neutron capture process],” Fuller explained to The Register.

'Systematic destruction of neutron stars'

“In the last milliseconds of the neutron star’s demise,” he said, “the amount of ejected neutron-rich material is sufficient to explain the observed abundances of heavy elements.” The theory relies on the existence of tiny black holes, but many astronomers believe they could be left over from the Big Bang, and now make up a small fraction of the dark matter in the universe.

“Since these events happen rarely, one can understand why only one in ten dwarf galaxies is enriched with heavy elements,” said Fuller. “The systematic destruction of neutron stars by primordial black holes is consistent with the paucity of neutron stars in the galactic center and in dwarf galaxies, where the density of black holes should be very high.”

The researchers’ calculation also revealed that the process might even explain some other mysterious phenomena observed by astronomers. “They are a distinctive display of infrared light (sometimes called a kilonova), a radio emission that may explain the mysterious Fast Radio Bursts from unknown sources deep in the cosmos, and the positrons detected in the galactic center by X‑ray observations,” said Fuller.

“Each of these represent long-standing mysteries. It is indeed surprising that the solutions of these seemingly unrelated phenomena may be connected with the violent end of neutron stars at the hands of tiny black holes.” ®

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