Primordial black holes may have helped to forge heavy elements

August 4, 2017, University of California - San Diego
Artist’s depiction of a neutron star. Credit: NASA

Astronomers like to say we are the byproducts of stars, stellar furnaces that long ago fused hydrogen and helium into the elements needed for life through the process of stellar nucleosynthesis.

As the late Carl Sagan once put it: "The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing . We are made of star stuff."

But what about the heavier elements in the periodic chart, elements such as gold, platinum and uranium?

Astronomers believe most of these "r-process elements"—elements much heavier than iron—were created, either in the aftermath of the collapse of massive stars and the associated supernova explosions, or in the merging of binary neutron star systems.

"A different kind of furnace was needed to forge gold, platinum, uranium and most other elements heavier than iron," explained George Fuller, a theoretical astrophysicist and professor of physics who directs UC San Diego's Center for Astrophysics and Space Sciences. "These elements most likely formed in an environment rich with neutrons."

In a paper published August 7 in the journal Physical Review Letters, he and two other theoretical astrophysicists at UCLA—Alex Kusenko and Volodymyr Takhistov—offer another means by which stars could have produced these heavy elements: tiny black holes that came into contact with and are captured by neutron stars, and then destroy them.

Neutron stars are the smallest and densest stars known to exist, so dense that a spoonful of their surface has an equivalent mass of three billion tons.

Tiny black holes are more speculative, but many astronomers believe they could be a byproduct of the Big Bang and that they could now make up some fraction of the "dark matter"—the unseen, nearly non-interacting stuff that observations reveal exists in the universe.

If these tiny black holes follow the distribution of dark matter in space and co-exist with neutron stars, Fuller and his colleagues contend in their paper that some interesting physics would occur.

They calculate that, in rare instances, a neutron star will capture such a black hole and then devoured from the inside out by it. This violent process can lead to the ejection of some of the dense neutron star matter into space.

"Small black holes produced in the Big Bang can invade a neutron star and eat it from the inside," Fuller explained. "In the last milliseconds of the neutron star's demise, the amount of ejected neutron-rich material is sufficient to explain the observed abundances of heavy elements."

"As the neutron stars are devoured," he added, "they spin up and eject cold neutron matter, which decompresses, heats up and make these elements."

This process of creating the periodic table's heaviest elements would also provide explanations for a number of other unresolved puzzles in the universe and within our own Milky Way galaxy.

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

In addition, the scientists calculated that ejection of nuclear matter from the tiny black holes devouring stars would produce three other unexplained phenomenon observed by astronomers.

"They are a distinctive display of infrared light (sometimes termed 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 at the hands of tiny black holes."

Explore further: New simulations could help in hunt for massive mergers of neutron stars, black holes

More information: Primordial black holes and r-process nucleosynthesis, Physical Review Letters (2017). journals.aps.org/prl/accepted/ … 5a1a918b69bd6d2e6077

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11 comments

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richk
not rated yet Aug 04, 2017
how/massive/are/the/black/holes/they/modelled?
cantdrive85
2 / 5 (12) Aug 05, 2017
Neutron stars are the smallest and densest stars known to exist, so dense that a spoonful of their surface has an equivalent mass of three billion tons.
Tiny black holes are more speculative,

LOL, it's all very speculative, imaginary conjecture even...
"We have to learn again that science without contact with experiments is an enterprise which is likely to go completely astray into imaginary conjecture." Hannes Alfvén
Kron
5 / 5 (4) Aug 05, 2017
It's all speculative, ofcourse, but a matter of which speculation fits observation best. Every branch of science is validated through experimental evaluation. There are certain theoretical models which at present have no way of being tested experimentally but tests are either on their way or, in the case that no possible experiments can be devised, they are rejected as wildly speculative.

On topic of article, it is most plausible that elements (especially the heavier variety) transmute from neutron matter. It is widely known that a neutron in free space decays into a hydrogen atom. I conjecture that inside of stars it is not the proton proton chain reaction that leads to helium production but rather quad neutron convergence that results in helium. I'd venture so far as to say that just as in free space neutrons decay into a proton and electron, the inverse occurs under the immense pressures in the cores of stars. Hydrogen converts to neutrons.
PoppaJ
not rated yet Aug 05, 2017
elements much heavier than iron—were created, either in the aftermath of the collapse of massive stars and the associated supernova explosions, or in the merging of binary neutron star systems.

Why is it either/or. Why not both? Tiny black holes are far less likely than in supernova. At least we have observed one of them.
Dingbone
Aug 05, 2017
This comment has been removed by a moderator.
cryptc
3 / 5 (2) Aug 05, 2017
How my brain sees this:

1. A neutron star (cookie) captures a black hole (Cookie Monster's attention).
2. The black hole (Cookie Monster) violently devours the neutron star (cookie).
3. Parts of the neutron star (cookie) fly in all directions.
Graeme
5 / 5 (1) Aug 05, 2017
Tiny black holes are much more likely to impact a normal star, as they have a much bigger surface area. So what effect would that have? Anyway I like the theory of neutron star collision making these R-process elements better.
Da Schneib
not rated yet Aug 06, 2017
@Graeme, what's surface area got to do with it? At the scales we are talking about stars are points.
Dingbone
Aug 06, 2017
This comment has been removed by a moderator.
nikola_milovic_378
Aug 06, 2017
This comment has been removed by a moderator.
emaalouf
not rated yet Aug 07, 2017
This seems far fetched particularly because when two neutron stars merge they warp space severely in effect stopping time at the event. A tiny black hole will take an infinite amount of time to reach the star which is past the time they merge and disperse into space?!

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