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Small Primordial Black Holes Implicated in Formation of Heavy Elements

Several new observations are indicating that primordial black holes, which are often viewed as speculative or hypothetical objects, may be responsible for more than a couple of unexplained and mysterious cosmic phenomena. Observations of deep space quasars – extremely luminous objects with highly active supermassive black holes at their centers – show that within less than a 100,000 years these black holes had already reached masses of a billion suns. This is much too quick for them to have formed from the conventional erroneous view of “gobbling up” material that falls into their inescapable horizon. Additionally, the latest detection of gravitational waves demonstrates black holes with median mass ranges that were not expected to result from stellar collapse. As evidenced in the observation of a "massive fail", which shows that the standard understanding of supernovae and black hole formation may not yet be completely understood.

These and other observations begin to piece together a pattern that black holes, specifically primordial black holes, may be responsible for the formation of various cosmological structures and high-energy phenomena, from quasars to galaxies and potentially the source of so-called dark matter. Although it is well understood that following the Big Bang the density of the universe was sufficient to produce a wide range of primordial black holes -- just think of all that energy confined to an extremely small volume, this possibility has been largely ignored.

Within one Planck time—approximately 10-43 seconds---after the Big Bang innumerable Planck mass black holes—at 10-5 grams---would have formed; at one second primordial black holes would have been forming at 10,000 solar masses. Such that black holes may be implicated in everything from baryogenesis (matter formation) to galactic and stellar formation. Now, a new study is addressing another outstanding mystery with primordial black holes – the conundrum of explaining were heavy elements come from

The conventional model of stellar thermonuclear synthesis, which is not at all fully delineated at this point and may be further revolutionized by the understanding of black hole dynamics, can explain the formation of elements up to iron, composed of 56 nucleons. Elements heavier than this, like gold, platinum, and uranium – cannot be explained by stellar collapse and thermonuclear fusion. Recent models have incorporated neutron stars, which as their name implies are incomprehensibly dense agglomerations of nuclei compacted into a tiny stellar core (a spoonful of a neutron star has an equivalent mass of three billion tons). Because of the dense packing of the nucleons, if there was some way for them to be ejected by the neutron stars they would readily form elements heavier than iron. The only problem is explaining how such material would be ejected.

One possibility is neutron star–to neutron star collisions, a rare event that would explain why some locations, like dwarf galaxies, have a noticeable paucity of “r-process” elements. Now, in a paper published August 7 in the journal Physical Review Letters, George Fuller, a theoretical astrophysicist and professor of physics who directs UC San Diego's Center for Astrophysics and Space Sciences has proposed a novel mechanism involving the merger of a neutron star with a small primordial black hole to describe how material can be ejected from the dense stellar core.

In rare instances, when a neutron star and small primordial black hole merge, the small black hole with begin to consume the neutron star, destabilizing it in such a way that it will begin ejecting cold neutron matter, which can then decompress, warm up, and undergo nucleosynthesis into the heaviest of elements.

"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 neutron stars at the hands of tiny black holes."

Read more at: https://physics.aps.org/articles/v10/89

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