Are Primordial Black Holes Antimatter Factories?

Could small, primordial black holes be efficient antimatter generators? It is well known that cool planetary bodies, surrounded by equal numbers of protons and electrons in thermal equilibrium, will eventually become positively charged. Why? Because electrons, with their low mass, have a higher velocity than the larger protons. Although they undergo the same gravitational acceleration, electrons are able to attain “escape velocity” more readily as the more massive protons get stuck in the gravitational well. The result? The planet has a net positive charge as more electrons, than proton escape into space.

Primordial black holes are thought to exist in our Universe (left-overs from the Big Bang), and although they may be small, they may influence ionized cosmic clouds in the same way, more electrons escape than protons left behind. However, should a threshold be reached, the extreme gravitational force surrounding the black hole could set up a powerful electrostatic field, kick-starting a strange quantum phenomenon that generates the electron’s anti-matter partner (the positron) from the vacuum of space…

Previously on Astroengine, I discussed the possibility that the Sun may change X-ray photons into a hypothetical component of dark matter, the weakly interacting axion. This is achieved through electromagnetic interactions, possibly allowing researchers to detect axions here on Earth (some researchers even want to generate a significantly large magnetic field to produce their own axions). The Sun could be a dark matter factory. Now it seems a cosmic exotic body (a primordial black hole) may be a factory for a more commonly accepted particle, the positron. The positron (e⁺) is the antimatter component of the electron (e^–).

Primordial black holes are theorized to have been created from the vast energy generated soon after the Big Bang. Since then, these cosmic vagabonds have floated around space, slowly evaporating due to Hawking Radiation, and if they were large enough when they were formed, it seems possible many still exist today. What’s more, these Big Bang relics may be a significant source of antimatter, interacting with clouds of interstellar ions.

According to research by Cosimo Bambi, an astrophysicist from Wayne State University, Detroit and colleagues from Italy and Russia, a small primordial black hole of less than 10²⁰g (about 0.00001% the mass of Earth) with an accretion rate close to the Eddington limit (the limit at where the gravitational force inwards equals the black hole radiation force outwards) can be the ideal condition for the black hole to generate antimatter. Accreting electrons will be repelled by black hole radiation, but attracted by gravity. The Eddington limit will trap accreting electrons at the horizon, forming an electrostatic shell around the black hole.

This is where a quirky quantum mechanical phenomenon steps in. The Schwinger mechanism basically predicts the production of electron-positron pairs within a strong electric field.

The Schwinger mechanism: Virtual pairs of charged particles from vacuum fluctuations can be separated to become real pairs when the potential energy over the Compton wavelength is greater than or equal to the rest mass energy. In the tunnelling interpretation the virtual pairs pass through the potential barrier lowered by the electric field. – From Schwinger Mechanism and Hawking Radiation as Quantum Tunnelling, Sang Pyo Kim, 2007.

Assuming the primordial black hole exists in a dense plasma environment (such as in the centre of galaxies) where proton densities exceed 10²⁴ protons/cm³ (or 1 g/cm³), it seems theoretically possible that a huge electric field may be possible spawning the generation of electron-positron pairs. Therefore (as investigated by Bambi), as a heavier proton falls into the horizon, an electron-positron pair can be generated. The positron (antimatter) is manufactured as a result of some strange quantum dynamics and ancient black holes.

The most exciting thing is that these excess positrons might be observable. If primordial black holes are hiding in the galactic core, or within the dense atmosphere of violent stars (such as Wolf-Rayets), the received cosmic rays from such events may contain more positrons than expected. These positrons would have energies of 1-100MeV and would be accompanied by gamma rays of similar energy.

I don’t fully understand the detail, but this is compelling work. If you’re feeling brave and want to learn more, check out the arXiv preprint online: Black holes as antimatter factories by Cosimo Bambi, Alexander D. Dolgov and Alexey A. Petrov.

Original source: arXiv blog