In 1971, Stephen Hawking suggested that a mini-black hole from the early universe could be lurking at the center of the sun. His proposal was extended in 1975 by Don Clayton and co-workers who suggested that the force generated by the impact of matter on such a black hole could explain the observed deficiency of neutrinos from the Sun’s electron flavor.
This shortage was known at the time as the solar neutrino problem. formulated by calculations by my early mentor, John Bahcall. Having a second energy source besides nuclear fusion would have naturally reduced the production of solar neutrinos from nuclear reactions and be responsible for the neutrino shortage. Meanwhile, better quantitative data from the Sudbury Neutrino Observatory in Canada, which awarded the 2015 Nobel Prize in Physics to Art McDonald, imply another solution to the solar neutrino problem, in terms of the transformation of neutrino flavors in the sun.
Nevertheless, could the Sun still harbor in its belly a primordial black hole that doesn’t contribute much to its brightness? After all, we know that 85% of the matter in the universe is invisible. Primordial black holes with masses similar to asteroids in the 1 to 100 kilometer range could be responsible for dark matter. If this is the nature of dark matter, is it possible that some stars have a primordial black hole trapped in their bellies? If so, what would their fate be?
It is easier to answer the second question. A black hole captured by a star can change its evolution and internal structure. The insides of stars can be diagnosed by their oscillations, much like using seismic signals to investigate the internal structure of the Earth. The unusual evolution and internal structure of stellar hosts of mini-black holes could be explored in the future.
Given the high speed of dark matter in the Milky Way, the probability that the sun has captured a primordial black hole is one in ten million. Nevertheless, given the hundreds of billions of stars in the Milky Way Galaxy, there could still be tens of thousands of Milky Way stars that have captured a mini-black hole. Due to the smaller characteristic velocity of dark matter in dwarf galaxies, most stars embedded in ultra-faint dwarf galaxies, such as Tucana III and Triangulum II, could have captured a mini-black hole.
After consuming their nuclear fuel, the core of Sun-like stars contracts to form a core white dwarfa metal sphere about the size of the Earth. Because the Earth’s radius is a hundred times smaller than the Sun’s radius, the average mass density of white dwarfs is about a million times greater than that of the Sun. The accretion rate of matter on an embedded mini-black could therefore potentially increase a million times ignite the white dwarf and caused a supernova explosion. Rare explosive transients of a new kind could be searched for in the data pipeline of the Rubin Observatory, which will become operational next year.
The effect of a mini-black hole would be even more dramatic if it were trapped in the core of a massive star with more than eight times the mass of the Sun. Such a core collapses to form a neutron star after the nuclear fuel is consumed. The neutron star’s density is similar to that of an atomic nucleus, a hundred trillion times higher than the average density of the Sun. In that case the rapid growth of matter could run the neutron star in a black hole, as I pointed out in 2014 paper with my former postdoc, Paolo Pani, who is currently a professor in Italy.
Under these conditions, the original black house can be thought of as a seed that grows to consume its host star and transform it into a stellar mass black hole. This channel could lead to black holes with the mass of a neutron star, an outcome not expected under normal astrophysical evolution.
Currently, the LIGO-Virgo-KAGRA observatory identifies compact objects as neutron stars or black holes based on their masses, identified by their gravitational wave signal. The existence of a neutron star channel to a black hole would cause confusion in this identification scheme and result in events where compact objects of neutron star mass are detected in gravitational waves but do not release electromagnetic radiation due to the absence of matter.
As I recently said in one paperjust accepted for publication in The Astrophysical Journal Letters, the growth of asteroid-mass mini-black holes is suppressed by quantum mechanics. This is because the size of their event horizon is smaller than the size of atoms.
If primordial black holes form dark matter, the nearest black hole would be in the solar system. Having a black hole close to home opens up the possibility of studying quantum gravity experimentally. A black hole with an event horizon the size of a proton would spontaneously radiate according to Hawking’s 1974 papera power of 1 gigawatt, usually in gamma photons with an energy of one hundred times the rest mass of the electron.
If we ever witness a black hole the mass of an asteroid in the solar system, it could be used as a testing ground for quantum gravity experiments at the subatomic scale. Understanding this will allow us to develop a predictive theory that unites quantum mechanics and gravity. Having such a theory would in turn inform us about what might have led to the Big Bang. And knowing that brings us closer to appreciating our cosmic roots.
