According to a study, dark matter, the mysterious material that exerts gravitational attraction but emits no light, might really be massive concentrations of ancient black holes produced at the very beginning of the universe.
This conclusion is based on an examination of the gravitational waves, or ripples in space-time, produced by two distant collisions of black holes and neutron stars.
The ripples — labeled GW190425 and GW190814 — were detected in 2019 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Washington and Louisiana, and the Virgo Interferometer near Pisa, Italy. A previous analysis suggested the ripples were produced by collisions between black holes between 1.7 and 2.6 times the mass of our sun and either a smaller neutron star or a much larger black hole.
However, one of the objects in each collision would be a solar-mass black hole, with roughly the mass of the sun.
“Solar-mass black holes are quite mysterious, as they are not expected from conventional astrophysics,” such as the star explosions, or supernovas, that crush larger stars into black holes, the study lead author, Volodymyr Takhistov of the University of California, Los Angeles, told Live Science in an email.
Instead, the authors of the study, which was published in the journal Physical Review Letters, argue that these solar mass black holes were generated during the Big Bang. Or they might have formed later when neutron stars were transmuted into black holes — either by swallowing primordial black holes or by absorbing certain proposed forms of dark matter, the mysterious matter that exerts gravitational attraction but does not interact with light, according to Takhistov.
Primordial black holes
If primordial black holes exist, they were most likely formed in large numbers within the first second of the Big Bang 13.77 billion years ago. They would have arrived in various shapes and sizes, with the smallest would have been microscopic and the greatest being tens of thousands of times the mass of our sun.
According to calculations, the smallest would have “evaporated” by now by emitting quantum particles via a process known as Hawking radiation, leaving only primordial black holes with masses of more than 1011 kilograms — around the mass of a small asteroid — to exist today.
Some astrophysicists believe that if these ancient black holes exist, they might make up the vast halos of “dark matter” that surround galaxies.
The researchers sought to see if they could tell the difference between primordial black holes and black holes generated from neutron stars, the glimmering remnants of supernovas left behind after their parent stars exploded after burning up all of their hydrogen in nuclear fusion events.
According to Live Science, astrophysicists determined that stars less than around five times the mass of the sun collapse to leaving behind a neutron star of ultra-dense matter with roughly the mass of our sun packed into a ball the size of a city.
According to this theory, the extreme gravity of some neutron stars would have continuously drawn dark matter particles; eventually, their gravity would have gotten so powerful that the neutron star and dark matter would have merged into a black hole, according to the study.
The study proposes that a neutron star attracted and merged with a small primordial black hole, which then settled at the neutron star’s centre of mass and fed off the surrounding matter until only the black hole remained.
Takhistov and his colleagues reasoned that black holes formed by the transformation of neutron stars would have to have the same mass distribution as the neutron stars from which they originated, which is determined by the sizes of their parent stars.
Taking this into account, the researchers examined the data from the 50 or so gravitational wave detections made to date and discovered that just two of them — GW190425 and GW190814 — involved objects with the right masses to be primordial black holes, according to the study authors.
The study is not conclusive: it is still feasible that the two collisions contained neutron stars of the detected masses or black holes transmuted from neutron stars of those masses. However, the scientists believe that the mass distribution of neutron stars predicted to exist in the universe makes this unlikely.
“Our work advances a powerful test to understand their origin and relation with dark matter,” Takhistov said. “In particular, this test demonstrates that black holes significantly heavier than about 1.5 solar-masses are very unlikely to be ‘transmuted’ black holes from neutron star disruptions.”
According to the study, if this is the case, it suggests that primordial black holes may exist and be a component of dark matter.
Takhistov believes that when more gravitational wave detections are made, the procedure will become more accurate: “The test is statistical in nature, so gathering more data will allow for a better understanding.”