Dark matter could be made of black holes from the beginning of time
An analysis of space-time ripples shows that the mystery substance is composed of primordial black holes.
According to a recent study, dark matter, the mysterious substance that exerts gravitational pull but emits no light, could actually be massive concentrations of ancient black holes produced at the very beginning of the universe.
That conclusion comes from an analysis of the gravitational waves, or ripples in space-time, produced by two distant collisions between 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, 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 could have developed later when neutron stars were transmuted into black holes — either by swallowing primordial black holes or by absorbing certain proposed types of dark matter, the mysterious substance that exerts gravitational attraction but does not interact with light, according to Takhistov.
Primordial black holes
Primordial black holes, if they exist, were likely created in vast numbers in the first second of the Big Bang about 13.77 billion years ago. They would have come in all sizes — the smallest would have been microscopic and the largest 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, allowing only primordial black holes with masses more than 1011 kilograms — about the mass of a small asteroid — to exist today.
Some astrophysicists believe that if these ancient black holes exist, they could make up a vast halos of “dark matter” that encircles galaxies.
The researchers wanted to learn if they could distinguish primordial black holes from black holes that had formed from neutron stars, the glimmering remnants of supernovas left behind when their parent stars exploded after using up all their hydrogen in nuclear fusion reactions.
According to Live Science, scientists calculated that stars smaller than about 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 this theory, the intense gravity of some neutron stars would have continuously attracted dark matter particles; eventually, their gravity would have become so powerful that the neutron star and dark matter would have collided into a black hole, according to the new study.
The study proposes that a neutron star attracted and fused 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 transmuted from neutron stars would have to follow the same mass distribution of the neutron stars they originated from, which depends on the sizes of their parent stars.
Taking this into account, the scientists reviewed the data from the 50 or so gravitational wave detections made to date and discovered that only two of them — GW190425 and GW190814 — involved objects with the necessary masses to be primordial black holes, according to the study authors.
The study is not conclusive: it is still possible that the two collisions involved neutron stars of the detected masses or black holes transmuted from neutron stars of those masses. But the mass distribution of neutron stars theorized to exist in the universe makes that unlikely, the authors wrote.
“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 thinks that when more gravitational wave detections are made, the method will become more accurate: “The test is statistical in nature, so gathering more data will allow for a better understanding.”