The black hole was detected using a gravitational lensing technique. (Getty)
A black hole has been detected that could be four times the size of the Sun “floating freely” through our galaxy alone.
Researchers at the University of California said there could be as many as 200 million free floating black holes in our Milky Way.
Black holes are invisible unless they are part of a stellar track or are surrounded by an accretion disk.
Scientists used gravitational microlenses, where the black hole distorts and illuminates the light coming from “behind” as seen from Earth.
Researchers are still unsure whether the object is a black hole or a neutron star (the heavy nucleus left after a supernova explosion).
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The team, led by graduate student Casey Lam and Jessica Lu, an associate professor of astronomy at UC Berkeley, estimated that the mass of the invisible compact object was 1.6 to 4.4 times that of Ground.
Because astronomers think the rest of a dead star must be heavier than 2.2 solar masses to collapse into a black hole, researchers warn that the object could be a neutron star instead of a black hole.
Neutron stars are also dense and very compact objects, but their gravity is balanced by the internal pressure of neutrons, which prevents a subsequent collapse of a black hole.
Whether it’s a black hole or a neutron star, the object is the first dark stellar remnant, a stellar “ghost”, discovered wandering the galaxy unmatched by another star.
Determining how many of these compact objects populate the Milky Way will help astronomers understand the evolution of stars, in particular how they die, and our galaxy.
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It could also reveal if any of the invisible black holes are primordial black holes, which some cosmologists believe occurred in large numbers during the Big Bang.
The analysis of Lam, Lu and his international team has been accepted for publication in The Astrophysical Journal Letters.
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The team also concluded that the probable population of black holes in the galaxy is 200 million, roughly what most theorists predicted.
Lam said the length of the lens event was the main indication that investigators were looking at a black hole. In 2020, he showed that the best way to look for black hole micro-lenses was to look for very long events.
Only 1% of detectable micro-lens events are likely to have black holes, he said, so looking at all events would be like looking for a needle in a haystack.
But Lam calculated that about 40 percent of micro-lens events lasting more than 120 days are likely to be black holes.
“The duration of the brightness event is an indication of the large amount of foreground lens that doubles the background star’s light,” he said.
“Long events are more likely because of black holes. It’s not a guarantee, though, because the duration of the glare episode depends not only on the massive mass of the foreground lens, but also on how quickly the foreground lens and the background star move relatively close to each other.
“However, by also obtaining measurements of the apparent position of the background star, we can confirm whether the foreground lens is really a black hole.”
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