Since the first observation of merging black holes by the Laser Interferometer Gravitational Wave Observatory (LIGO) in 2015, astronomers have been repeatedly surprised by their large masses.
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Though they emit no light, black hole mergers are observed through their emission of gravitational waves ripples in the fabric of spacetime that were predicted by Einstein’s theory of general relativity.
Physicists originally expected that black holes would have masses less than about 40 times that of the Sun, because merging black holes arise from massive stars, which can’t hold themselves together if they get too big.
The LIGO and Virgo observatories, however, have found many black holes with masses greater than that of 50 suns, with some as massive as 100 suns.
Numerous formation scenarios have been proposed to produce such large black holes, but no single scenario has been able to explain the diversity of black hole mergers observed so far, and there is no agreement on which combination of formation scenarios is physically viable.
This new study, published in the Astrophysical Journal Letters, is the first to show that both large and small black hole masses can result from a single pathway, wherein the black holes gain mass from the expansion of the universe itself.
Astronomers typically model black holes inside a universe that cannot expand. “It’s an assumption that simplifies Einstein’s equations because a universe that doesn’t grow has much less to keep track of,†said Kevin Croker, a professor at the UH MÄnoa Department of Physics and Astronomy.
“There is a trade off though: predictions may only be reasonable for a limited amount of time.â€
To investigate this hypothesis, the researchers simulated the birth, life, and death of millions of pairs of large stars. Any pairs where both stars died to form black holes were then linked to the size of the universe, starting at the time of their death. As the universe continued to grow, the masses of these black holes grew as they spiraled toward each other.
The result was not only more massive black holes when they merged, but also many more mergers. When the researchers compared the LIGO Virgo data to their predictions, they agreed reasonably well.
The agreement between the new model and our current data comes from simply acknowledging that realistic black holes don’t exist in a static universe. The researchers were careful to stress, however, that the mystery of LIGO–Virgo’s massive black holes is far from solved. ■