Since the first observation of merging black holes in 2015, astrophysicists have been repeatedly surprised by their large masses. They originally expected that black holes would have masses less than about 40 times that of the Sun. The LIGO and Virgo observatories, however, have found many black holes with masses greater than 50 solar masses, with some as massive as 100 solar masses. A 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.
A view of the accretion disk around the supermassive black hole, with jet-like structures flowing away from the disk. The extreme mass of the black hole bends spacetime, allowing the far side of the accretion disc to be seen as an image above and below the black hole. Image credit: Science Communication Lab, DESY.
“Astronomers typically model black holes inside a non-expanding Universe. It’s an assumption that simplifies Albert Einstein’s equations because a Universe that doesn’t grow has much less to keep track of,” said Professor Kevin Croker, an astronomer in the Department of Physics and Astronomy at the University of Hawai’i at Manoa.
“There is a trade-off though: predictions may only be reasonable for a limited amount of time.”
Because the individual events detectable by LIGO-Virgo only last a few seconds, when analyzing any single event, this simplification is sensible. But these same mergers are potentially billions of years in the making.
During the time between the formation of a pair of black holes and their eventual merger, the Universe grows profoundly.
If the more subtle aspects of Einstein’s theory are carefully considered, then a startling possibility emerges: the masses of black holes could grow in lockstep with the Universe, a phenomenon that Professor Croker and colleagues call ‘cosmological coupling.’
The most well-known example of cosmologically-coupled material is light itself, which loses energy as the Universe grows.
“We thought to consider the opposite effect. What would LIGO-Virgo observe if black holes were cosmologically coupled and gained energy without needing to consume other stars or gas?” said Professor Duncan Farrah, an astronomer in the Department of Physics and Astronomy and the Institute for Astronomy at the University of Hawai’i at Manoa.
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 scientists compared the LIGO-Virgo data to their predictions, they agreed reasonably well.
“This new model is important because it doesn’t require any changes to our current understanding of stellar formation, evolution, or death,” they said.
“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.”
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Kevin S. Croker et al. 2021. Cosmologically Coupled Compact Objects: A Single-parameter Model for LIGO-Virgo Mass and Redshift Distributions. ApJL 921, L22; doi: 10.3847/2041-8213/ac2fad
