Over the past 6 years, gravitational wave observatories have been detecting black holeA black hole is a place in space where the pull of gravity is so strong not even light can escape it. Astronomers classify black holes into three categories by size: miniature, stellar, and supermassive black holes. Miniature black holes could have a mass smaller than our Sun and supermassive black holes could have a mass equivalent to billions of our Sun.”>black hole mergers, verifying a major prediction of Albert Einstein’s theory of gravity. But there is a problem — many of these black holes are unexpectedly large. Now, a team of researchers from the University of Hawaiʻi at Mānoa, the University of ChicagoFounded in 1890, the University of Chicago (UChicago, U of C, or Chicago) is a private research university in Chicago, Illinois. Located on a 217-acre campus in Chicago’s Hyde Park neighborhood, near Lake Michigan, the school holds top-ten positions in various national and international rankings. UChicago is also well known for its professional schools: Pritzker School of Medicine, Booth School of Business, Law School, School of Social Service Administration, Harris School of Public Policy Studies, Divinity School and the Graham School of Continuing Liberal and Professional Studies, and Pritzker School of Molecular Engineering.”>University of Chicago, and the University of Michigan at Ann Arbor, have proposed a novel solution to this problem: black holes grow along with the expansion of the universe.
Since the first observation of merging black holes by the Laser Interferometer Gravitational-Wave Observatory (LIGOThe Laser Interferometer Gravitational-Wave Observatory (LIGO) is a large-scale physics experiment and observatory supported by the National Science Foundation and operated by Caltech and MIT. It’s designed to detect cosmic gravitational waves and to develop gravitational-wave observations as an astronomical tool. It’s multi-kilometer-scale gravitational wave detectors use laser interferometry to measure the minute ripples in space-time caused by passing gravitational waves. It consists of two widely separated interferometers within the United States—one in Hanford, Washington and the other in Livingston, Louisiana.”>LIGO) in 2015, astronomers have been repeatedly surprised by their large masses. Though they emit no light, black hole mergers are observed through their emission of gravitational wavesGravitational waves are distortions or ripples in the fabric of space and time. They were first detected in 2015 by the Advanced LIGO detectors and are produced by catastrophic events such as colliding black holes, supernovae, or merging neutron stars.”>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 Mānoa Department of Physics and Astronomy. “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 Croker and his team 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,” said research co-author and UH Mānoa Physics and Astronomy Professor Duncan Farrah. “What would LIGO—Virgo observe if black holes were cosmologically coupled and gained energy without needing to consume other stars or gas?”
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. “I have to say I didn’t know what to think at first,” said research co-author and University of Michigan Professor Gregory Tarlé. “It was a such a simple idea, I was surprised it worked so well.”
According to the researchers, this new model is important because it doesn’t require any changes to our current understanding of stellar formation, evolution, or death. 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.
“Many aspects of merging black holes are not known in detail, such as the dominant formation environments and the intricate physical processes that persist throughout their lives,” said research co-author and NASAEstablished in 1958, the National Aeronautics and Space Administration (NASA) is an independent agency of the United States Federal Government that succeeded the National Advisory Committee for Aeronautics (NACA). It is responsible for the civilian space program, as well as aeronautics and aerospace research. It’s vision is “To discover and expand knowledge for the benefit of humanity.””>NASA Hubble Fellow Dr. Michael Zevin. “While we used a simulated stellar population that reflects the data we currently have, there’s a lot of wiggle room. We can see that cosmological coupling is a useful idea, but we can’t yet measure the strength of this coupling.”
Research co-author and UH Mānoa Physics and Astronomy Professor Kurtis Nishimura expressed his optimism for future tests of this novel idea, “As gravitational-wave observatories continue to improve sensitivities over the next decade, the increased quantity and quality of data will enable new analysis techniques. This will be measured soon enough.”
Reference: “Cosmologically Coupled Compact Objects: A Single-parameter Model for LIGO–Virgo Mass and Redshift Distributions” by Kevin S. Croker, Michael Zevin, Duncan Farrah, Kurtis A. Nishimura and Gregory Tarlé, 3 November 2021, The Astrophysical Journal Letters.