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| The first rendered image of a black hole, illuminated by infalling matter. In this study, researchers have proposed a model where these objects can gain mass without the addition of matter: they can cosmologically couple to the growth of the universe itself. (Image Credit: Jean-Pierre Luminet, “Image of a Spherical Black Hole with Thin Accretion Disk,” Astronomy and Astrophysics 75 (1979): 228–35.) |
Over the last six years, gravitational wave observatories have been detecting black hole mergers, proving a crucial prediction of Albert Einstein's theory of gravity. But there is a problem—many of these black holes are unusually large. A team of researchers from the University of Hawaiʻi at Mānoa, the University of Chicago, and the University of Michigan at Ann Arbor have offered a creative solution to this problem: Black holes develop along with the expansion of the universe.
Since the first sighting of merging black holes by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015, astronomers have been constantly shocked by their large masses. Though they produce no light, black hole mergers are witnessed by their production of gravitational waves—-ripples in the fabric of spacetime that were predicted by Einstein's theory of general relativity. Physicists initially thought that black holes would have masses smaller than around 40 times the Sun since merging black holes come from huge stars, which can't keep themselves together if they grow too enormous.
HOWEVER, the LIGO and Virgo observatories have identified several black holes with masses more extensive than that of 50 suns, with some as enormous as 100 suns. Numerous formation scenarios have been suggested to generate such large black holes. Still, no one theory has explained the variety of black hole mergers seen so far. There is no consensus on which combination of formation scenarios is physically plausible. This new research, published in the Astrophysical Journal Letters, is the first to prove that both large and tiny black hole masses may come from a single route, whereby the black holes acquire mass through the expansion of the universe itself.
Astronomers often simulate black holes within a universe that cannot grow. "It's an assumption that simplifies Einstein's equations because a world that doesn't expand has much less to keep track of," said Kevin Croker, a UH Mānoa Department of Physics and Astronomy professor. "There is a trade-off though: Predictions may only be realistic for a short length of time."
Because the individual events observable by LIGO—Virgo only last a few seconds, this reduction is appropriate when evaluating any one occasion. But these identical mergers are possibly billions of years in the making. The cosmos develops drastically during the interval between the birth of a pair of black holes and their ultimate union. Suppose the more delicate features of Einstein's theory are appropriately explored. In that case, a shocking option emerges: The masses of black holes may expand in lockstep with the cosmos, a process that Croker and his colleagues term cosmic coupling.
The most well-known example of a cosmologically-coupled substance is light, which loses energy as the universe expands. "We thought to examine the reverse impact," said study co-author and UH Mānoa Physics and Astronomy Professor Duncan Farrah. "What would LIGO—Virgo see if black holes were cosmologically connected and acquired energy without having to devour other stars or gas?"
The researchers recreated the birth, life, and death of millions of pairs of large stars to examine this idea. Any couples where both stars perished to produce black holes were then connected to the size of the universe, commencing at the moment of their demise. As the cosmos continued to develop, the masses of these black holes swelled as they spiraled toward one other. The outcome was not just more huge black holes when they merged, but also many more mergers. When the researchers compared the LIGO—Virgo data to their predictions, they agreed pretty well. "I have to confess I didn't know what to believe at first,"' said study co-author and University of Michigan Professor Gregory Tarlé. "It was such a basic concept; I was astonished it worked so well."
According to the researchers, this new model is essential since it doesn't need modifications to our present knowledge of star origin, evolution, or death. The agreement between the new model and our current facts comes from simply accepting that actual black holes don't exist in a static world. However, the researchers were cautious to highlight that the riddle of LIGO—huge Virgo's black holes is far from solved.
"Many elements of merging black holes are not understood in detail, such as the prevailing creation conditions and the complicated physical processes that endure throughout their existence," stated study co-author and NASA Hubble Fellow Dr. Michael Zevin. "Despite the fact that we utilized a great population based on existing data, there is still space for improvement. Cosmic connection is beneficial, but we don't yet know how strong this coupling really is."
He is confident that this new theory will be tested in the future, according to co-author and UH Manoa Physics and Astronomy Professor Kurtis Nishimura, "Over the next decade, as gravitational-wave observatories continue to improve their sensitivity, more and better data will be available for analysis. Measurement of this will take place soon enough."
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