Scientists typically simulate black holes in models where the universe does not expand. Physicists do this because it simplifies Einstein's equations and makes the data easier to analyze. The newly proposed model, however, suggests that black hole growth is influenced exactly by the expansion of the universe.
Since scientists observed the merging of black holes for the first time 6 years ago, observatories around the world have been detecting more and more of the same objects. Like any new discovery about the universe, black hole mergers gave us more questions than answers. Scientists have since faced a new problem – some of the observed black holes are impossibly large.
Scientists believe that the growth of black holes is connected to the expansion of the universe
Merging black holes
The discovery of black hole mergers confirmed a long-standing prediction from Einstein’s theory of general relativity. Since there are no light emissions from black holes, the only way to find them and observe the processes around or within is through gravitational waves. Their existence was proposed decades ago and only confirmed in 2015.
Unexpectedly large merging black holes
Since 2015, observatories around the world have been detecting new merging black holes but not all have had identical characteristics. Some mergers continue to impress scientists with their mass.
Back in the day, physicists predicted that a black hole would not have a mass greater than 40 times that of our sun. Now we know that this estimate is far from the truth. Astronomers have found multiple black holes with masses between the mass of 50 and 100 suns.
How can black holes become this large?
Over the past few years, scientists have proposed more than one or two scenarios about how these objects came to be. While some ideas have enough context to be considered, there is one main question that prevents physicists from reaching an agreement. None of the proposed models is viable enough to explain the diversity of merging black holes.
A team of physicists has proposed an alternative view on the formation and growth of black holes. It explains that both small and large black holes can form in similar ways because their growth depends on something else – the expansion of the universe.
Physicists decided to go sideways and test a new approach. In most cases, black hole models include objects in a hypothetical universe that does not grow. The issue with this traditional method is that our universe continues to grow profoundly and such models are nothing more than temporary. And this is why mergers with masses far greater than the 40 suns do not fit into such models.
Is black hole growth connected to the expansion of the universe?
Instead, scientists decided to look at those objects from the point of view of an evergrowing universe. They note that black hole mergers must have existed for hundreds of millions or even billions of years. At the start, the pair of black holes formed but in order to reach that merging state, they grew as the universe expanded.
Does this new model change anything?
Apart from giving the world a simple, yet viable explanation, this new model does not re-write the long-standing understanding of the evolution of the universe. It simply means that the traditional models in a static universe are not entirely correct.
Far from solved
Physicists noted that the mysteries of black hole mergers are far from solved but based on their simulations, it is likely that black hole growth is connected to the expansion of the universe. At least for now, there is no better model to explain this phenomenon.
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• Croker, K. S. (n.d.). Cosmologically Coupled Compact Objects: A Single-parameter Model for LIGO–Virgo Mass and Redshift Distributions. The Astrophysical Journal Letters.
• Irving, M. (2021, November 5). New model suggests black holes gain mass as the universe expands. New Atlas.
• Phys.org. (2021, November 3). New Study proposes expansion of the universe directly impacts black hole growth.
• Sci-News. (n.d.). Black Holes Grow along with Universe’s Expansion, Study Suggests.