Black Holes May Gain Mass From the Expansion of the Universe Itself

Spherical Black Hole Image

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. Credit: Jean-Pierre Luminet, “Image of a Spherical Black Hole with Thin Accretion Disk,” Astronomy and Astrophysics 75 (1979): 228–35.

Over the past 6 years, gravitational wave observatories have been detecting Comparison of Black Hole Merger Observations With Predictions

Comparison of black hole merger observations with predictions from the new model. The horizontal axis shows the total mass of both black holes in any individual merger, relative to the Sun’s mass. The vertical axis gives a measure of how far into the past the merger was observed, where a redshift (denoted z) of 1 corresponds to when the Universe was half of its current size and z = 0 is today. The LIGO—Virgo observations are displayed as black crosses, with smaller crosses representing measurements with smaller uncertainties. Predictions for black holes in a static (not expanding) universe are shown in the orange region, with the darker shading representing more predicted objects. These are contrasted to predictions for cosmologically coupled black holes in a growing universe, which are shown in the blue region. Credit: University of Hawai`i, University of Chicago, University of Michigan at Ann Arbor

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

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…

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