ANN ARBOR—Searching through existing data spanning 9 billion years, a University of Michigan physicist and colleagues have uncovered the first evidence of "cosmological coupling"—a newly predicted phenomenon in Einstein's theory of gravity, possible only when black holes are placed inside an evolving universe. 

Gregory Tarlé, U-M professor of physics, and researchers from the University of Hawaii and other institutions across nine countries studied supermassive black holes at the heart of ancient and dormant galaxies to develop a description of them that agrees with observations from the past decade. Their findings are published in two journal articles, one in The Astrophysical Journal and the other in The Astrophysical Journal Letters.

The first study found that these black holes gain mass over billions of years in a way that can't easily be explained by standard galaxy and black hole processes, such as mergers or accretion of gas. According to the second paper, the growth in mass of these black holes matches predictions for black holes that not only cosmologically couple, but also enclose vacuum energy—material that results from squeezing matter as much as possible without breaking Einstein's equations, thus avoiding a singularity. 

With singularities removed, the paper then shows that the combined vacuum energy of black holes produced in the deaths of the universe's first stars agrees with the measured quantity of dark energy in our universe. 

"We're really saying two things at once: that there's evidence the typical black hole solutions don't work for you on a long, long timescale, and we have the first proposed astrophysical source for dark energy," said Duncan Farrah, University of Hawaii astronomer and lead author on both papers. 

"What that means, though, is not that other people haven't proposed sources for dark energy, but this is the first observational paper where we're not adding anything new to the universe as a source for dark energy: Black holes in Einstein's theory of gravity are the dark energy." 

These new measurements, if supported by further evidence, redefine our understanding of what a black hole is. 

Nine billion years ago

In the first study, the team determined how to use existing measurements of black holes to search for cosmological coupling. 

"My interest in this project was really born from a general interest in trying to determine observational evidence that supports a model for black holes that works regardless of how long you look at them," Farrah said. "That's a very, very difficult thing to do in general, because black holes are incredibly small, they're incredibly difficult to observe directly, and they are a long, long way away." 

Black holes are also hard to observe over long timescales. Observations can be made over a few seconds, or tens of years at most—not enough time to detect how a black hole might change over the lifetime of the universe. To see how black holes change over a scale of billions of years is a bigger task. 

"You would have to identify a population of black holes and determine their distribution of mass billions of years ago. Then you would have to see the same population, or an ancestrally connected population, at present day and again be able to measure their mass," Tarlé said. "That's a really difficult thing to do." 

Because galaxies can have life spans of billions of years, and most galaxies contain a supermassive black hole, the team realized that galaxies held the key, but choosing the right types of galaxies was essential.

Please read the rest of the ariticle on the U-M News website.

More Information:
Gregory Tarlé

Study No. 1 in The Astrophysical Journal: A Preferential Growth Channel for Supermassive Black Holes in Elliptical Galaxies at z ≲ 2

Study No. 2 in The Astrophysical Journal Letters: Observational Evidence for Cosmological Coupling of Black Holes and Its Implications for an Astrophysical Source of Dark Energy