Observations have found black holes spanning 10 orders of magnitude in mass across most of cosmic history. The assembly of stellar and supermassive black hole (SMBH) mass in galaxies can help to diagnose the origins of locally observed correlations between SMBH mass and stellar mass.
A team of researchers led by experts at the University of Hawai’i at Mānoa have found the first proof of “cosmological coupling,” a newly predicted phenomenon in Einstein’s theory of gravity, which is only feasible when black holes are positioned inside an evolving universe. The findings offer insight into what might exist inside real black holes.
Kevin Croker, a professor of physics and astronomy who led this ambitious study, said, “When LIGO heard the first pair of black holes merge in late 2015, everything changed. The signal was in excellent agreement with predictions on paper, but extending those predictions to millions or billions of years? Matching that model of black holes to our expanding universe? It wasn’t at all clear how to do that.”
The findings are published in two papers.
In the first study, the team determined how to use existing measurements of black holes to search for cosmological coupling. The scientists understood that galaxies held the key since they can have billions of years’ lifetimes and because most galaxies contain a supermassive black hole. But, picking the appropriate kinds of galaxies was crucial.
Study co-author Sara Petty, a galaxy expert at NorthWest Research Associates, said, “We decided that by focusing only on black holes in passively evolving elliptical galaxies, we could help to sort this thing out.”
Scientists could claim that other known processes couldn’t easily produce changes in the galaxies’ black hole masses by focusing only on elliptical galaxies with no recent activity. The scientists then used these populations to look at how the mass of their centre black holes altered over the previous 9 billion years.
The scientists found that the black holes’ masses were less relative to their current masses the further back in time they looked. The black holes were 7 and 20 times bigger than they were 9 billion years ago, which was so significant that the scientists assumed cosmic coupling might be too responsible.
In other words, the study found that these black holes accumulate mass over billions of years in a difficult way to explain by the standard galaxy and black hole processes, such as mergers or gas accretion.
In the second investigation, the team looked into the possibility that cosmic coupling may account for the growth in black holes observed in the first study.
Croker said, “You can think of a coupled black hole like a rubber band, stretched along with the universe as it expands. As it stretches, its energy increases. Einstein’s E = mc2 tells you that mass and energy are proportional, so the black hole mass increases, too.”
“How much the mass increases depends on the coupling strength, a variable- referred to as k.”
Because the mass growth of black holes from cosmological coupling depends on the size of the universe, and the universe was smaller in the past, the black holes in the first study must be less massive by the correct amount for the cosmological coupling explanation to work.
The results show that the mass expansion of these black holes is consistent with predictions for black holes that not only couple cosmologically but also contain vacuum energy, which is created by compressing matter as much as possible without defying Einstein’s equations and preventing a singularity.
The researchers studied three groups of elliptical galaxies, each representing a collection of five different black hole populations, from periods when the universe was roughly one-half and one-third the size it is now. They found that k was roughly positive 3 in each comparison.
In 2019, Croker, a graduate student, and Joel Weiner, a mathematics professor at UH Mnoa, predicted this value for black holes with vacuum energy rather than a singularity.
The implication is profound: Croker and Weiner had already demonstrated that if k is 3, then the total contribution of all known black holes to the universe’s dark energy density is almost constant, as suggested by dark energy measurements.
The team used the latest measurements of the rate of earliest star formation provided by the James Webb Space Telescope and found that the numbers line up.
The research then demonstrates that the measured amount of dark energy in our universe fits with the combined vacuum energy of black holes formed in the deaths of the universe’s first stars when singularities are not present.
UH, Mānoa astrophysicists Duncan Farrah, a faculty member at the Institute for Astronomy and the Department of Physics and Astronomy, said, “We’re 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.”
“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.”
Scientists noted, “The studies provide a framework for theoretical physicists and astronomers to test further—and for the current generation of dark energy experiments such as the Dark Energy Spectroscopic Instrument and the Dark Energy Survey—to shed light on the idea.”
Farrah said, “If confirmed, this would be a remarkable result, pointing the way towards the next generation of black hole solutions.”
Croker added, “This measurement, explaining why the universe is accelerating now, gives a beautiful glimpse into the real strength of Einstein’s gravity. A chorus of tiny voices spread throughout the universe can work together to steer the entire cosmos. How cool is that?”
- Duncan Farrah et al. A Preferential Growth Channel for Supermassive Black Holes in Elliptical Galaxies at z ≲ 2. The Astrophysical Journal. DOI 10.3847/1538-4357/acac2e
- Duncan Farrah et al. Observational Evidence for Cosmological Coupling of Black Holes and its Implications for an Astrophysical Source of Dark Energy. The Astrophysical Journal Letters. DOI 10.3847/2041-8213/acb704