Observations throughout most of cosmic history have found black holes with masses of 10 orders of magnitude. The assembly of stellar and supermassive black hole (SMBH) mass in galaxies can help diagnose the origin of locally observed correlations between SMBH mass and stellar mass.
A team of researchers led by experts from the University of Hawai’i at Mānoa has found the first evidence of “cosmological coupling,” a newly predicted phenomenon in Einstein’s theory of gravity that is only feasible when black holes are in reside in 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 merging at the end of 2015, everything changed. The signal matched the predictions on paper very well, but extending those predictions to millions or billions of years? Match that model of black holes to our expanding universe? It was not at all clear how to do that.”
The findings have been published in two articles.
In the first study, the team determined how existing measurements of black holes could be used to search for cosmological coupling. The scientists understood that galaxies held the key, as they can have lifespans of billions of years and most galaxies contain a supermassive black hole. But choosing the right types of galaxies was crucial.
Study co-author Sara Petty, a galaxy expert at NorthWest Research Associates, said: “We decided we could help solve this problem by focusing only on black holes in passively evolving elliptical galaxies.”
Scientists might argue that other known processes cannot easily cause changes in the masses of the galaxies’ black holes by focusing only on elliptical galaxies with no recent activity. The scientists then used these populations to look at how the masses of their middle black holes changed over the past 9 billion years.
The scientists found that the masses of the black holes were less relative to their current masses as they looked further back in time. The black holes were 7 and 20 times bigger than they were 9 billion years ago, which was so significant that the scientists assumed that cosmic coupling might be too responsible.
In other words, the study found that these black holes accumulate mass over billions of years in a way that is difficult to explain through the standard processes of galaxies and black holes, such as mergers or gas accretion.
In the second study, the team explored the possibility that cosmic coupling could explain the black hole growth observed in the first study.
croker said, “You can think of a linked black hole as 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 mass of the black hole is also increasing.”
“How much the mass increases depends on the coupling strength, a variable called k.”
Because the mass growth of black holes due to cosmological coupling depends on the size of the universe, and because the universe was smaller in the past, the black holes in the first study must be sufficiently less massive to support the explanation of cosmological coupling. let work.
The results show that the mass expansion of these black holes is consistent with predictions for black holes that not only cosmologically couple, but also contain vacuum energy, which is created by compressing matter as much as possible without defying Einstein’s equations and creating a prevent singularity.
The researchers studied three groups of elliptical galaxies, each representing a collection of five different populations of black holes, from periods when the universe was about half and a third the size it is today. They found that k was about positive 3 in every equation.
In 2019, Croker, a graduate student, and Joel Weiner, a math 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 shown that when k is 3, the total contribution of all known black holes to the dark energy density of the universe is nearly constant, as suggested by dark energy measurements.
The team used the latest measurements of the rate of earliest star formation from the James Webb Space Telescope and found that the numbers match.
The research then shows that the measured amount of dark energy in our universe corresponds to the combined vacuum energy of black holes formed at the death of the universe’s first stars when no singularities are present.
UH, Mānoa astrophysicists Duncan Farrah, a faculty member in the Institute of Astronomy and the Department of Physics and Astronomy, said: “We’re saying two things at once: that there’s evidence that the typical black hole solutions don’t work for you on a long, long time scale, and we have the first proposed astrophysical dark energy source.”
“What that means, though, isn’t that other people haven’t proposed sources of dark energy, but this is the first observational paper in which we add nothing new to the universe as a source of 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.”
Farra said, “If confirmed, this would be a remarkable result, pointing the way to the next generation of black hole solutions.”
Crocker added, “This measurement, which explains why the universe is now accelerating, gives a nice glimpse into the real force of Einstein’s gravity. A chorus of small voices scattered across the universe can work together to direct the entire cosmos. How cool is that?”
Magazine references:
- Duncan Farrah et al. A preferred 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
