Cosmological coupling — a newly-predicted phenomenon in Albert Einstein’s theory of gravity — allows black holes to grow in mass without consuming gas or stars, according to a team of astrophysicists from the University of Hawai’i at Mānoa and elsewhere.

An artist’s impression of a supermassive black hole. Image credit: University of Hawai’i at Mānoa.

“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 Dr. Duncan Farrah, an astrophysicist at the University of Hawai’i at Mānoa and lead author of two new studies.

“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, will redefine our understanding of what a black hole is.”

In their first study, published in the Astrophysical Journal, Dr. Farrah and colleagues determined how to use existing measurements of black holes to search for cosmological coupling.

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 throughout 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 identify their distribution of mass billions of years ago,” said Dr. Gregory Tarlé, a physicist at University of Michigan.

“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. 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 authors realized that galaxies held the key, but choosing the right types of galaxy was essential.

“There were many different behaviors for black holes in galaxies measured in the literature, and there wasn’t really any consensus,” said Dr. Sara Petty, a galaxy expert at NorthWest Research Associates.

“We decided that by focusing only on black holes in passively evolving elliptical galaxies, we could help to sort this thing out.”

Elliptical galaxies are enormous and formed early. They are fossils of galaxy assembly.

Astronomers believe them to be the final result of galaxy collisions, enormous in size with upwards of trillions of old stars.

By looking at only elliptical galaxies with no recent activity, the researchers could argue that any changes in the galaxies’ black hole masses couldn’t easily be caused by other known processes.

Using these populations, they then examined how the mass of their central black holes changed throughout the past 9 billion years.

If mass growth of black holes only occurred through accretion or merger, then the masses of these black holes would not be expected to change much at all.

However, if black holes gain mass by coupling to the expanding Universe, then these passively evolving elliptical galaxies might reveal this phenomenon.

The scientists found that the further back in time they looked, the smaller the black holes were in mass, relative to their masses today.

These changes were big: the black holes were anywhere from 7 to 20 times larger today than they were 9 billion years ago — big enough that the researchers suspected cosmological coupling could be the culprit.

In their second study, published in the Astrophysical Journal Letters, the astrophysicists investigated whether the growth in black holes measured in the first study could be explained by cosmological coupling alone.

“Here’s a toy analogy. You can think of a coupled black hole like a rubber band, being stretched along with the Universe as it expands,” said Dr. Kevin Croker, an astrophysicist at the University of Hawai’i at Mānoa.

“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 the researchers call k.

“The stiffer the rubber band, the harder it is to stretch, so the more energy when stretched. In a nutshell, that’s k,” Dr. Croker said.

Because 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 in order for the cosmological coupling explanation to work.

The researchers examined five different black hole populations in three different collections of elliptical galaxies, taken from when the Universe was roughly one half and one third of its present size.

In each comparison, they measured that k was nearly positive 3.

In 2019, this value was predicted for black holes that contain vacuum energy, instead of a singularity.

The conclusion is profound: the scientists had already shown that if k is 3, then all black holes in the Universe collectively contribute a nearly constant dark energy density, just like measurements of dark energy suggest.

Black holes come from dead large stars, so if you know how many large stars you are making, you can estimate how many black holes you are making and how much they grow as a result of cosmological coupling.

The team used the very latest measurements of the rate of earliest star formation provided by the NASA/ESA/CSA James Webb Space Telescope and found that the numbers line up.

The new studies provide a framework for theoretical physicists and astronomers to further test — 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.

“If confirmed this would be a remarkable result, pointing the way towards the next generation of black hole solutions,” Dr. Farrah said.

“This measurement, explaining why the Universe is accelerating now, gives a beautiful glimpse into the real strength of Einstein’s gravity,” Dr. Croker said.

“A chorus of tiny voices spread throughout the Universe can work together to steer the entire cosmos. How cool is that?”

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Duncan Farrah et al. 2023. A Preferential Growth Channel for Supermassive Black Holes in Elliptical Galaxies at z ≲ 2. ApJ 943, 133; doi: 10.3847/1538-4357/acac2e

Duncan Farrah et al. 2023. Observational Evidence for Cosmological Coupling of Black Holes and its Implications for an Astrophysical Source of Dark Energy. ApJL 944, L31; doi: 10.3847/2041-8213/acb704