Key takeaways
- Researchers discovered the first evidence of a phenomenon called “cosmological coupling,” where black holes grow in mass over billions of years, fitting predictions from Einstein’s theory of gravity.
- The study found that black holes in old, inactive galaxies have grown significantly in mass over the last 9 billion years, suggesting they interact with the expanding universe.
- The research proposes that black holes could be the source of dark energy, which makes up about 70% of the universe’s energy, without adding anything new to the universe.
- By studying elliptical galaxies with little recent activity, scientists were able to rule out other processes that could explain the black holes’ mass increase, supporting the idea of cosmological coupling.
- If confirmed, this discovery would change our understanding of black holes and their role in the universe, offering new insights into the nature of dark energy and the evolution of the cosmos.
A University of Michigan physicist and colleagues discovered the first evidence of “cosmological coupling”—a newly predicted phenomenon in Einstein’s theory of gravity that can only occur when black holes are placed within an evolving universe—after searching through existing data spanning 9 billion years.
Gregory Tarlé, a physics professor at the University of Michigan, with colleagues from the University of Hawaii and other institutions in nine nations researched supermassive black holes at the center of old and inactive galaxies to construct a description that matches observations made in the last decade. Their findings have been published in two scientific articles: The Astrophysical scientific and The Astrophysical Journal Letters.
The first research discovered that these black holes develop mass over billions of years in ways that are not easily explained by normal galaxy and black hole processes like mergers or gas accretion. According to the second paper, the mass growth of these black holes is consistent with predictions for black holes that not only cosmologically couple but also enclose vacuum energy—material created by squeezing matter as much as possible without breaking Einstein’s equations, thereby avoiding a singularity.
With singularities eliminated, the article demonstrates that the cumulative vacuum energy of black holes formed after the deaths of the universe’s initial stars is consistent with the known amount of dark energy in our universe.
“We’re really saying two things at once: there’s evidence that 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, lead author of both articles and an astronomer at the University of Hawaii.
“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 corroborated by more data, will change our understanding of what a black hole is.
Nine billion years ago
In the first investigation, the scientists identified how to use current black hole observations to seek for cosmic 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 stated. “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 similarly difficult to witness over extended timeframes. Observations can last only a few seconds or tens of years, which is insufficient time to discern how a black hole may alter throughout the course of the universe. It is more difficult to understand how black holes evolve over billions of years.
“You’d have to identify a population of black holes and calculate their mass distribution billions of years ago.” Then you would have to observe the same population, or an ancestrally related group, in the current day and measure their mass again,” Tarlé explained. “That’s a really difficult thing to do.”
Because galaxies may live for billions of years and most include a supermassive black hole, the researchers concluded that galaxies held the answer, but selecting the appropriate sorts of galaxies was critical.
“There were many different behaviors for black holes in galaxies measured in the literature, and there wasn’t really any consensus,” said research co-author Sara Petty, a galaxy specialist 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 massive and formed early. They resemble remains of galaxy construction. Astronomers believe they are the end consequence of galaxy mergers, behemoths composed of billions of ancient stars.
“These galaxies are old, don’t produce many new stars, and have very little gas remaining between them. Tarlé replied, “There is no food for black holes.”
By focusing primarily on elliptical galaxies with no recent activity, the researchers could argue that any variations in the galaxies’ black hole masses could not be easily explained by other known processes. Using these populations, the scientists investigated how the mass of their center black holes has evolved over the last 9 billion years.
If black hole mass development happened only through accretion or merging, the masses of these black holes would be anticipated to remain relatively constant. However, if black holes gain mass by interacting with the expanding cosmos, these passively developing elliptical galaxies might disclose this process.
The researchers discovered that the further back in time they examined, the smaller the black holes were in mass compared to their masses today. These changes were significant: black holes were 7 to 20 times bigger now than they were 9 billion years ago, leading the researchers to hypothesize cosmic coupling.
Unlocking black holes
of the second investigation, the team looked into whether the rise of black holes observed in the first study could be explained only by cosmic coupling.
“This is a toy analogy. Kevin Croker, a University of Hawaii theoretical astrophysicist and research co-author, compared a connected black hole to a rubber band that is stretched along with the expansion of the universe. “As it expands, its energy grows. According to Einstein’s E = mc2, mass and energy are proportionate, hence the mass of the black hole grows as well.
The amount by which the mass rises is determined by the coupling strength, which the researchers refer to as k.
“The firmer the rubber band, the more difficult it is to stretch, requiring more energy when stretched. In a word, that’s okay,” Croker remarked.
Because the mass development of black holes due to cosmic coupling is proportional to the size of the universe, and the cosmos was smaller in the past, the black holes in the first study must be less massive by the appropriate amount for the cosmological coupling explanation to work.
The researchers looked at five separate black hole populations in three different groupings of elliptical galaxies from when the universe was around one-half to one-third its current size. In each comparison, they found that k was virtually positive 3.
Croker, then a doctoral student, and Joel Weiner, a mathematics professor at the University of Hawaii, anticipated this figure for black holes with vacuum energy rather than a singularity four years ago.
The result is profound: Croker and Weiner had previously demonstrated that if k is 3, all black holes in the universe contribute a roughly constant dark energy density, as dark energy measurements indicate.
“Is it enough?” Tarlé spoke. “Are the black holes made over time enough to account for 70% of the energy in the universe today?”
Black holes form from dead huge stars, thus if you know how many large stars you’re producing, you can calculate how many black holes you’re producing and how much they’re growing as a result of cosmic coupling. Using the most recent measurements of the rate of early star formation supplied by the James Webb Space Telescope, the team discovered that the numbers line up.
According to the researchers, their findings give a foundation for theoretical physicists and astronomers to investigate further, as well as enabling the current generation of dark energy experiments, such as the Dark Energy Spectroscopic Instrument and the Dark Energy Survey, to shed light on the concept.
“If cosmological coupling is confirmed, it would mean that black holes never entirely disconnect from our universe, that they continue to exert a major influence on the evolution of the universe into the distant future” Tarlé stated.
“The nature of dark energy is one of the most critical unresolved questions in modern physics. It accounts for the vast bulk of the universe’s energy, around 70%. And now we have observable evidence as to where it came from, why it is 70%, and why it is here now. It’s quite thrilling!”
