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First observational proof of the association between black holes and dark energy.

First observational proof of the association between black holes and dark energy.

Over a wide range of cosmic history, observations have revealed the presence of black holes with a mass range spanning 10 orders of magnitude. The accumulation of stellar and supermassive black hole (SMBH) mass in galaxies can provide insights into the origins of the SMBH mass-stellar mass correlations observed locally.

Artist’s impression of a supermassive black hole. Cosmological coupling allows black holes to grow in mass without consuming gas or stars. Credit: Institute for Astronomy

A team of experts from the University of Hawai’i at Mānoa has detected the first evidence of “cosmological coupling,” a phenomenon recently predicted in Einstein’s theory of gravity, which can only occur when black holes are present within an evolving universe. The researchers’ findings shed light on the potential contents of actual 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 initial study, the team explored how to detect cosmological coupling by utilizing existing measurements of black holes. They recognized that galaxies were crucial in this quest because they could exist for billions of years and contain supermassive black holes. However, the team needed to select the right type of galaxies to study.

Sara Petty, a galaxy specialist at NorthWest Research Associates and co-author of the study, stated, “We determined that we could help untangle this issue by focusing solely on black holes in passively evolving elliptical galaxies.”

By focusing exclusively on elliptical galaxies with no recent activity, scientists could argue that other known processes would not easily produce changes in the black hole masses of these galaxies. The researchers then examined how the mass of the central black holes in these populations had changed over the last 9 billion years.

The scientists discovered that the black holes’ masses were lower relative to their current masses as they looked back further in time. The black holes were seven to twenty times larger than they were nine billion years ago, indicating that cosmic coupling could be responsible for their growth.

In other words, the research found that these black holes had accumulated mass over billions of years in a way that standard galaxy and black hole processes, such as mergers or gas accretion, could not easily explain.

In the second study, the team delved deeper into the possibility that cosmic coupling could account for the observed growth in black holes.

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.”

The team discovered that for cosmological coupling to explain the mass growth of black holes, the black holes in the first study must be less massive by a certain amount, since the mass growth of black holes from cosmological coupling depends on the size of the universe, which was smaller in the past. The study found that the mass growth of these black holes aligns with predictions for black holes that couple cosmologically and contain vacuum energy, produced by compressing matter as much as possible without contradicting Einstein’s equations and causing a singularity.

The researchers examined three sets of elliptical galaxies, each consisting of five distinct black hole populations, from times when the universe was about one-half and one-third of its current size. They discovered that k was around positive 3 for each comparison. In 2019, graduate student Croker and mathematics professor Joel Weiner at UH Mānoa predicted this value for black holes with vacuum energy rather than a singularity.

The implication is significant: if k equals 3, then the total contribution of all known black holes to the universe’s dark energy density is almost constant, as dark energy measurements suggest. The team used the latest measurements of the rate of earliest star formation from the James Webb Space Telescope and found that the results aligned with their findings.

The research indicates that the measured amount of dark energy in our universe corresponds to the combined vacuum energy of black holes formed in the deaths of the universe’s first stars, when singularities are absent.

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.”

According to the scientists, these studies offer a framework that can be used by theoretical physicists and astronomers to conduct further testing. They also suggest that current dark energy experiments, including the Dark Energy Spectroscopic Instrument and the Dark Energy Survey, could help to clarify this concept.

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?”

Journal References:

  1. 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
  2. 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
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