Researchers discover evidence of 'cosmological coupling' in black holes, shedding light on dark energy

Physicists from the University of Michigan, in collaboration with researchers from the University of Hawaii and other international institutions, have made a groundbreaking discovery providing evidence for "cosmological coupling" in black holes. 

"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," lead author Duncan Farrah, an astronomer from the University of Hawaii, said. "Black holes in Einstein's theory of gravity are the dark energy." 

Searching through extensive data spanning 9 billion years, a physicist from the University of Michigan, Gregory Tarlé, and a team of researchers from the University of Hawaii and multiple countries, have uncovered compelling evidence of "cosmological coupling" in black holes. This newly predicted phenomenon, derived from Einstein's theory of gravity, is only possible when black holes are situated within an evolving universe. The team's findings, detailed in two separate articles published in The Astrophysical Journal and The Astrophysical Journal Letters, provide crucial insights into the behavior of black holes and their connection to dark energy. In the first study, the researchers focused on supermassive black holes located at the centers of ancient and dormant galaxies. 

They discovered that these black holes gain mass over billions of years in a manner that cannot be easily explained by conventional processes like mergers or gas accretion. The second paper delves deeper into the growth of these black holes and demonstrates how their mass aligns with predictions for black holes that exhibit cosmological coupling and enclose vacuum energy. Vacuum energy is a material resulting from the compression of matter to the point of not breaking Einstein's equations, effectively avoiding a singularity.

This groundbreaking research challenges our existing understanding of black holes and their behavior. The study aimed to find observational evidence supporting a model for black holes that remains consistent regardless of the timescale. However, observing black holes over billions of years presents significant challenges due to their small size, distance, and limited observation timescales. The team realized that examining galaxies, which can have lifespans of billions of years and often contain supermassive black holes, held the key to understanding black hole evolution. 

The researchers focused on passively evolving elliptical galaxies, which are ancient and do not form many new stars. These galaxies were chosen because they contain trillions of old stars and very little gas, making them ideal candidates to study the mass changes in their central black holes. By observing only these galaxies, the team could argue that any variations in black hole masses could not be easily attributed to known processes. They then examined how the mass of the central black holes in these galaxies changed over the past 9 billion years.

The results revealed a significant trend: The black holes' masses were smaller in the past compared to their current sizes. In fact, the black holes today are 7 to 20 times larger than they were 9 billion years ago. These substantial changes supported the notion of cosmological coupling as the underlying cause. In the second study, the team delved deeper into the concept of cosmological coupling. They compared five different populations of black holes in three collections of elliptical galaxies, representing different stages of the universe's size. 

The measurements indicated that a variable called the coupling strength, denoted as k, was approximately positive 3. This value aligns with a prediction made four years prior by University of Hawaii astrophysicist Kevin Croker and mathematician Joel Weiner, suggesting that black holes containing vacuum energy contribute to a nearly constant dark energy density. The implications of these findings are profound. If the cosmological coupling theory is confirmed, it would mean that black holes continuously influence the universe's evolution and contribute significantly to dark energy. By accounting for the rate of earliest star formation, the researchers estimate that the black holes' growth over time could explain up to 70% of the energy in the universe today.

 "The question of the nature of dark energy is perhaps the most important unanswered question in contemporary physics" Gregory Tarlé said. "It's the majority, 70% of the energy of the universe. And now we finally have observational evidence of where it comes from, why 70%, and why it's here now. It's very exciting!" 

This groundbreaking research not only enhances our understanding of black holes but also provides a framework for further investigation by theoretical physicists and astronomers. Current and upcoming dark energy experiments, such as the Dark Energy Spectroscopic Instrument and the Dark Energy Survey, hold the potential to shed more light on this fascinating phenomenon. If confirmed, cosmological coupling could fundamentally transform our comprehension of the universe's evolution and the role black holes play within it.