Black holes linked to the universe’s expansion

Scientists have long pondered the nature of dark energy, which makes up roughly 70% of the universe yet remains largely mysterious. Recent studies are now linking this elusive force to black holes, hinting at a potential connection that may illuminate one of the universe’s deepest mysteries.

Dark energy has fascinated scientists since its discovery. It drives the universe's accelerating expansion and may share characteristics with the energy that spurred the Big Bang almost 14 billion years ago. This ancient energy expanded the infant universe, forming all known matter. Some researchers now speculate that dark energy might have a similar origin, possibly produced by black holes.

In an innovative study published in the Journal of Cosmology and Astroparticle Physics, a team led by researchers from Arizona State University is exploring this potential link using data from the Dark Energy Spectroscopic Instrument (DESI). Positioned on the Mayall telescope at Kitt Peak National Observatory, DESI’s 5,000 robotic lenses capture data from distant galaxies, gathering insight into the universe's expansion and the changes in dark energy over time.

Kevin Croker, lead author of the study and assistant research scientist at Arizona State University, explains, “If black holes contain dark energy, they can couple to and grow with the expanding universe, causing its growth to accelerate.” Though the exact mechanics remain unclear, Croker emphasizes that there is evidence of this phenomenon occurring.

The first year of DESI’s planned five-year survey reveals intriguing clues: the density of dark energy appears to have increased over time, aligning with the growth and mass of black holes. This correlation offers a glimpse into the nature of dark energy.

The researchers found that as new black holes formed from collapsing stars, dark energy grew at a corresponding rate. Duncan Farrah, associate professor of physics at the University of Hawai’i and co-author of the study, notes, “The two phenomena were consistent with each other.

As new black holes were made in the deaths of massive stars, the amount of dark energy in the universe increased in the right way. This makes it more plausible that black holes are the source of dark energy.”

This work builds on previous research investigating “cosmological coupling” in black holes, or the idea that black holes and the expansion of the universe are interconnected. A 2023 study by many of the same researchers examined supermassive black holes within galactic centers. Their findings encouraged scientists to explore this potential connection across different black holes throughout the universe.

Brian Cartwright, co-author and astrophysicist, explains, “Those papers investigate the link between dark energy to black holes by their rate of growth. Our new paper links black holes to dark energy by when they are born.” This distinction is crucial. In previous studies, supermassive black holes, which are much older, were the focus. Now, researchers are studying younger black holes that emerged when the formation of stars—and thus, black holes—was well underway.

Rogier Windhorst, an interdisciplinary scientist for the James Webb Space Telescope (JWST) and professor of earth and space exploration at Arizona State University, points out, “This occurs much later in the universe and is informed by recent measurements of black hole production and growth as observed with the Hubble and Webb space telescopes.”

For Croker, one of the essential questions now is where these black holes are and how they’ve moved over the last 8 billion years. Scientists are actively working to pinpoint their locations and track their journeys across the cosmos. Answering these questions will bring researchers closer to understanding the link between black holes and dark energy.

The DESI data, which offers unprecedented accuracy, is essential to this investigation. By examining tens of millions of distant galaxies, DESI enables scientists to study how the universe expands and how dark energy behaves over time. This new evidence from DESI not only strengthens the connection between black holes and dark energy but also challenges researchers to think differently about the role black holes may play in cosmic evolution.

Co-author Steve Ahlen, professor emeritus of physics at Boston University, initially approached the project with skepticism. “When I first got involved with the project, I was very skeptical. But I maintained an open mind throughout the entire process, and when we started doing the cosmology calculations, I said, ‘Well, this is a really nice mechanism for making dark energy,’” he said.

The collaboration and the mounting data are reshaping how scientists understand dark energy, marking a pivotal shift in the field. DESI’s precision enables researchers to observe dark energy on a level never before possible, suggesting that the black hole hypothesis could reshape our understanding of dark energy.

Ahlen adds, “This will only bring more depth and clarity to our understanding of dark energy, whether that continues to support the black hole hypothesis or not. I think as an experimental endeavor, it’s wonderful. You can have preconceived notions or not, but we’re driven by data and observations.”

Ultimately, this work heralds a new era for dark energy research. What was once solely a theoretical question has now transformed into a practical, experimental one. Gregory Tarlé, professor emeritus of physics at the University of Michigan and co-author of the study, underscores this point, saying, “Fundamentally, whether black holes are dark energy, coupled to the universe they inhabit, has ceased to be just a theoretical question. This is an experimental question now.”

As DESI continues its survey, researchers expect new discoveries that could either confirm or challenge the hypothesis that black holes produce dark energy. In either case, the findings promise to deepen our knowledge of the universe’s structure, evolution, and the forces that drive it forward.

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