UH Mānoa: Evidence mounts for dark energy from black holes

Astronomy researchers have found new evidence that can help answer one of the biggest mysteries in space: where dark energy comes from. The evidence from their new experiment points to black holes.

Scientists believe that about 14 billion years ago, during the Big Bang, an unidentified energy made the universe expand rapidly and created everything we know. Today, there’s still a similar mysterious energy called “dark energy” that makes up about 70% of the universe. But no one knows exactly what dark energy is or where it comes from.

The Big Bang started when ancient dark energy “fell apart” into hot matter. Black holes appear to be converting hot matter back into dark energy, like pressing it back together, almost like a mini Big Bang in reverse.

Scientists from five institutions including the University of Hawaiʻi at Mānoa revealed those findings in a new study published in the Journal of Cosmology and Astroparticle Physics.

The findings pair with recent research that identifies dark energy with black holes by their rate of growth while this research considers when the black holes were produced.

Almost 14 billion years ago, at the very beginning of the Big Bang, a mysterious energy drove an exponential expansion of the infant universe and produced all known matter, according to the prevailing Inflationary Universe theory.

That ancient energy shared key features of the current universe’s dark energy, which is the largest mystery of our time by at least one objective standard: It makes up the majority—roughly 70%—of the universe, but scientists don’t know exactly what it is.

“If you ask yourself the question, ‘Where in the later universe do we see gravity as strong as it was at the beginning of the universe?’ the answer is at the center of black holes,” said Tarlé, a professor at the University of Michigan and co-author of the study. “It’s possible that what happened during inflation runs in reverse, the matter of a massive star becomes dark energy again during gravitational collapse—like a little Big Bang played in reverse.”

The new study claims to strengthen the case for this scenario with recent data from the Dark Energy Spectroscopic Instrument (DESI): 5,000 robotic eyes mounted on the Mayall telescope at the Kitt Peak National Observatory on the land of the Tohono O’odham Nation.

“If black holes contain dark energy, they can couple to and grow with the expanding universe, causing its growth to accelerate” said Kevin Croker, the lead author of the team’s new study and who is both an UH affiliate faculty member and an assistant research scientist at Arizona State University. “We can’t get the details of how this is happening, but we can see evidence that it is happening.”  

Data from the first year of DESI’s planned five-year survey shows tantalizing evidence that the density of dark energy increased in time. This provides a compelling clue supporting this idea of what dark energy is, the researchers said, because that increase in time agrees with how the amount and mass of black holes increased in time.

“When I first got involved with the project, I was very skeptical,” said Steve Ahlen, professor at Boston University and co-author. “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.'”

The difference a DESI makes

To search for evidence of dark energy from black holes, the team used tens of millions of distant galaxies measured by DESI. Peering billions of years into the past, these data can be used to determine how fast the universe is expanding with exquisite precision. In turn, these data can be used to infer how the amount of dark energy is changing in time.

The team compared these data to how many black holes were being made in the deaths of large stars across the history of the Universe. “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,” said Duncan Farrah, associate professor of physics at the University of Hawai`i and co-author of the study. “This makes it more plausible that black holes are the source of dark energy.”  

This research complements a growing body of literature studying the possibility of cosmological coupling in black holes. A 2023 study that reported cosmological coupling in supermassive black holes within galactic centers, involving many of the authors on this paper, has encouraged other teams to search for the effect in black holes across all the different places they can be found in the Universe.  

“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.” said Brian Cartwright, astrophysicist, co-author and former General Counsel of the US Securities and Exchange Commission.  

A key difference in the new paper is that the majority of the relevant black holes are younger than those previously examined. These black holes were born in an epoch when star formation—which tracks black hole formation—was well underway, rather than just beginning.

“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” said Rogier Windhorst, co-author, interdisciplinary scientist for the James Webb Space Telescope, and regents’ and foundation professor at Arizona State University.

Science demands more avenues of inquiry and observations, and now that DESI is online, this exploration for dark energy is just getting started. “The next question is where these black holes are, and how they have been moving around for the past eight billion years. Scientists are working to constrain this right now,” said Croker.

“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,” Ahlen said. “I think as an experimental endeavor, it’s wonderful. You can have preconceived notions or not, but we’re driven by data and observations.”

Regardless of what those future observations bring, the work happening now represents a sea change in dark energy research, the team said.

“Fundamentally, whether black holes are dark energy, coupled to the universe they inhabit, has ceased to be just a theoretical question,” Tarlé said. “This is an experimental question now.”