According to the prevailing inflationary universe theory, at the very beginning of the Big Bang, a mysterious energy drove an exponential expansion of the infant Universe and produced all known matter. That ancient energy shared key features of the current Universe’s dark energy. 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. 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.’ New research is strengthening the case for this scenario with recent data from the Dark Energy Spectroscopic Instrument (DESI).
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A view of the accretion disk around the supermassive black hole, with jet-like structures flowing away from the disk. The extreme mass of the black hole bends spacetime, allowing the far side of the accretion disc to be seen as an image above and below the black hole. Image credit: Science Communication Lab, DESY.
“If black holes contain dark energy, they can couple to and grow with the expanding Universe, causing its growth to accelerate,” said Dr. Kevin Croker, an astronomer at Arizona State University.
“We can’t get the details of how this is happening, but we can see evidence that it is happening.”
The 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, 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 Boston University’s Professor Steve Ahlen.
“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’.”
To search for evidence of dark energy from black holes, the astronomers used tens of millions of distant galaxies measured by DESI.
The instrument peers billions of years into the past and collects data that 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 researchers compared these data to how many black holes were being made in explosions 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 Dr. Duncan Farrah, a physicist at the University of Hawai’i.
“This makes it more plausible that black holes are the source of dark energy.”
The research complements a growing body of literature studying the possibility of cosmological coupling in black holes.
A 2023 study reported cosmological coupling in supermassive black holes within galactic centers.
That study 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,” said Dr. Brian Cartwright, an astrophysicist with Healthpeak Properties Inc. and former general counsel of the U.S. Securities and Exchange Commission.
“Our new paper links black holes to dark energy by when they are born.”
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 Arizona State University’s Professor Rogier Windhorst.
“The next question is where these black holes are, and how they have been moving around for the past 8 billion years. Scientists are working to constrain this right now,” Dr. Croker said.
Science demands more avenues of inquiry and observations, and now that DESI is online, this exploration for dark energy is just getting started.
“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,” Professor 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.
“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,” said University of Michigan’s Professor Gregory Tarlé.
The study was published in the Journal of Cosmology and Astroparticle Physics.
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Kevin S. Croker et al. 2024. DESI dark energy time evolution is recovered by cosmologically coupled black holes. JCAP 10: 094; doi: 10.1088/1475-7516/2024/10/094
This article was adapted from an original release by the University of Michigan.
