For decades, scientists have puzzled over a major cosmic puzzle: why is the expansion of the universe accelerating, not slowing down, under the influence of gravity? The search for an answer has led groups of scientists around the world to search for an invisible force called “dark energy” that could explain the observation. New research may have finally given us the breakthrough we’ve been waiting for.
Two new papers published by a group of 17 international scientists offer the first observational evidence for the existence of a source of dark energy. According to the authors, after analyzing data spanning 9 billion years of cosmic evolution, the most likely answer is black holes — but not in the way you probably understand them.
“If the theory holds, then this is going to revolutionize the whole of cosmology because at last, we’ve got a solution for the origin of dark energy that’s been perplexing cosmologists and theoretical physicists for more than 20 years,” said in a statement research co-author Chris Pearson from the British company RAL Space.
The key to the discovery was tracking the rate of black hole growth relative to their position in the history of the universe. The researchers found that black holes embedded in ancient galaxies that formed in the early universe—which are now dead and thus don’t form new material to feed their black holes—were more massive than could be explained by the traditional methods of growth, which are eating stars and merging with other black holes. The researchers also found that the black holes were getting more massive in relative lockstep with the expansion of the universe. This, the researchers wrote, is known as “cosmological coupling.”
“We thus propose that stellar remnant black holes are the astrophysical origin of dark energy,” the authors wrote in the study.
This finding requires us to think about black holes a little differently than usual. Black holes are usually thought of as containing a gravitational singularity. The same idea suggests that instead of a singularity, there is vacuum energy inside a black hole. Vacuum energy stems from the idea that instead of being a complete void, the vacuum actually has a complex structure at the quantum level.
While this discovery certainly blows the imagination, it actually fits neatly into the current model of the universe. This means that there is no need to imagine some external force causing the universe to expand, and removes the claim that black holes must contain a gravitational singularity, which remains a burning problem in physics.
“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,” first author Duncan Farrah from the University of Hawaii, which led the research, said. “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.”