Image: MARK GARLICK via Getty Images

For decades, a major cosmic mystery has puzzled scientists: Why is the universe's expansion accelerating, rather than slowing down due to gravity? The search for an answer has led groups all over the world to look for an invisible force dubbed dark energy that could explain this observation. Now, new research may have finally given us just the breakthrough we've been waiting for. 

A pair of new papers published by a team of 17 international scientists offers the first observational evidence of a source for dark energy. According to the team, after poring over data covering 9 billion years of cosmic evolution, the most likely answer is black holes—but not how 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," study co-author Chris Pearson, from RAL Space in the UK, said in a statement.

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 conclusion requires us to think of black holes a little differently than we normally might. Typically, black holes are envisioned as containing a singularity, where even gravity breaks down. What this idea assumes is that instead of a singularity, black hole interiors contain vacuum energy. Vacuum energy stems from the idea that, rather than a vacuum being a total void, it actually has a complex structure on the quantum scale. 

As an American Astronomical Society blog on the new research explains: "Traditional singularity-containing black holes would have a coupling strength of 0, while vacuum-energy black holes would have a coupling strength of 3. Ultimately, the team found the coupling strength to be around 3.11, and they ruled out the possibility of zero coupling at 99.98% confidence."

While this discovery is certainly mind-blowing, it actually fits neatly into existing models of the universe. It means there is no need to conceive of some external force causing the universe to expand, and it does away with the requirement that black holes contain singularities, which remain a thorny 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 Hawai`i, 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."