Credits: NASA/ESA and G. Bacon (STScI)
Black holes are the dark figures of the universe, and many millions of them wander unseen in our galaxy alone. These cosmic heavyweights famously destroy anything that wanders too close, tearing stars and other objects apart with their immense gravitational pull.
But this may not be the end of the story of those doomed objects. A new study published on October 28 in Journal of Astroparticle Cosmology and Physics provides observational evidence that some of the matter falling into black holes could be converted into dark energy, the mysterious pressure that is accelerating the expansion of the universe and makes up 70% of the cosmos.
Related: What we know (and what we don’t know) about dark energy
This result depends on a strange possible property of black holes that form when a massive star runs out of fuel and collapses. Called “cosmological coupling,” this theory states that black holes are woven into the fabric of the cosmos in such a way that they grow as the universe expands – a feature noticeably absent from the standard definition of black holes, which says that they they can only gain mass by fusing or devouring other objects.
Cosmological coupling
It’s a bizarre concept that dates back to 1915. That year Albert Einstein introduced general relativity, a set of cosmic equations with some shocking implications for the behavior of the universe.
The equations have given rise to new phenomena such as black holes and the expansion of the universe, both of which are well observed. But since the equations are based on some factors that are not completely known (because we have not precisely defined all the properties of the universe), they can have several possible solutions depending on what those undefined values might be.
That’s how we I could achieve cosmological coupling: There are some conditions that could allow black holes to “couple” with cosmic expansion.
If black holes actually interacted with the evolving space-time around them in this way, gaining mass and energy proportionally as the universe expanded, dark energy could result entirely from the process.
Distribution of dark energy
This goes against standard thinking, which suggests that dark energy is evenly distributed throughout the cosmos.
“I would say this is the number one misconception,” says Kevin Croker, a research assistant at Arizona State University who conducted the recent study. “There is a very clear, if controversial, argument that it doesn’t matter where dark energy is located; it only matters that you have dark energy somewhere. So it could be uniformly scattered, or it could be in many tiny spots,” as would be the case if it were produced by black holes scattered across space and time.
To test the idea, a team of scientists analyzed tens of millions of distant galaxies measured by the Dark Energy Spectroscopic Instrument (DESI) installed at the Kitt Peak National Observatory near Tucson, Arizona.
By measuring the speed at which galaxies are receding at different distances, related to different cosmic ages, scientists have determined how the expansion rate of the universe has changed over time under the influence of dark energy. Then the team cross-referenced this data with information on how quickly black holes formed over the same enormous amount of time.
If their theory about black holes producing dark energy is correct, black hole formation and dark energy density should be related; as more black holes form, dark energy should increase in strength.
This is exactly what they saw.
Far from proven
But not everyone is convinced.
Prevailing theories about dark energy predict a so-called cosmological constant, in which the mysterious cosmic pressure has a constant strength, which neither grows nor weakens over time, and is uniformly spread throughout the cosmos. Dark energy expert Yun Wang, a senior researcher at Caltech IPAC, warns that measurements are not limited enough to strongly support the evolution of dark energy over time. And cosmologically coupled black holes might not even exist. “This is speculation at the moment,” he says. “So this theory is possible, but not yet plausible.”
Further studies could help shift the needle.
“The next question is to ask where the black holes are, adding their positions into the calculation,” Croker says. “There are all kinds of interesting surprises that can happen there, and we’re exploring that right now.”
Confirming the theory would redefine our understanding of the expansion of the universe and open up many new questions. But even if future observations disprove the theory, the research will still push the boundaries of current scientific knowledge and open the doors to new discoveries.
“This will only bring more depth and clarity to our understanding of dark energy, whether or not it continues to support the black hole hypothesis,” said Steve Ahlen, professor emeritus of physics at Boston University and co-author of the article, in a press release. “I think as an experimental effort it’s wonderful. You may or may not have preconceived notions, but we are guided by data and observations.
