From the winding down of clocks to the death of stars, everything seems destined to eventually grind to a halt. But there’s one really, really big thing to which this doesn’t seem to apply, at all. That’s the Universe itself, getting bigger and bigger all the time.
According to our physical descriptions of how the cosmos should behave, that growth should be slowing down. Instead, measurements show it is speeding up, driven by a mysterious force known as dark energy. And it’s a dilly of a huge pickle.
But there’s something in the Universe that might explain it, some scientists argue: the massive, dense, growing concentrations of matter we call black holes.
“If black holes contain dark energy, they can couple to and grow with the expanding universe, causing its growth to accelerate,” says astrophysicist Kevin Croker of Arizona State University. “We can’t get the details of how this is happening, but we can see evidence that it is happening.”
We don’t know what it is, but calculations suggest whatever is responsible for the accelerated expansion it makes up an estimated 70 percent of the matter-energy distribution of the Universe.
What we see as expansion today also may not have always been consistent.
According to current theory, an early period of growth in the cosmos was the period of inflation. Just after the Big Bang, the Universe went from nothing to a fairly large something in a fraction of a second. Then it grew relatively slowly for a time, until about 5 billion years ago when expansion became dominated by dark energy.
Whatever caused the Universe to initially inflate, slow, and accelerate had to overcome the extreme gravity contained within a cosmic sum of matter squeezed into one spot.
“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,” explains physicist Gregory Tarlé of the University of Michigan.
“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 idea that black holes might be implicated in dark energy emerges from a relatively recently proposed concept called cosmological coupling, a proposal that emerges from attempts to resolve black hole weirdness with interpretations of general relativity.
According to the hypothesis, the extreme distortions of space associated with black holes is linked, or coupled, to the expansion of the Universe. As the Universe grows, so too do black holes; and as black holes grow, so does the Universe.
Last year, a team of researchers published a paper demonstrating the plausibility of cosmological coupling by studying the supermassive black holes at the centers of ‘dead’ galaxies. These are galaxies that have exhausted their fuel, both for growing new stars, and feeding the growth of the supermassive black hole.
Any growth demonstrated by the black holes in these galaxies could therefore not be attributed to normal growth mechanisms, but cosmological coupling. Finding there were indeed signs of growth, the team felt their idea was validated.
frameborder=”0″ allow=”accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” allowfullscreen>
In a new paper, Croker and his colleagues have investigated the link between black holes and dark energy – not as the black hole grows, but when it is born. They used the Dark Energy Spectroscopic Instrument to study the formation rate of black holes from the core collapse of massive stars at the end of their lifespan, much later in the lifetime of the Universe than when the supermassive black holes formed, and compared this with the expansion 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,” says physicist Duncan Farrah of the University of Hawai’i.
“This makes it more plausible that black holes are the source of dark energy.”
According to the theory of cosmological coupling, black holes convert normal matter into dark energy. The team’s calculations not only reproduced an expansion rate for the Universe consistent with current measurements, but provide an explanation for another problem: we have not been able to find all of the normal matter that should be in the Universe.
The rate of formation of black holes gives a dark energy conversion rate that is consistent with the amount of missing normal matter.
The work handily offers solutions to several outstanding questions in one fell swoop – pushing it towards the top of the pile for explanations for the mysterious force pushing the Universe apart.
“Fundamentally, whether black holes are dark energy, coupled to the universe they inhabit, has ceased to be just a theoretical question,” Tarlé says. “This is an experimental question now.”
The research has been published in the Journal of Cosmology and Astroparticle Physics.