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 % of the matter-energy distribution of the Universe.

What we see as expansion today also may not have always been consistent

A diagram showing the two distinct epochs of accelerated expansion. (Coldcreation/Wikimedia Commons, CC BY-SA 3.0)

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 notion that black holes might be linked to dark energy emerges from a relatively new theory known as cosmological coupling. This concept attempts to reconcile the unique properties of black holes with general relativity.

According to this hypothesis, black holes' extreme distortions of space could be coupled to the Universe's expansion. As the Universe expands, so might black holes; and as black holes grow, they could drive cosmic expansion.

A study published last year provided compelling evidence supporting cosmological coupling. Researchers analyzed supermassive black holes at the centers of "dead" galaxies—galaxies that no longer form new stars and whose supermassive black holes no longer grow through typical means. Any observed growth in these black holes could not be due to conventional factors, leaving cosmological coupling as the likely explanation.

Their findings suggested that these black holes were indeed growing, reinforcing the plausibility of the concept.

In a recent study, Croker and his team explored a deeper link between black holes and dark energy, focusing on black hole formation.

Using the Dark Energy Spectroscopic Instrument, they analyzed the rate at which black holes form through the core collapse of massive stars, which occurs much later in the Universe's lifetime compared to the formation of supermassive black holes. They then compared these formation rates to the expansion rate of the Universe.

"The two phenomena aligned," says physicist Duncan Farrah of the University of Hawai'i. "As new black holes formed from dying massive stars, the amount of dark energy increased accordingly. This lends credibility to the idea that black holes could indeed be the source of dark energy."

Cosmological coupling theory suggests that black holes could convert ordinary matter into dark energy. The researchers’ calculations aligned with current measurements of cosmic expansion and offered a solution to another conundrum: the missing “normal” matter that theoretical models predict but we haven’t yet found.

The rate at which black holes form implies a conversion rate to dark energy that matches the amount of unaccounted normal matter, potentially solving multiple outstanding puzzles at once and placing this explanation high among potential answers to the dark energy mystery.

“Ultimately, the question of whether black holes are linked to dark energy, tied to the universe itself, is no longer just a theoretical matter," Tarlé says. "It’s become an experimental question."

The implications of this research could fundamentally alter our understanding of the Universe and its eventual fate.

Study: DESI Dark Energy Time Evolution is Recovered by Cosmologically Coupled Black Holes (DOI: 10.1088/1475-7516/2024/10/094)

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