From the gradual winding down of clocks to the death of stars, everything in the universe seems destined to eventually stop. However, one truly colossal exception defies this trend entirely: the Universe itself, which continues expanding at an ever-increasing rate.
Current physics suggests that this cosmic growth should actually be decelerating. Instead, observations reveal the opposite: the expansion is accelerating, driven by a force known as dark energy. This phenomenon poses a profound mystery.
One potential explanation for this lies in an unexpected place: black holes. Some scientists propose that these massive, dense objects may harbor dark energy, directly contributing to the Universe’s accelerating expansion.
"If black holes contain dark energy, they could be tied to the expansion of the universe, growing alongside it and causing acceleration," says astrophysicist Kevin Croker from Arizona State University. "While we can’t yet understand the mechanics of this process, we see strong evidence that it may be happening."
Although we don’t fully understand dark energy, estimates suggest it constitutes around 70% of the Universe's total matter-energy content.
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| A diagram showing the two distinct epochs of accelerated expansion. (Coldcreation/Wikimedia Commons, CC BY-SA 3.0) |
Intriguingly, the expansion rate we observe today may not have been consistent throughout cosmic history. According to prevailing theories, the Universe underwent an early period of rapid growth known as inflation right after the Big Bang, expanding from virtually nothing to a substantial size in a fraction of a second. It then slowed down for billions of years until around 5 billion years ago, when dark energy began to dominate, accelerating expansion once more.
Whatever forces influenced this cycle of inflation, deceleration, and subsequent acceleration had to overcome the extreme gravitational pull exerted by all matter concentrated at the beginning of time.
"If we ask, 'Where in the modern Universe do we observe gravity as intense as it was initially?' the answer is inside black holes," explains physicist Gregory Tarlé of the University of Michigan. "Perhaps, in a reverse of cosmic inflation, the gravitational collapse of massive stars generates dark energy – like a small Big Bang running 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)
The Secret Life of the Universe:
An Astrobiologist's Search for the Origins and Frontiers of Life
“A highly informative survey of the current state of astrobiology.” —Literary Review
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