The rate of expansion of the universe is accelerating across the cosmos, driven by a mysterious force known as dark energy — but maybe not at the edges of black holes, new research suggests.
Rather than implying that dark energy doesn't act at the boundaries of black holes, this idea suggests that this mysterious universe-dominating force is the only energy at play at event horizons.
The concept may help solve a longstanding problem in cosmology called the "Hubble tension," which arises from radically different estimations of the universe's rate of expansion, known as the Hubble constant, or the Hubble parameter.
Perhaps even more significantly to non-theoretical physicists, this research means that black holes, their outer boundaries, or "event horizons," and the dark energy-driven expansion of space could all be stranger and tougher to understand than we feared.
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This new mind-boggling idea has been suggested by theoretical physicist Nikodem Poplawski from the University of New Haven in Connecticut. He said that, even though the space around black holes is expanding, albeit differently than in the rest of the cosmos, the black holes themselves aren't growing because of this.
"The rate of expansion of the universe at the event horizon of every black hole is constant, yet the size of the event horizon, and thus black hole itself, does not increase as the universe expands," Poplawski told Space.com. "One may ask, How is it possible that the event horizon does not grow, but space there grows? It is because the expansion of space causes points very close to the event horizon to move away from it."
Poplawski added that some people have suggested that black holes might be growing and increasing their mass without any matter accretion due to the expansion of the universe. He argued that his results show that this explanation of black hole growth, especially as it applies to supermassive black holes that grew incredibly fast in the early universe, is not valid.
Almost black holes?
Researchers first conceived of black holes as solutions to Einstein's 1915 theory of gravity, called general relativity, were proposed, most notably by German physicist and astronomer Karl Schwarzschild.
General relativity states that objects with mass cause the very fabric of space and time, united as a single entity called space-time, to "warp." The larger the mass, the greater the warp in space-time it generates. As gravity arises from this warping, that explains why the more mass an object has, the more intense the gravitational influence it exerts on its surroundings.
Black holes are born from the idea of an infinite amount of mass concentrated in an infinitesimally small space, known as a singularity. According to the equations of general relativity, this singularity, where all physics breaks down, would be bounded by a non-physical surface at which not even light could move fast enough to escape. This is the event horizon, and its existence means nothing escapes a black hole. Thus, we can never hope to "see" what lies inside a black hole.
Because of the extreme warping of time around a black hole, we can also never hope to see the event horizon itself.
"The event horizon forms after an infinite time has elapsed on Earth," Poplawski said. "What we observe aren't black holes but 'almost black holes.'"
Thus, when a star collapses at the end of its life to birth a black hole, what we see is not the black hole but the final instant of that transformation. As if that concept were not already strange enough, Poplawski thinks that event horizons are even weirder: Dark energy exists there, but space around event horizons seems to just ignore it.
"The rate of the expansion of the universe, the Hubble parameter, is constant and can be either positive or zero at the event horizons of black holes," Poplawski said. "This must be the case, because if the rate of the expansion of the universe at an event horizon were not constant, the pressure and space-time curvature would be infinite. That would not be measurable; thus, it would be unphysical."
As mind-bending (and space-bending) as Poplawski's theory is, it could actually resolve an issue that has been troubling scientists for decades.
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Hubble trouble no more?
In the late 1990s, two separate teams of astronomers used measurements of the distance to Type Ia supernovas to determine that not only is the universe expanding, as evidence collected by Edwin Hubble showed in the early 20th century, but that expansion is also accelerating.
The term "dark energy" was coined at that time to describe whatever aspect of the universe is driving that acceleration. Since then, scientists have determined that in the current epoch of the cosmos we live in, dark energy dominates dark matter and everyday matter, accounting for about 68% of the energy and matter in the universe.
Currently, the simplest explanation for dark energy is the "cosmological constant," a measure of the energy density of the vacuum. However, as you've probably now realized, nothing is really simple in cosmology.
When the value of the cosmological constant is calculated from quantum field theory, the result is greater than what is obtained when we look at distant Type Ia supernovas and stars that alternate in brightness called Cephid variables, which are both known as "standard candles" because of their utility in measuring cosmic distances.
By some estimations, the difference between the two values is as great as 121 orders of magnitude — that is, 10 followed by 120 zeroes. It is little wonder that some physicists call the cosmological constant "the worst prediction in the history of physics."
This problem, referred to as the Hubble tension, has only grown worse as quantum field theory and cosmology have improved and astronomy has become more robust; surprisingly, the values have continued to diverge.
The only way that both estimations of the Hubble parameter could be correct is if the rate of expansion of the universe didn't proceed evenly across the cosmos, with some regions expanding much more rapidly than others.
One idea is that our galaxy, the Milky Way, is located in an underdense "bubble" of the universe — a "Hubble bubble," if you like — that affects local distance measures, causing them to deliver a low Hubble parameter value. On the other hand, quantum field theory isn't limited by the local universe and considers the whole cosmos, thus delivering a high value that is averaged across all of space.
Now, Poplawski's hypothesis offers another way in which certain regions of the cosmos could be accelerating at different rates.
"The rate of expansion is the same at all event horizons, but in other parts of the universe, it depends on the matter and spatial curvature there, so it is different," he explained. "Therefore, different parts of the universe have different rates of expansion. This explains the observed Hubble tension."
Could Poplawski's theory of universal expansion moving at a constant rate at event horizons be verified observationally with astronomy?
Sadly, he thinks that is doubtful. Standard candles like Type Ia supernovas and Cephid variable stars don't exist at the edge of event horizons. That means the astronomical methods of determining the Hubble parameter are pretty much useless in this case.
Additionally, there is that whole time-warping thing and the fact that light can't escape a black hole to consider. The only way to measure the Hubble parameter here may be to take a one-way trip into the black hole.
"Strictly speaking, we cannot measure the Hubble parameter at the event horizon because as we see the black hole, the horizon hasn’t formed yet," Poplawski said. "However, an observer falling into a black hole will cross the event horizon within a finite time and could theoretically measure the Hubble parameter while crossing it.
"However, they would not be able to send that information back to Earth, as nothing can escape from the event horizon to outer space."
Poplawski, therefore, believes that, unless a revolutionary method of measuring the Hubble parameter comes along, black holes' closely guarded secrets will remain shrouded in mystery.
Poplawski's research is featured in a pre-peer-reviewed paper on the preprint website arXiv.
