The rate of expansion of the universe is accelerating throughout the universe, driven by a mysterious force known as dark energy — but perhaps not at the edges of black holes, new research suggests.
Rather than suggesting that dark energy does not operate at the boundaries of black holes, this idea suggests that this mysterious force dominating the universe is only energy in the event horizon game.
The concept may help resolve a long-standing problem in cosmology called the “Hubble tension,” which arises from radically different estimates of the universe’s expansion rate, known as the Hubble constant or Hubble parameter.
Perhaps even more significantly for non-theoretical physicists, the research means that black holes, their outer boundaries or “event horizons,” and the dark energy-driven expansion of the universe may be stranger and more difficult to understand than we feared.
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This stunning new idea was proposed by theoretical physicist Nikodem Poplawski of 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 universe, the black holes themselves are not growing because of it.
“The expansion rate of the universe at the event horizon of each black hole is constant, yet the size of the event horizon, and thus the black hole itself, does not increase as the universe expands,” Poplawski told Space.com. “Someone may ask: How is it possible that the event horizon is not growing, but the space there is growing? This 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 can grow and increase in mass without any mass accretion due to the expansion of the universe. He claimed that his results show that this explanation for black hole growth, especially as it relates to supermassive black holes that grew incredibly fast in the early universe, is not valid.
Almost black holes?
Scientists first introduced black holes as a solution to Einstein’s 1915 theory of gravity, called general relativity, which was primarily proposed by the German physicist and astronomer Karl Schwarzschild.
General relativity states that objects with mass cause the very fabric of space and time, unified as a single entity called space-time, to “warp”. The larger the mass, the larger the space-time warp it generates. How gravity arises from this distortion explains why the more mass an object has, the more intense the gravitational influence it has on its surroundings.
Black holes are born from the idea of ​​an infinite amount of matter concentrated in an infinitesimal space, known as a singularity. According to the equations of general relativity, this singularity, where all physics breaks down, would be bounded by an unphysical surface where even light could not travel fast enough to escape. This is the event horizon, and its existence means that nothing escapes the black hole. So we can never hope to “see” what lies inside a black hole.
Also, due to the extreme curvature of time around a black hole, we can never hope to see the event horizon itself.
“The event horizon is formed after an infinite amount of time has passed on Earth,” Poplawski said. “What we observe are not black holes, but ‘almost black holes’.”
So when a star collapses at the end of its life to give birth to a black hole, what we see is not a black hole, but the final moment of that transformation. As if the concept wasn’t weird enough, Poplawski thinks event horizons are even weirder: dark energy exists, but the space around event horizons seems to simply ignore it.
“The expansion rate of the universe, the Hubble parameter, is constant and can be either positive or zero at the event horizon of black holes,” Poplawski said. “It has to be, because if the expansion rate of the universe over the event horizon wasn’t constant, the pressure and curvature of spacetime would be infinite. That wouldn’t be measurable, so it would be unphysical.”
As mind-bending (and space-bending) as Poplawski’s theory is, it might actually solve a problem that has plagued scientists for decades.
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No more problems with Hubble?
In the late 1990s, two separate teams of astronomers used distance measurements to Type Ia supernovae to determine that the universe was not only expanding, as evidence gathered by Edwin Hubble in the early 20th century had shown, but that the expansion was also accelerating.
The term “dark energy” was coined at the time to describe any aspect of the universe that drives this acceleration. Since then, scientists have found that in the current epoch of the universe 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 have probably realized by now, there is nothing simple about cosmology.
When the value of the cosmological constant is calculated from quantum field theory, the result is larger than what we get when we look at distant Type Ia supernovae and stars that alternate in brightness called Cepheid variables, both of which are known as “standard candles” because their utility in measuring cosmic distances.
According to some estimates, the difference between the two values ​​is as much as 121 orders of magnitude – that is, 10 followed by 120 zeros. No wonder some physicists call the cosmological constant “the worst prediction in the history of physics.”
This problem, referred to as the Hubble tension, has only gotten worse as quantum field theory and cosmology have improved and astronomy has become more robust; surprisingly, the values ​​continue to diverge.
The only way that both estimates of the Hubble parameter could be correct is that the rate of expansion of the universe would not be uniform across the universe, with some regions expanding much faster than others.
One idea is that our galaxy, the Milky Way, is in an underdense “bubble” of space—the “Hubble bubble,” if you will—that affects local distance measurements, causing them to give a low value for the Hubble parameter. On the other hand, quantum field theory is not limited by the local universe and takes the entire universe into account, thus providing a high value that is averaged across the entire universe.
Now, Poplawski’s hypothesis offers another way that certain regions of the universe 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 mass and spatial curvature there, so it’s 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 by observations with astronomy?
Unfortunately, he thinks it’s doubtful. Standard candles like Type Ia supernovae and Cephid variable stars do not exist at the edge of the event horizon. This means that astronomical methods of determining the Hubble parameter are essentially useless in this case.
Plus there’s the whole time warp thing and the fact that light can’t escape from a black hole. The only way to measure the Hubble parameter here might be a one-way trip to the black hole.
“Strictly speaking, we can’t 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 at a finite time and could theoretically measure the Hubble parameter as it passes through.
“However, they would not be able to send this information back to Earth because nothing can escape from the event horizon into space.”
Poplawski therefore believes that unless a revolutionary method of measuring the Hubble parameter comes along, the closely guarded secrets of black holes will remain shrouded in mystery.
Poplawski’s research is featured in a peer-reviewed article on the arXiv preprint website.