The rate of expansion of the universe is accelerating across the cosmos, driven by a mysterious force known as dark energy – but perhaps not at the edges of black holes, a new study suggests.
Rather than implying that dark energy does not act at the boundaries of black holes, this idea suggests that this mysterious force that dominates the universe is only energy at play in event horizons.
The concept could 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, this research means that black holes, their outer limits or “event horizons” and the dark energy expansion of space may all be stranger and more harder to understand than we feared.
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This surprising new idea was suggested by theoretical physicist Nikodem Poplawski of the University of New Haven in Connecticut. He said that, although the space around black holes is expanding, although unlike the rest of the cosmos, the black holes themselves are not growing because of it.
“The rate of expansion of the universe at the event horizon of any 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. “One might ask, how is it possible that the event horizon does not grow, but the space there does? 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 accretion of matter 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 extremely fast in the early universe, is not valid.
Almost black holes?
Researchers first conceived of black holes as a solution to Einstein’s 1915 theory of gravity, called general relativity, was proposed, most notably by the 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 greater the mass, the greater the distortion in space-time it creates. As gravity arises from this distortion, it explains why the more mass an object has, the stronger the gravitational influence it exerts on its surroundings.
Black holes arise from the idea of an infinite mass 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 that not even light could move fast enough to escape. This is the event horizon, and its existence means that nothing escapes a black hole. Thus, we can never hope to “see” what is inside a black hole.
Because of the extreme time distortion around a black hole, we also 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 the black hole, but the last moment of that transformation. As if this concept weren’t strange enough, Poplawski thinks event horizons are even stranger: dark energy is there, but the space around the event horizon seems to simply ignore it.
“The expansion rate of the universe, the Hubble parameter, is constant and can be positive or zero at the event horizons of black holes,” Poplawski said. “This must be the case, because if the rate of expansion of the universe over an event horizon were not constant, the pressure and curvature of space-time would be infinite. This would not be measurable; thus, it would be non-physical.”
As ingenious (and space-bending) as Poplawski’s theory is, it may actually solve an issue that has puzzled scientists for decades.
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Hubble no more problems?
In the late 1990s, two separate teams of astronomers used distance measurements of type Ia supernovae to determine that not only is the universe expanding, as evidence gathered by Edwin Hubble in the early 20th century showed, but that expansion is also accelerating.
The term “dark energy” was coined at the time to describe whatever aspect of the universe is driving that acceleration. Since then, scientists have determined that in the current age of the cosmos we live in, dark energy dominates dark matter and everyday matter, making up 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, nothing is really simple in cosmology.
When the value of the cosmological constant is calculated from quantum field theory, the result is larger than that obtained when looking at distant Type Ia supernovae and stars that alternate in brightness called Cepheid variables, which are known as “standard candles” because they their usefulness in measuring cosmic distances.
According to some estimates, the difference between the two values is as large 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 powerful; surprisingly, the values have continued to change.
The only way that both estimates of the Hubble parameter could be correct is if the rate of expansion of the universe did not proceed evenly throughout the cosmos, with some regions expanding much faster than others.
One idea is that our galaxy, the Milky Way, is located in a low-lying “bubble” of the universe—a “Hubble bubble,” if you will—that affects local distance measures, causing them to give a value of low of the Hubble parameter. On the other hand, quantum field theory is not limited by the local universe and takes the entire cosmos into consideration, thus giving a high value that is averaged across space.
Now, Poplawski’s hypothesis offers another way in which some 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 the 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.”
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Can Poplawski’s theory of universal expansion moving at a constant rate across event horizons be observationally verified with astronomy?
Alas, he thinks this is doubtful. Standard candles such as Type Ia supernovae and Cepheid variable stars do not exist at the edges of the event horizon. This means that astronomical methods of determining the Hubble parameter are almost useless in this case.
Plus there’s that whole time warp thing and the fact that light can’t escape a black hole to consider. The only way to measure the Hubble parameter here would be to take 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 within a finite time and could theoretically measure the Hubble parameter as it crosses it.
“However, they would not be able to send that information back to Earth, as nothing can escape the event horizon in outer 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 presented in a peer-reviewed paper on the preprint website arXiv.