The universe’s expansion rate is accelerating across the cosmos, driven by a mysterious force known as dark energy, but perhaps this acceleration isn’t happening at the boundaries of black holes, new research suggests.
Rather than implying that dark energy does not move around the boundaries of black holes, this idea suggests that this mysterious force that dominates the universe is, just energy plays games on event horizons.
The concept could help resolve a long-standing problem in cosmology called the “Hubble tension,” which stems from radically different estimates of the universe’s expansion rate and is known as the Hubble constant or Hubble parameter.
Perhaps even more important to non-theoretical physicists is that this research means that black holes, their outer boundaries or “event horizons,” and the expansion of space driven by dark energy may be stranger and harder to understand than we feared.
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This mind-boggling new idea was put forward by theoretical physicist Nikodem Poplawski of the University of New Haven in Connecticut, who said that while the space around black holes is expanding at a rate different from the rest of the cosmos, the black holes themselves are not growing because of it.
“At the event horizon of any black hole, the rate of expansion of the universe is constant, but the size of the event horizon, and therefore 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 is not growing, but the space there is growing? That’s 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 could grow and increase their mass without any accumulation of matter due to the expansion of the universe. He argued that his results show that this explanation of black hole growth is not valid, especially when applied to supermassive black holes that grow incredibly fast in the early universe.
Almost black holes?
Black holes were first thought of by researchers as a solution to Einstein’s theory of gravity, called general relativity, proposed in 1915, and the first person to propose this theory was the German physicist and astronomer Karl Schwarzschild.
General relativity states that objects with mass cause the fabric of space and time, combined into a single entity called spacetime, to become “distorted.” The greater the mass, the greater the distortion it creates in spacetime. Since gravity results from this distortion, this explains why the more mass an object has, the more intense the gravitational pull it exerts on its surroundings.
Black holes arise from the idea of an infinite amount of mass concentrated in an infinitely small space, known as a singularity. According to the equations of general relativity, this singularity, where all physics collapses, will be bounded by a non-physical surface beyond which not even light can travel fast enough to escape. This is the event horizon, and its existence means that nothing can escape a black hole. Therefore, we can never hope to “see” what is inside a black hole.
Because of the extreme warping of time around a black hole, we can never hope to see the event horizon itself.
“The event horizon forms after an infinite amount of time has passed on Earth,” Poplawski said. “What we observe are not black holes, but ‘nearly’ black holes.”
So when a star collapses at the end of its life and gives birth to a black hole, what we see is not a black hole, but the final moments of that transformation. As if that concept wasn’t weird enough, Poplawski thinks event horizons are even weirder: Dark energy is there, but the space around the event horizons seems to 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 expansion rate of the universe were not constant at an event horizon, the pressure and the curvature of space-time would be infinite. This would not be measurable; therefore, it would not be physical.”
As mind-boggling (and space-bending) as Poplawski’s theory is, it could actually solve a problem that has vexed scientists for decades.
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Is the Hubble problem gone?
In the late 1990s, two separate teams of astronomers measured the distance to Type Ia supernovae and determined that the universe was not only expanding, but also accelerating, as evidence gathered by Edwin Hubble in the early 20th century had shown.
The term “dark energy” was coined at the time to describe any aspect of the universe that drives this acceleration. Scientists have since determined that in the current era 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.
The simplest explanation for dark energy right now is the “cosmological constant,” a measure of the energy density of the vacuum. But, as you’ve probably realized by now, nothing in cosmology is ever truly simple.
When the value of the cosmological constant is calculated from quantum field theory, the result is larger than the value obtained when looking at distant Type Ia supernovae and stars with varying brightness called Cephid variables, known as “standard candles” because of their usefulness in measuring cosmic distances.
By some estimates, the difference between the two values is as much as 121 orders of magnitude—that is, 10 followed by 120 zeros. Not surprisingly, some physicists have called the cosmological constant “the worst prediction in the history of physics.”
This problem, called the Hubble tension, has gotten worse as quantum field theory and cosmology have developed and astronomy has become more robust; surprisingly, the values have continued to diverge.
The only way both estimates of the Hubble parameter could be correct is if the expansion rate of the universe were not uniform across the cosmos, with some regions expanding much faster than others.
One idea is that our galaxy, the Milky Way, is in a low-density “bubble” of the universe — a “Hubble bubble” if you like — and that this affects local distance measurements, causing them to give a low Hubble parameter value. Quantum field theory, on the other hand, is not limited to the local universe and considers the entire cosmos, so it gives a high value averaged over all space.
Poplawski’s hypothesis offers another way to think that certain parts of the universe could be accelerating at different rates.
“The expansion rate is the same across all event horizons, but in other parts of the universe, it depends on the matter there and the spatial curvature, so it’s different,” he explained. “Therefore, different parts of the universe have different expansion rates. This explains the observed Hubble stretch.”
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Can Poplawski’s theory that the universal expansion moves at a constant speed across the event horizons be confirmed by astronomical observations?
Unfortunately, he thinks this is doubtful. Standard candles such as Type Ia supernovae and Cephid variable stars do not exist at the edge of their event horizons. This means that astronomical methods of determining the Hubble parameter are pretty much useless in this case.
Also, the whole time warping thing and the fact that light can’t escape from a black hole have to be taken into account. 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 when we see the black hole, the horizon hasn’t formed yet,” Poplawski said. “However, an observer falling into the black hole will cross the event horizon in a finite time and theoretically could measure the Hubble parameter as it passes.
“However, since nothing can escape across the event horizon into space, they cannot send this information back to Earth.”
Poplawski therefore believes that unless a revolutionary method is found to measure the Hubble parameter, the closely guarded secrets of black holes will remain a mystery.
Poplawski’s research appears in a peer-reviewed paper on the preprint website arXiv.