The rate at which the universe is expanding is accelerating across the cosmos, driven by a mysterious force known as dark energy. However, new research suggests that this may not be the case at the edges of black holes.

Rather than suggesting that dark energy does not act at the boundaries of black holes, this idea suggests that this mysterious, universe-dominating force *only energy* that occur at event horizons.

The concept could help solve a long-standing problem in cosmology, the “Hubble tension.” This tension arises from radically different estimates of the rate of expansion of the universe, also known as the Hubble constant or the Hubble parameter.

For non-theoretical physicists, this research is perhaps even more important: black holes, their outer boundaries, or “event horizons,” and the expansion of space caused by dark energy may all be stranger and harder to understand than we feared.

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This new, mind-boggling idea was proposed by theoretical physicist Nikodem Poplawski of the University of New Haven in Connecticut. He said that while space around black holes is expanding, albeit differently than the rest of the cosmos, the black holes themselves are not growing as a result.

“The rate of expansion of the universe at the event horizon of each black hole is constant, but 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 expand, but the space there does? That 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 may grow and increase their mass without any accretion of matter by 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?

Scientists saw black holes as solutions to Einstein’s 1915 theory of gravity, general relativity, and the German physicist and astronomer Karl Schwarzschild in particular came up with this theory.

General relativity states that objects with mass cause the fabric of space and time, united as a single entity called space-time, to “warp.” The greater the mass, the greater the curvature in space-time it generates. Since gravity is created by this curvature, this 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 infinitely small space, known as a singularity. According to the equations of general relativity, this singularity, where all physics collapses, would be bounded by a nonphysical surface that even light cannot travel fast enough to escape. This is the event horizon, and its existence means that nothing escapes a black hole. Therefore, we can never hope to “see” what is inside a black hole.

Because time around a black hole is extremely curved, we can never count on seeing the event horizon itself.

“The event horizon forms after an infinite amount of time has passed on Earth,” Poplawski said. “What we’re observing are not black holes, but ‘almost black holes.'”

So when a star collapses at the end of its life and forms a black hole, we don’t see the black hole, we see the final moment of that transformation. As if that concept wasn’t strange enough, Poplawski thinks event horizons are even stranger: dark energy exists there, but the space around event horizons just seems to ignore it.

“The rate of expansion 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 at an event horizon were not constant, the pressure and the space-time curvature would be infinite. That would not be measurable; it would be unphysical.”

As mind-boggling (and space-expanding) as Poplawski’s theory is, it could actually solve a problem that scientists have been struggling with for decades.

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## No more problems with Hubble?

In the late 1990s, two separate teams of astronomers used measurements of the distance to Type Ia supernovae to determine that the universe is not only expanding, as evidence gathered by Edwin Hubble in the early 20th century showed, but that the expansion is also accelerating.

The term “dark energy” was coined at the time to describe what aspect of the universe is driving this acceleration. Scientists have since determined that in the current era of the cosmos in which we live, 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. But as you probably realize by now, nothing in cosmology is really simple.

When the value of the cosmological constant is calculated using quantum field theory, the result is larger than what is obtained when we consider distant Type Ia supernovae and stars that fluctuate in brightness, called Cephid variables. These stars are both called “standard candles” because of their usefulness in measuring cosmic distances.

By some estimates, the difference between the two values is as large as 121 orders of magnitude — that is, 10 followed by 120 zeros. It’s no wonder that some physicists are calling the cosmological constant “the worst prediction in the history of physics.”

This problem, called the Hubble voltage, has only gotten worse as quantum field theory and cosmology have improved and astronomy has become more robust. Surprisingly, however, the values have continued to diverge.

The only way both estimates of the Hubble parameter could be correct is if the expansion of the universe were not uniform across the universe, with some regions expanding much faster than others.

One idea is that our galaxy, the Milky Way, is in an under-dense “bubble” of the universe — a “Hubble bubble,” if you will — that biases local distance measurements, giving them a low Hubble parameter value. Quantum field theory, on the other hand, is not constrained by the local universe and considers the entire cosmos, giving a high value that is averaged over all of space.

Poplawski’s hypothesis offers another way in which certain parts of the universe could accelerate at different rates.

“The expansion rate 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. “That’s why different parts of the universe have different expansion rates. This explains the observed Hubble stress.”

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Can Poplawski’s theory that the universal expansion moves at a constant speed along the event horizons be verified by observations using astronomy?

Unfortunately, he thinks this is doubtful. Standard candles such as Type Ia supernovae and Cephid variable stars do not exist at the edge of event horizons. That means that astronomical methods for determining the Hubble parameter are virtually useless in this case.

Then there’s the whole time-warping thing and the fact that light can’t escape a black hole. The only way to measure the Hubble parameter here might 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 the horizon hasn’t formed yet when we see the black hole,” Poplawski said. “However, an observer falling into a black hole passes through the event horizon in a finite amount of time and could theoretically measure the Hubble parameter as he passes through it.

“But they can’t send that information back to Earth because nothing can escape from the event horizon into space.”

Poplawski is therefore convinced that unless a revolutionary method for measuring the Hubble parameter emerges, the well-kept secrets of black holes will remain shrouded in mystery.

Poplawski’s research can be found in a pre-peer-reviewed article on the preprint website arXiv.