Strange physics at the edges of black holes may help solve the lingering ‘Hubble problem’

The rate of expansion of the universe is accelerating across the cosmos, driven by a mysterious force known as dark energy – but it may not be at the edges of black holes, according to new research.

Rather than implying that dark energy does not act on the boundaries of black holes, this idea suggests that this mysterious force dominates the universe. energy only playing at the events area of ​​interest.

The concept could help resolve a long-standing problem in cosmology known as the “Hubble Tension,” which arises from differing 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 boundaries, or “event mountains,” and the dark, energy-driven expansion of space may be even stranger and harder to understand than that we were afraid of.

Related: Forbidden black holes and ancient stars are hidden in these ‘little red dots’ (image)

This new idea was proposed 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 it is different than the rest of the cosmos, the black holes themselves are not growing because of this.

“The expansion rate of the universe at the exit horizon of each black hole is constant, but the size of the exit horizon, and therefore the black hole itself, does not increase as the universe expands,” Poplawski told “One may ask, How is it possible that the event horizon does not grow, but that space does? It is because the expansion of space puts points very close to the event horizon to move away from.”

Poplawski added that some people have suggested that black holes could be growing and increasing in mass without any material being credited due to the expansion of the universe. He argued that his findings show that this explanation for black hole growth, particularly as it relates to supermassive black holes that grew extremely rapidly early in the universe, is not valid.

Almost black holes?

Black holes were originally conceived by researchers as solutions to Einstein’s 1915 theory of gravity, known as general relativity, particularly by the German physicist and astronomer Karl Schwarzschild.

General relativity states that objects with mass cause the fabric of space and time, unified as a single entity called space-time, to “compact.” The greater the mass, the greater the density in space-time it generates. As gravity arises from this war, that explains why the more mass an object has, the stronger the gravitational influence it exerts on its surroundings.

Black holes are created from the idea that an infinite amount of mass is concentrated in an infinitesimal space, called a singularity. According to the equations of general relativity, this singularity, in which all physics would break down, would be bounded by an unphysical surface at which even light might not be able to move fast enough to escape. This is the event horizon, and its existence means that nothing escapes a black hole. Therefore, we cannot expect to “see” what is inside a black hole.

Because of the crunch of time around a black hole, we cannot expect 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 ‘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 final instant of that transformation. As if that concept wasn’t already strange enough, Poplawski thinks the events’ interests are even stranger: There’s dark energy, but space doesn’t seem to surround the horizon of the events ignore it.

telescope image of a black hole, shown as a fuzzy donut of yellow-orange light around a black center

telescope image of a black hole, shown as a fuzzy donut of yellow-orange light around a black center

“The expansion rate of the universe, the Hubble parameter, is constant and can be positive or zero on the departure from the black holes,” said Poplawski. “This must be so, because if the expansion rate of the universe at the departure horizon were not constant, the pressure and curvature of space-time would be infinite. That would not be measurable; therefore, it would be unphysical. “

Along with mind bending (and space bending) and Poplawski’s theory, it may solve a question that has been plaguing scientists for decades.

Related: Our expanding Universe: Age, history & other facts

Hubble trouble is not more?

In the late 1990s, two separate teams of astronomers used measurements of the distance to a Type Ia supernova to determine not only that the universe is expanding, as evidence collected by Edwin Hubble in the early 20th century showed, but that the 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 dark matter and everyday matter in the current era of the cosmos we live in is dominated by dark energy, which accounts 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, nothing is really simple in cosmology.

A bright point of light emits thin beams of light in the cardinal directions.  It is surrounded by a dense mass of gaseous nebula and the black star of space.A bright point of light emits thin beams of light in the cardinal directions.  It is surrounded by a dense mass of gaseous nebula and the black star of space.

A bright point of light emits thin beams of light in the cardinal directions. It is surrounded by a dense mass of gaseous nebula and the black star of space.

When the value of the cosmological constant is calculated from quantum field theory, the result is larger than what is obtained when we look at distant Type Ia supernovae and stars that alternate in brightness known as Cephid variables, both “standard candles” because of their convenience 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. Little wonder that some physicists call the cosmological constant “the worst prediction in the history of physics.”

This problem, known as the Hubble tension, has only gotten worse as quantum field theory and cosmology have improved and astronomy has become stronger; surprisingly, the values ​​have diverged.

The only way that both estimates of the Hubble parameter could be correct is if the rate of expansion of the universe did not progress evenly across the cosmos, with some regions expanding much faster than others.

One idea is that our galaxy, the Milky Way, is located in a rarefied “bubble” of the universe—a “Hubble bubble,” if you will—that interferes with local distance measures, causing them to deliver the value of the Hubble parameter low. On the other hand, quantum field theory is not limited to the local universe and considers the entire cosmos, delivering a high value averaged over the entire 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 in all areas of interest, but in other parts of the universe, it depends on the material 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 tension observed on Hubble.”


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Could Poplawski’s theory of universal expansion moving at a constant rate at event horizons be verified observationally with astronomy?

Sadly, he thinks that’s doubtful. Standard candles such as Type Ia supernovae and Cephid variable stars are not at the edge of the event horizon. This means that the astronomical methods of determining the Hubble parameter are useless in this case.

Plus, there’s that whole war thing and the fact that light can’t escape a black hole to think about. Perhaps the only way to measure the Hubble parameter here is a one-way trip into the black hole.

“Precisely, we cannot measure the Hubble parameter at the event horizon because as we see the black hole, the horizon is not yet formed,” said Poplawski. “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.

“However, they wouldn’t be able to send that information back to Earth, because nothing can escape from the event horizon into outer space.”

Poplawski believes, therefore, that unless a revolutionary method to measure the Hubble parameter comes, the secrets of black holes will remain shrouded in mystery.

Poplawski’s research appears in a pre-peer-reviewed paper on the preprint website arXiv.

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