# The surprising behavior of black holes in an expanding universe

The behavior of black holes in an expanding universe has captured the attention of physicists, revealing fascinating insights into the nature of these enigmatic cosmic entities. Recent research, published on the arXiv preprint server, has shed light on the surprising implications of Einstein's equations when applied to black holes within an expanding universe. This was reported by SSPDaily.

Nikodem Popławski, a Distinguished Lecturer at the University of New Haven, conducted a study that uncovered a compelling conclusion. According to Popławski's findings, the rate of the universe's expansion at the event horizon of every black hole must be a constant. This implies that dark energy, also known as the cosmological constant, is the only energy present at the event horizon.

The significance of this discovery lies in the avoidance of unphysical scenarios. If the pressure of matter and the curvature of spacetime were to be infinite at a black hole's horizon, it would violate physical principles. With the observation that the expansion rate remains constant at all event horizons, despite variations in mass or other properties, a deeper understanding of black hole behavior in an expanding universe emerges.

Black holes possess a unique quality – while remarkably simple in terms of their properties (mass, electric charge, and angular momentum), their existence gives rise to extraordinary characteristics. An event horizon, a nonphysical surface surrounding a black hole at a critical distance, separates the region from which nothing can escape, even light itself.

The inception of black holes dates back to 1916 when Karl Schwarzschild, a German soldier suffering from pemphigus, predicted their existence. Utilizing Einstein's equations of general relativity, Schwarzschild envisioned these massive, nonrotating, perfectly round objects within an empty and unchanging universe. He was the first to recognize the event horizon, a boundary proportional to a black hole's mass that marks the point of no return for objects within it.

Interestingly, Schwarzschild's calculations unveiled an apparent singularity at the center of these black holes, potentially indicating a breakdown in our understanding of gravity as described by Einstein's laws. Modern studies have revealed that supermassive black holes likely reside at the centers of most galaxies. For instance, Sagittarius A* encapsulates the heart of our Milky Way, boasting a mass over four million times that of the sun. The first direct image of a black hole was obtained in 2019, depicting a black spot surrounded by a luminous halo, located in Messier 87, approximately 55 million light-years away.

Building upon Schwarzschild's groundwork, Popławski explored the behavior of black holes in a more nuanced context: an expanding universe. Adopting a model proposed by the British mathematician and cosmologist George McVittie in 1933, he investigated the structure of spacetime around a massive, centrally symmetric object. This formulation suggested that near the mass, spacetime mimics Schwarzschild's solution, featuring an event horizon. In contrast, at larger distances, akin to our current universe, the cosmological expansion becomes apparent. The rate of expansion at the event horizon, determined by the Hubble parameter, was found to be a constant, with a direct relationship to the cosmological constant or dark energy density.

Notably, this concept aligns with the observed Hubble tension, a notable disparity between different measurements of the Hubble parameter. Depending on whether late-universe or early-universe techniques are employed, these approaches yield divergent results. Popławski suggests that this tension arises naturally from an accurate analysis of black hole spacetime within Einstein's general theory of relativity in an expanding universe.

Moreover, the implications of black hole horizons expanding at different rates necessitate a positive value for the cosmological constant. Sailentally, without this constant, a closed universe would exhibit oscillatory behavior rather than the observed current acceleration. These findings provide the simplest explanation for the observed acceleration of the universe.

While the surface boundary of a star also experiences the expansion of the universe, gravitational and electromagnetic forces counteract the expansive forces, preventing the star's physical expansion. In contrast, an event horizon entirely consists of points in space, lacking matter or energy. Consequently, a constant rate of expansion within the event horizon is not unexpected. Importantly, the event horizon itself, along with the black hole, does not experience expansion, as points of space exterior to the event horizon move away from it.

Real black holes may have rotation, but if the rate of rotation is slow, Popławski's conclusions can be reasonably applied. Nevertheless, measuring the Hubble parameter at an event horizon currently presents technical challenges that require further developments in observational techniques.

Interestingly, even though an observer at the event horizon could potentially measure the Hubble parameter, they would never be able to communicate their findings beyond the event horizon's grasp. Falling beyond the event horizon renders any exchange of information with the wider universe impossible. Remarkably, these conclusions relate to Popławski's earlier hypothesis, proposed in 2010, suggesting that every black hole is indeed a wormhole – an Einstein-Rosen bridge – connecting to an entirely new universe beyond its event horizon.

"The event horizon is a doorway from one universe to another," Popławski asserts. "This doorway does not grow with the expansion of the universe. If this occurs for the event horizon of the black hole forming a universe, it should also work for the event horizons of other black holes in that universe."

By delving into the behavior of black holes in an expanding universe, scientists continue to unravel the mysteries and intricate dynamics of these celestial objects. Popławski's work pushes the boundaries of understanding, shedding light on the significant role played by dark energy and providing profound insights into the nature of our universe.