Valerio Faraoni. Photo: Bishops’ University

Local physics prof talks black holes and the expanding universe

By William Crooks

Local Journalism Initiative

In a July 29 interview, Valerio Faraoni, a professor of theoretical physics at Bishop’s University, shared exciting developments in his research on black holes, furthering our understanding of these mysterious cosmic objects and their relationship with the expanding universe. Faraoni delved into the intricacies of his recent findings and their potential implications for cosmology.

Faraoni began by highlighting a recent collaboration with a colleague from Italy, culminating in a paper published just last week. This paper explores black holes in the context of an expanding universe, a topic that has intrigued scientists for decades. “The masses of black holes at the centres of galaxies increase with the expansion of the universe,” Faraoni said, summarizing his findings. This observation ties into a larger question about how black holes evolve as the universe itself expands and accelerates.

Black holes are found in various contexts within the universe. There are stellar mass black holes, with masses comparable to that of the sun, and supermassive black holes at the centres of galaxies. Faraoni noted that we have extensive observational evidence for these black holes, including gravitational waves and radio pictures of regions near black hole horizons. “This is not speculative stuff. This is pretty much established,” he emphasized.

To explain further, a stellar mass black hole is formed when a massive star collapses under its own gravity after exhausting its nuclear fuel. These black holes typically have a mass several times that of our sun. On the other hand, supermassive black holes reside at the centres of galaxies and can have masses ranging from millions to billions of times that of the sun. Gravitational waves are ripples in space-time caused by violent astrophysical processes, such as the collision of black holes. The horizon of a black hole, or event horizon, is the boundary beyond which nothing, not even light, can escape the gravitational pull of the black hole.

A central theme of Faraoni’s research addresses whether a black hole can remain static within an expanding universe. “We started with the calculation, writing down the equations to describe a horizon that is exactly static in a universe that expands,” he said. The results were unexpected and intriguing: a static black hole horizon in an expanding universe becomes a singularity, a point where space-time breaks apart and physical quantities go to infinity, which is not possible in physics.

To clarify, a static black hole horizon would mean that the boundary of the black hole remains unchanged over time, even as the universe around it expands. A singularity is a location in space-time where densities become infinite, and the laws of physics as we know them cease to function. This finding suggests that the existence of a static black hole horizon in an expanding universe would lead to these problematic singularities.

This leads to a crucial conclusion: a black hole’s horizon cannot remain completely static in an expanding universe. Faraoni noted, “This horizon becomes a singularity, and that’s not possible. Singularities should not exist, and we have seen the horizon through radio pictures.”

Faraoni’s work is grounded in both theoretical calculations and observational evidence. About two years ago, his team found some evidence that the mass of black holes at the centres of galaxies grows in a way that might explain away dark energy, a mysterious force driving the universe’s accelerated expansion. “We proposed that if the interior of black holes expands in a certain way, the… dark energy could be relegated inside the horizon of these black holes,” he explained. This hypothesis, though highly speculative, could potentially revolutionize our understanding of dark energy and black holes.

Dark energy is a form of energy that permeates all of space and tends to accelerate the expansion of the universe. Despite making up about 68 per cent of the universe, its exact nature remains one of the biggest mysteries in cosmology. Faraoni’s hypothesis suggests that if the interiors of black holes expand in a specific manner, they might be able to account for the effects attributed to dark energy.

The theoretical part of this work involves complex calculations. Faraoni described an experiment where light rays or small objects, when sent towards a static black hole horizon in an expanding universe, never reach the horizon. “These objects never get there. They stop because they cannot complete the motion,” he said. This impossibility indicates a fundamental problem with the concept of a static horizon in such a dynamic universe.

The next steps in Faraoni’s research involve exploring how black hole horizons evolve over time in an expanding universe. “Given what we know about the universe and how it expands now, we need to discover exactly how these horizons evolve,” he said. This work will require a combination of theoretical mathematics and observational data.

Faraoni emphasized the importance of observing the sky to find phenomena that match theoretical predictions. “If there is something like what we predicted, it will motivate further theoretical work,” he said. This interplay between theory and observation is crucial for advancing our understanding of the universe.

Faraoni reiterated that a particularly exciting aspect of his work is its potential to shed light on dark energy. Faraoni’s hypothesis that black holes could somehow encapsulate dark energy offers a tantalizing possibility for future research. “Explaining what is dark energy is the biggest question in theoretical physics,” he noted.

However, Faraoni remains cautious about this speculative idea. “This hypothesis needs to be scrutinized a million times,” he said. Despite the uncertainties, the possibility of connecting black holes and dark energy opens up new avenues for understanding the cosmos.