Can the Interior of a Black Hole Be as Big as the Universe?

There is an interesting question that has puzzled both physics enthusiasts and laypeople alike: can the interior of a black hole be as vast as the universe? To explore this, it is essential to understand the nature of black holes and the principles of space-time in our universe.

Understanding Black Holes and Space-Time

When Dr. John Schonland took over as Director of AERE Harwell from Sir John (later Lord Cockcroft), the latter famously declared a nuclear particle to be as large as a barn, emphasizing the importance of precise terminology in scientific discourse. Similarly, the size and nature of a black hole are often misunderstood. According to Marios Christodoulou and Carlo Rovelli (2019), the black hole at the center of our galaxy is incredibly vast, spanning approximately 2 million kilometers across. However, as the speaker casually mentioned in the lecture during the Reactor School, this massive figure should be seen in context since the orbit of Mercury is roughly 60 times larger. This black hole is estimated to be around a billion years old and has been continuously growing throughout its existence, with a volume that could accommodate about a million solar systems!

Exploring the Interior of Black Holes

While the interior of a black hole appears to be almost incomprehensible to us, this is where intriguing questions about the fabric of space and time come into play. The inside of a black hole is governed by the principles of general relativity, a theory that revolutionized our understanding of gravity and the structure of the universe. Inside a black hole, the concept of space and time as we know them becomes distorted and can even become interchanged.

Consider the case of a non-dynamical or static black hole. Picture a black hole that is neither growing nor shrinking. Once you cross the event horizon, the singular point becomes the focal point, and time behaves differently. For an observer who enters the black hole, the singularity is reached after a finite amount of time. Yet, for the observer already at the singularity, the distance to the event horizon is experienced in the form of time. Thus, the separation in time becomes a separation in space, leading to a complicated yet fascinating interplay between time and space.

Some physicists argue that if a black hole were to last forever, the amount of space that collapses toward the singularity would be infinite. This idea is mathematically sound but faces significant challenges. First, black holes are not static; they grow and evolve over time due to the accretion of matter and energy. Hence, they will never achieve a state of infinite size. Second, the concept of multiple dimensions beyond the familiar three-dimensional space is complex and often abstract. It is as if the black hole has turned inside out, much like the surface of a deflating balloon.

Is There a Limit to Black Hole Size?

The size of a black hole is closely tied to its mass, and the more massive a black hole, the stronger its gravitational field. However, there is no theoretical limit to how much mass a black hole can contain. Theoretically, if the mass inside a black hole were equivalent to the mass outside it, it could potentially curve spacetime to an extreme, almost like a TARDIS from the sci-fi series Doctor Who - but taken to an extreme. In this scenario, the curvature of spacetime would be so intense that it could create the illusion of infinite space, although this remains purely speculative.

Recent studies in astrophysics and theoretical physics continue to shed light on the nature of black holes, and the question of their interior space continues to captivate our imagination. While the answer to whether the interior of a black hole can be as vast as the universe remains elusive, the pursuit of this question drives our understanding of the cosmos and the limitations of our scientific knowledge.