Question Discussion: Why can't a black hole engulf the entire universe? What limits its gravitational reach?

Question URL at ResearchGate

Question Resolved: I acknowledge the question regarding the potential for a black hole to engulf the entire universe and the limits of its gravitational reach. It's important to clarify that mass compression increases gravitational density rather than gravitational reach. For example, when a star's mass is compressed to form a black hole, the resulting mass density reaches nuclear matter density. This immense density increases the gravitational strength near the event horizon but does not extend its gravitational influence indefinitely. The diminishing influence with distance and the gravitational boundaries within galaxies collectively limit the reach of a black hole's gravitational pull. Therefore, while the gravitational strength near the singularity is extreme, it does not imply an infinite gravitational reach capable of engulfing the entire universe.

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Since a singularity has properties that reach infinity, one might assume its gravitational influence is also infinite. When a star collapses into a black hole, its mass is compressed into an extremely small volume, leading to a significant increase in gravitational strength near the singularity. For example, a star with a mass of approximately 3.978 ×10³⁰ kg compressed into a volume of 10⁻⁸ m³ results in a gravitational strength multiplier of 3.978 ×10³⁸. This enormous increase in gravitational strength raises the question: why doesn't this immense gravitational pull extend to engulf the entire universe?

However, several factors limit a black hole's gravitational reach:

1. Distance and Diminishing Influence: 

Typically, the gravitational influence of a black hole, like any mass, diminishes with distance. While the gravitational pull is extremely strong near the event horizon, it weakens as one moves farther away. However, with a gravitational strength multiplier of 3.978 ×10³⁸, the black hole could engulf enough matter within its reach and correspondingly increase its gravitational strength further. This suggests that the black hole could potentially extend its gravitational reach indefinitely, challenging the idea of diminishing influence over distance.

2. Cosmic Expansion: 

The effect of cosmic expansion is not applicable to a black hole within a galaxy, as dark energy's influence is negligible within galactic scales. A black hole cannot be isolated from a galaxy unless it engulfs the entire galaxy, so cosmic expansion does not apply within this context.

3. Gravitational Boundaries: 

While black holes are typically found within galaxies, their gravitational influence near the event horizon is immense. The gravitational boundaries of their host galaxies pose limits, but with a gravitational strength multiplier as high as 3.978 ×10³⁸, the black hole could potentially overcome these boundaries by engulfing enough mass to further increase its gravitational pull indefinitely.

Despite the singularity's extreme gravitational strength near its event horizon and the potential for an ever-increasing gravitational pull, the limits posed by the gravitational boundaries of galaxies and cosmic structures may be challenged by the immense gravitational strength of a black hole, suggesting the possibility of extending its reach indefinitely.