Question

What is black hole in-universe?

Answer 1

A black hole is a region of space once something enters it can never leave, not even light.

Explanation:

Albert Einstein published his field equations of General Relativity in 1915. Soon after Karl Schwarzschild solved the field equations for the vacuum around a massive body.

The Schwarzschild solution has a singularity where time stops and nothing can escape. It is called the Schwarzschild radius `r_s`.

`r_s = (2GM)/c^2`

Where `g` is the gravitational constant, `M` is the mass of the body and `c` is the speed of light.

This means that any mass which gets compressed to a radius below `r_s` is effectively isolated from the rest of the universe. Nothing which enters this radius can escape, not even light. This is a black hole.

The Sun is too small to collapse into a black hole, but we can calculate `r_s`. The mass of the Sun `M=1.989*10^30Kg`, `GM=1.327*10^20`, `c=299,792,458 m \/ s`. So, `r_s=2,953m`. This is less than 3 kilometers!

So, Schwarzschild predicted black holes in 1915. More recently objects have been discovered which are so small and massive that they can only be black holes.