Can a black hole be a wormhole or can a wormhole exist within a black
No. A black hole cannot be a wormhole. But, yes, a wormhole might exist inside a black hole. The "might" part results from the fact that our understanding of black holes is theoretical. There's nothing wrong with a good theoretical understanding, but it helps if it can be validated empirically.
The most likely candidate for a black hole with a wormhole in its interior is the so-called Kerr black hole.
A Kerr black hole is a type of black hole that possesses only mass
and angular momentum (but not electrical charge – the third possible
property of a black hole). In other words, a Kerr black hole is an
uncharged black hole that rotates about a central axis. It is named
after the New Zealand mathematician Roy Kerr who, in 1963, became the
first person to solve the field equations of Einstein's general theory
of relativity for a situation of this kind.
Kerr black holes are probably the commonest in nature, since the
massive stars from which they typically form possess rotation (but no
overall charge) before they collapse at the end of their lives. By the
principle of conservation of angular momentum, much of this spin is
then retained by the black hole following the star's terminal
A Kerr black hole has the following distinct regions:
ring singularity inner and outer event horizons ergosphere static
limit (the boundary between the ergosphere and normal space)
Inside a Kerr black hole
At each event horizon the roles of space and time are reversed; so, in
the case of a Kerr black hole space and time swap places twice. The
singularity is ring-shaped and, except if approached on its equatorial
plane, is repulsive. This fact is simply the result of the equations
of Kerr's metrical geometry. The singularity is also a temporal one,
so that it can be avoided. In theory, it is possible to escape from a
black hole, although not by the same way you went in. On leaving the
black hole you would find yourself either in a region of "negative
space," the physical meaning of which is unclear, or in an entirely
However, they do come with a hazard warning. Enter them at your peril. However, for the purposes of a science-fiction story they can be assumed to be safe enough for your characters to travel them unmolested. This, of course, is pure hand-waving necessary for the story's sake.
Everything that has been said here is purely theoretical. The Kerr
solution is very unstable, corresponding as it does to a black hole in
complete isolation. The addition of extraneous matter, such as even
the approach of a would-be traveler, could be enough to destabilize
the Kerr solution and make travel through the black hole unrealistic.
To properly investigate the feasibility of journeys past or through
the singularities of black holes we need to be able to take quantum
effects into account. However, this will require a quantum theory of
gravity – one of the chief goals of contemporary theoretical physics.
Source: Kerr black hole