What does parallel universe mean?

Parallel universes are a popular feature among science fiction writers, but the idea of them is theoretical.

This suggests that there may be other universes that coexist with ours, some of which may even be independent of it and whose possibility of communication with us is unknown.

The physical constants in our universe are very finely tuned; the universe as we know it would not exist if any of the constants had a slightly different value. This is one feature of our universe.

Certain theories about parallel universes state that they have different values for physical constants; for example, in some universes stars cannot form; in other theories, light travels faster than light or has different geometries; in science fiction, hyperspace is a parallel universe where two points can be reached faster than light.

Another idea is what happens when an event happens. According to theory, the universe splits into two at the decision point, with each universe having a distinct outcome. Let's say there are two options.

Theories also exist that suggest black holes serve as entryways to other universes.

A parallel universe, also known as a parallel reality or alternate universe, is a hypothetical self-contained separate reality coexisting with one's own. This concept suggests the existence of multiple universes, each with its own set of physical laws, dimensions, and potentially different outcomes for events and decisions.