However, in a multiverse there are multiple universes - perhaps an immeasurable number - which may or may not contain humans or life at all. In fact, some of them may be so hostile that chemistry or physics as we know them do not function properly.
Travel between universes is possible for extremely advanced civilizations through the use of portals, dark energy, time maneuvering, or other space-warping mechanisms. Additionally, cosmic entities can traverse through universes easily. Most universes shall eventually "die" as the second law of thermodynamics ensures that energy becomes less and less usable over time. This may not always be the case with alternate physical laws, though.