The multiverse hypothesis and the concept of multiple universes depends on the conception of reality as a four-dimensional space-time continuum in which "everything that could possibly happen does happen in some universe.
The prominent theoretical physicist, mathematician, and string theorist Brian Greene explains the multiverse hypothesis in detail.
The multiverse hypothesis has been criticized for a number of reasons:
Some physicists claim that the multiverse hypothesis is not falsifiable, and therefore not scientific. In response, defenders claim that the multiverse is testable and some suggest it may be more scientific than certain other untestable theories.
Some proponents of the theory say there are an infinite number of conceivable universes.
According to quantum mechanics, universes without any classical space and time, idealised as sets of quantum wavefunctions, cannot be directly observed because they do not interact with anything in our universe and would have no effect on future events in our universe.
Brian Greene explains our universe would be affected by the ripples from bubbles of another universe that it bumps into as it passes by. However multiverse models are problematic because they can not explain why the Big Bang happened at all or why the laws of physics are as they are today, which seem to follow from mathematical constants.
The American theoretical physicist and string theorist Brian Greene discussed nine types of multiverses.
A multiverse of a somewhat different kind has been envisaged within string theory and its higher-dimensional extension, M-theory. But we will discuss these theories extensively in another video.