The concept of the multiverse is a speculative idea that suggests the existence of multiple universes beyond our own. It is important to note that the multiverse theory encompasses a wide range of ideas, and there is no universally accepted version or concrete evidence for its existence. However, I can provide an overview of some of the proposed ideas regarding the nature of the multiverse.
Level I Multiverse: Inflationary Cosmology - This idea is based on the theory of cosmic inflation, which suggests that the early universe underwent a rapid expansion. According to this view, our universe is just one "bubble" or region within a larger eternally inflating multiverse. Each bubble represents a separate universe with its own physical properties.
Level II Multiverse: Many-Worlds Interpretation of Quantum Mechanics - This concept arises from the interpretation of quantum mechanics known as the Many-Worlds Interpretation (MWI). It suggests that every quantum event spawns multiple universes, each representing a different outcome. For example, in a quantum measurement, every possible result is realized in a separate universe.
Level III Multiverse: String Theory Landscape - This idea is based on the framework of string theory, which proposes that fundamental particles are not point-like but rather tiny strings. In the context of string theory, it is believed that there are numerous solutions or configurations of the fundamental strings that could give rise to different universes with distinct physical laws and properties. This ensemble of universes is referred to as the "string theory landscape."
Level IV Multiverse: Mathematical Universe Hypothesis - This hypothesis suggests that the multiverse is not just a collection of physically distinct universes but encompasses all mathematically possible structures. According to this view, all consistent mathematical structures exist as separate universes, regardless of whether they have a physical realization.
Regarding what holds the multiverses or what encompasses them, different ideas have been proposed. In some models, such as the inflationary multiverse, the multiverses are thought to be expanding within a larger spacetime or meta-universe. In other models, the multiverses may exist in a higher-dimensional space or be part of a complex interconnected structure.
However, it is important to note that the nature and structure of the multiverse, if it exists, are highly speculative and still the subject of active research and debate. At present, there is no conclusive evidence to confirm or refute the existence of the multiverse.