The multiverse is the hypothetical set of all universes.[a] Together, these universes are presumed to comprise everything that exists: the entirety of space, time, matter, energy, information, and the physical laws and constants that describe them. The different universes within the multiverse are called "parallel universes", "other universes", "alternate universes", or "many worlds". One common assumption is that the multiverse is a "patchwork quilt of separate universes all bound by the same laws of physics."[1]

The concept of multiple universes, or a multiverse, has been discussed throughout history, with origins in ancient Greek philosophy. It has evolved over time and has been debated in various fields, including cosmology, physics, and philosophy. Some physicists argue that the multiverse is a philosophical notion rather than a scientific hypothesis, as it cannot be empirically falsified. In recent years, there have been proponents and skeptics of multiverse theories within the physics community. Although some scientists have analyzed data in search of evidence for other universes, no statistically significant evidence has been found. Critics argue that the multiverse concept lacks testability and falsifiability, which are essential for scientific inquiry, and that it raises unresolved metaphysical issues.

Max Tegmark and Brian Greene have proposed different classification schemes for multiverses and universes. Tegmark's four-level classification consists of Level I: an extension of our universe, Level II: universes with different physical constants, Level III: many-worlds interpretation of quantum mechanics, and Level IV: ultimate ensemble. Brian Greene's nine types of multiverses include quilted, inflationary, brane, cyclic, landscape, quantum, holographic, simulated, and ultimate. The ideas explore various dimensions of space, physical laws, and mathematical structures to explain the existence and interactions of multiple universes. Some other multiverse concepts include twin-world models, cyclic theories, M-theory, and black-hole cosmology.

The anthropic principle suggests that the existence of a multitude of universes, each with different physical laws, could explain the fine-tuning of our own universe for conscious life. The weak anthropic principle posits that we exist in one of the few universes that support life. Debates around Occam's razor and the simplicity of the multiverse versus a single universe arise, with proponents like Max Tegmark arguing that the multiverse is simpler and more elegant. The many-worlds interpretation of quantum mechanics and modal realism, the belief that all possible worlds exist and are as real as our world, are also subjects of debate in the context of the anthropic principle.

If the multiverse is all of the possible universes, then God is as all-encompassing as anything could be, is the greatest possible being that is necessary, self-existent, transcending any particular cosmos, and exhibiting all possible moral virtue. The "Multiverse" is a local set of many unique Universes under a given continuous series of laws and is also the second in a series of Archverses, being succeeded by the Metaverse. Uniqueness isn't mandatory but is guaranteed due to the sensitivity of conditions during Universe formation. The biggest multiverse is registered as MLTI-577, and the smallest one is knowm as MLTI-183.

Each Universe in a Multiverse, as stated above, shares in common laws of physics, systems of nature, sets of elementary particles and all other laws of the parent Multiverse. However, since each Universe varies in initial conditions, the entire timeline is affected and entire cosmological structures are "reshuffled", so to speak.

We can use our own Universe as an example of ways other Universes vary in more than just timelines. For example, if the initial conditions are different enough, the ratio of matter to dark matter can be affected, the ratio between matter and anti-matter can change, and sometimes, it is possible that even constants can change by very tiny amounts, like the fine structure constant being very slightly different.

Finite multiverses average in size around 8 quadrillion light-years, making our Multiverse slightly above average in diameter. Multiverses are also 4 dimensional, meaning that even though Universes are quite far away from each other, despite not moving much, and there being copious amounts in each Multiverse, they still very rarely collide with one another and have a lot of space, whilst moving through 4 axis.

Similar to Universes, Multiverses have their own entire natural systems and structures, just like Universes. They are a byproduct of their Universes, so if a particularly strange and "mutated" Universe forms naturally within its Multiverse, it will have a strong impact on its surrounding ecosystem. However, most Multiverses have similar Universes, therefore they also appear to be homogeneous.