Universe
Multiverse hypotheses

Some speculative theories have proposed that our universe is but one of a set of disconnected universes, collectively denoted as the multiverse, challenging or enhancing more limited definitions of the universe. Max Tegmark developed a four-part classification scheme for the different types of multiverses that scientists have suggested in response to various problems in physics. An example of such multiverses is the one resulting from the chaotic inflation model of the early universe.

Another is the multiverse resulting from the many-worlds interpretation of quantum mechanics. In this interpretation, parallel worlds are generated in a manner similar to quantum superposition and decoherence, with all states of the wave functions being realized in separate worlds. Effectively, in the many-worlds interpretation the multiverse evolves as a universal wavefunction. If the Big Bang that created our multiverse created an ensemble of multiverses, the wave function of the ensemble would be entangled in this sense. Whether scientifically meaningful probabilities can be extracted from this picture has been and continues to be a topic of much debate, and multiple versions of the many-worlds interpretation exist. The subject of the interpretation of quantum mechanics is in general marked by disagreement.

The least controversial, but still highly disputed, category of multiverse in Tegmark's scheme is Level I. The multiverses of this level are composed by distant spacetime events "in our own universe". Tegmark and others have argued that, if space is infinite, or sufficiently large and uniform, identical instances of the history of Earth's entire Hubble volume occur every so often, simply by chance. Tegmark calculated that our nearest so-called doppelgänger is 10 metres away from us (a double exponential function larger than a googolplex). However, the arguments used are of speculative nature.

It is possible to conceive of disconnected spacetimes, each existing but unable to interact with one another. An easily visualized metaphor of this concept is a group of separate soap bubbles, in which observers living on one soap bubble cannot interact with those on other soap bubbles, even in principle. According to one common terminology, each "soap bubble" of spacetime is denoted as a universe, whereas humans' particular spacetime is denoted as the universe, just as humans call Earth's moon the Moon. The entire collection of these separate spacetimes is denoted as the multiverse.

With this terminology, different universes are not causally connected to each other. In principle, the other unconnected universes may have different dimensionalities and topologies of spacetime, different forms of matter and energy, and different physical laws and physical constants, although such possibilities are purely speculative. Others consider each of several bubbles created as part of chaotic inflation to be separate universes, though in this model these universes all share a causal origin.
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