The various universes within a multiverse are called "parallel universes,” "other universes,” or "alternate universes.” It is possible to classify a multiverse based on what types of universes it contains and how they relate to each other (see Altverse).

Dimensionality

The dimensionality of a multiverse depends on the specific interpretation of the multiverse taken.

A simple conception of a multiverse is 5-Dimensional space, with several four dimensional universe-"lines" arranged in a stack in a 5 dimensional space.

One can also construct a multiverse with the same dimensionality as the universes it contains; if the universes are finitely large, they can simply sit inside a larger space like bubbles in a volume of water. Provided that the multiverse is negatively curved, one can even fit infinitely many infinitely large universes of the same dimensionality inside. In this model, each infinite universe is a horohyperball bounded by a horohypersphere. Using a Poincaré hyperball model to model the geometry of such a multiverse, every representation of an infinite universe would be a hyperball tangent to the outer boundary of the unit hyperball.

The most extreme conception of the multiverse has them be infinite dimensional, with the coordinates of a point in the multiverse representing the complete state of the universe located at that position (such a multiverse can also be called a phaseverse).

Multiverses as Generalization

A feature common among many cosmologies is to take a multiverse as a generalization or abstraction of the universes it contains: for example, whereas a single universe might have a single mathematical basis for its laws of physics, a fixed number of dimensions, a single set of fundamental constants, and an initial condition, a multiverse can be constructed in which every universe has three of these four held constant but the other is free to vary between universes. One operation by which this may be done is the power-set, which has the useful property in that the "size", in one sense, of a multiverse containing infinitely large universes is one beth number larger than that of those universes.

In this model, it then becomes feasible to introduce a megaverse as a structure that behaves like a "multiverse of multiverses", perhaps losing another feature in common among the universes it contains, and to continue ad infinitum in what is called an archverse chain.

Tegmark Multiverses

While his writing is inconsistent with the description given above, it is nonetheless common for discussions of the multiverse to bring up the work of physicist Max Tegmark, who proposes that cosmology may be layered up to four levels deep, each one requiring fewer assumptions about physics than before. As such, these levels are outlined below for whatever reference purposes they may be needed for.

Type I

A Type I multiverse, the least like a multiverse as written above, is a natural consequence of an infinite, isotropic, homogenous universe: if the probability of a cosmological region – be it a cubic centimeter or a Hubble volume – having certain contents in a certain arrangement after the Big Bang is nonzero, then an infinite universe would contain infinitely many such regions. Popularly, this argument is taken to mean that an exact copy of our own observable universe exists somewhere else in the universe. It can be crudely estimated that identical regions the size of our Hubble volume are spaced apart by approximately 10^10^115 meters, identical regions 100 light-years in radius are spaced apart by approximately 10^10^91 meters, and identical copies of a person are spaced apart by approximately 10^10^29 meters.