multiverse

Any of various concepts of the universe according to which the region that can be directly observed by us, now or in the future, is only part of an enormous ensemble of such regions, or island universes. The whole is the multiverse: the region we can observe is our universe or the observable universe.

The inflationary theory of the Big Bang implies that the observable universe could occupy one ‘bubble’ in a vast sea of such bubbles. Since the boundary of our bubble is receding at the speed of light, owing to the expansion of space, we cannot observe the greater universe beyond.

In some variants of such cosmology theories, the fundamental constants - the strengths of the fundamental forces - are different from bubble to bubble. In many such bubbles, the combination of fundamental constants does not lead to a long-lived and complex universe like ours - the bubble may expand too slowly and collapse immediately after the Big Bang; or it may expand too fast, being filled with cooling, thinning gas that is never able to form stars and planets.

In brane cosmology, island universes move through a higher-dimensional space, and can collide, triggering Big Bangs in each. It has also been proposed that multiple universes share a single space whose number of dimensions varies from region to region. Our universe occupies a sinkhole of a mere three spatial dimensions and one time dimension.

In the babyverse theory, new universes are born within black holes in those universes that develop in the right way for such black holes to be formed. Such a babyverse can swell without limit relative to observers inside it, while being imperceptible to observers outside the black hole.

A radically different theory involving a multiverse is the many-worlds interpretation of quantum mechanics. This is an attempt to deal with the paradoxical situation that in quantum mechanics an object often seems to exist in two or more different states at the same time. For example, it may be impossible to define which of two slits an electron passes through. The many-world interpretation deals with this by saying that at every such quantum-mechanical event, the universe splits into two: in one universe the electron passes through one slit, in the other it passes through the other slit. This splitting of the universe occurs at every instant, as quantum-mechanical events occur in innumerable places, so that unimaginably huge numbers of branching universes exist.

Still more exotic types of multiverse are being considered by present-day cosmologists.