a collapsing universe can reappear from a nonsingular state and thus give birth to a new universe, perhaps ad infinitum

. The “cyclic” part of Penrose's cosmology comes from noticing that the future of one ever-expanding universe can be smoothly bridged to the past of another big-bang universe by means of such a conformal manifold.

 In this case, the big bang is not a true beginning, but only a sort of phase change from one “epoch” to another (Penrose 2010).

According to the pre-big-bang model the universe is not only eternal into the future, it is also eternal into the past, the two cosmic phases (contracting and expanding) being separated by a non-singular big bang.

As a mechanism for generating the huge number of universes, multiverse physicists make use of the eternal inflation scenario. Moreover, the multiverse is closely associated with anthropic reasoning: we find ourselves in our universe, with its particular physical laws and content of particles, not because other universes are impossible or improbable, but because our kind of life cannot exist in other universes.