The idea of multiple universes, or parallel universes that exist next to our own universe, has been discussed by physicists, cosmologists, and philosophers. Many different approaches and theories have been developed to consider the nature of such universes within a physical framework. If such multiverses do exist beyond our own, then one can easily imagine that the element of time could be very different in other universes.
One important theory is based on the picture of cosmic inflation. The idea of cosmic inflation, introduced by physicist Alan Guth at the beginning of the 1980s, describes a phase of rapid expansion of the universe in its very early stages. The concept behind this scenario is that the tension of the vacuum manifests itself in a repulsive gravitational force, and as a result the universe is blown up like a balloon. In more detail, a transition between different vacuum states, starting from a state with repulsive gravity and passing over to its state today, marks the beginning and end of inflation, while the vacuum energy is being released into a hot fireball of particles. After this, the normal cosmological evolution takes place. This so-called vacuum decay is not simultaneous for the overall universe as a result of quantum fluctuations during the decay process. This means that some regions have not yet reached the end of inflation, while others including our own have, resulting in island universes that can also be pictured as bubbles. This process is called eternal inflation and has been studied in detail by Alex Vilenkin; similar approaches also based on inflation have been proposed and discussed by Andrei Linde.
These island universes can be viewed as multiple universes, each following its own laws with its own time. A single universe has its own timeline, completely independent of any neighboring universe. It is possible to define a general “time” that includes all universes, although there are still problems in making any direct predictions. Such island universes can be completely different from our universe or be similar. An infinite number of such universes may exist, and it is possible that elsewhere our universe is repeated, complete with exact copies of ourselves!
Another possibility to describe and interpret multiverses comes from quantum mechanics (QM). Here, the many-worlds view of Hugh Everett and the so-called Copenhagen interpretation are two heavily discussed topics. In QM, any observable values (e.g., a subatomic particle’s position and velocity) cannot be measured simultaneously, as stated in the well-known uncertainty principle of physicist Werner Heisenberg. This view is completely at variance with our common experience. In the quantum mechanical world, however, a physical object like an electron can occupy only states that have defined values (eigenvalues) with a certain probability. This probability is described by a wavefunction. Any measurement influences the system, thereby forcing the electron into one of the defined states (wavefunction collapse). In the many-worlds interpretation, the ensemble of possible states represents different universes. This means, if we measure that the electron is located in state A, it will be in state B in another universe. Note that all possible outcomes happen at the same time. Put most simply, one could say that any measurement leads to a number of universes presented by the different possible states.
The Copenhagen interpretation differs from the many-worlds view. Here, a measurement is taken over all possible universes (with different timelines), where we find ourselves in a universe with a high probability.
Many other theories have been proposed, some perhaps more plausible than others, regarding this fascinating subject. For now, all are necessarily consigned to the realm of speculation until such time as we gain the ability to test them by direct measurements.
See also Cosmogony; Cosmology, Inflationary; Histories, Alternative; Time and Universes; Universes, Baby; Worlds, Possible
Deutsch, D. (1997). The fabric of reality. New York: Penguin.
Guth, A. H. (1997). The inflationary universe: The quest for a new theory of cosmic origins. Cambridge, MA: Helix/Perseus.
Vilenkin, A. (2006). Many worlds in one: The search for other universes. New York: Hill and Wang.