A Scientific Possibility
The concept of a multiverse, which involves the existence of multiple parallel universes, is a widely used plot device in science fiction. The Oscar-winning movie “Everything Everywhere All At Once” is a recent example of this trope. Despite its fictional roots, the idea of a multiverse is scientifically plausible.
It is important to clarify that the concept of a multiverse is not a product of fanciful imaginations of science fiction enthusiasts. Rather, it is a consequence of our current understanding of the fundamental laws of physics.
There are several versions of the multiverse, with the most well-known version originating from quantum mechanics, which governs the behavior of particles and atoms. Quantum mechanics suggests that particles can exist in multiple states simultaneously until they are observed, at which point they take on a definite state. One interpretation of this theory posits that all unobserved quantum possibilities are realized in other parallel universes.
An Outcome of Cosmic Inflation
The second type of multiverse, known as the cosmological multiverse, is a result of cosmic inflation. To explain why the universe looks almost uniform everywhere, physicist Alan Guth suggested in 1981 that there was a period of rapid expansion in the early universe. During this expansion, known as inflation, space expanded at a rate faster than the speed of light, pushing apart any two points.
Inflation theory also predicted the existence of primordial seeds that eventually formed structures like stars and galaxies. In 2003, this was confirmed by the observation of small temperature fluctuations in the cosmic microwave background radiation, which is the residual light from the Big Bang. This was further confirmed with great accuracy by space experiments such as WMAP and Planck.
The success of cosmic inflation theory led most cosmologists to consider it as the primary theory for explaining the early universe.
Eternal Inflation and String Theory
Cosmic inflation, which is a period of accelerated expansion in the early universe, led to the stretching and smoothing of space over large scales. However, inflation must eventually end to allow the universe to evolve into what it is today. As physicists realized, some regions of space-time could continue to inflate even after inflation ended in other regions, leading to the creation of a multiverse of universes.
This process of inflating universes producing even more inflating universes is known as “eternal inflation,” which was first proposed by physicists Paul Steinhardt and Alex Vilenkin in 1983. For many years, it was considered only a curious consequence of cosmic inflation until it was combined with an idea from string theory in the early 21st century to offer an explanation of why physical laws are what they are today.
Although string theory is still unproven, it is the best candidate for a theory of everything that unites quantum mechanics and gravity. To describe our present universe, string theory requires ten or more dimensions, with six or more of these dimensions “compactified” in a way that they cannot be observed.
The Observability Challenge of the Multiverse
The multiverse faces a major challenge in terms of observability. Even if it exists, is it possible to observe the other universes? The answer is no for the quantum multiverse as different universes cannot communicate with each other. However, the answer is different for the inflationary multiverse as neighboring universes could potentially collide with each other, leaving relics and imprints in our observable universe.
A research collaboration led by Hiranya Peiris and Matthew Johnson showed that such collisions could leave imprints on the cosmic microwave background that can be searched for, but these signatures have not yet been detected.
Theoretical challenges also exist. Some theorists argue that most of the universes in the string landscape are mathematically inconsistent and exist in a “swampland” of solutions. It is difficult to find solutions of string theory that permit cosmic inflation, which makes the concept of the inflationary multiverse controversial.
The Observability of the Multiverse
The compatibility of string theory and cosmic inflation is a topic of heated debate among string theorists and cosmologists. While some believe that string theory can explain the phenomenon of inflation, others argue that it is not possible. The outcome of this debate could have a profound impact on the future of theoretical physics.
The fundamental premise of cosmic inflation is that it is the driving force behind the smooth and uniform universe that we observe today. However, it has never been thoroughly investigated whether the process of cosmic inflation can actually begin in the first place. This is due to the complexity of the equations involved in describing the beginning of the process. Nevertheless, research teams around the world are working on this issue, and modern high-performance computing is being utilized to tackle these previously unsolvable equations.
The multiverse concept faces an enormous challenge in terms of its observability. If other universes exist, is it possible to observe them? The answer is no for the quantum multiverse, as different universes are unable to interact with each other. However, for the inflationary multiverse, the answer is “possibly.” Because neighboring universes occupy the same physical space, they could collide with each other and leave relics or imprints in our observable universe. Research conducted by Hiranya Peiris and Matthew Johnson suggests that such collisions should leave marks on the cosmic microwave background. However, such signatures have yet to be discovered.
Some theorists contend that most universes in the string landscape are mathematically inconsistent and therefore unable to exist as our universe does. These universes exist in a “swampland” of solutions. The solutions of string theory that allow for cosmic inflation appear to be particularly difficult to find within this swampland.