One of the unexpected benefits of the explosion of superhero movies and television programs is the popularization of the "Multiverse". This is a topic that has been discussed by theoretical physicists for decades (It was actually a key component of some of my own research a few years ago, and continues to be one of my interests for future research.), but in recent years it has also become a topic of discussion among the general public. In fact the inspiration for today's article was a chat I had with some old classmates, who were not scientists but were curious about the real world theories on multiverses.  

Obviously a full technical discussion of the various multiverse theories would be far too long for a simple article on a popular scientific blog, but as a first step I thought that I would discuss the different categories of multiverse theories. The classification system I will use is one that was promoted by Tegmark and others, and seems to have become fairly well accepted in the research literature.

Before delving too deeply into Tegmark's taxonomy, we should start with the general concept of a multiverse. In its most general form, the multiverse is the idea that there may be other universes beyond our own. We define "The Universe" as being everything that is somehow connected to our own immediate region of space and time. (It is worth mentioning here as well the subtle difference between the "observable universe", which is everything that can affect us and that we can observe, and the "extended universe" which is everything that is connected to the observable universe. Some authors consider the extended universe as the simplest multiverse, but this definition is debated and is purely philosophical in nature).

These universes that comprise the multiverse could be entirely different from ours, with different laws of physics, different physical constants, and even different shapes and numbers of dimensions. The concept of the multiverse has been proposed to explain some of the most profound mysteries of the universe, such as the fine-tuning of physical constants, the nature of dark matter, and the origins of the universe itself.

Contrary to popular belief, there isn't a single theory of the multiverse, or even a single category of theory that we can refer to as a multiverse theory. Although there is some debate about what we mean by a multiverse, it is generally agreed that there are four categories of multiverse that can be discussed and debated.

Level I: The "Quilted" Multiverse

The simplest multiverse models are the "quilted" multiverses. This level posits that our universe is finite but has repeating patterns due to its vast size. This means that if we were to travel far enough in one direction, we would eventually reach a region that looks exactly like the one we left. This level of the multiverse is based on the idea that the observable universe is limited in size due to the speed of light and the age of the universe. It's also supported by the cosmic microwave background radiation, which shows a nearly uniform distribution of energy across the universe.

This type of multiverse is also the most likely to be true, since we already know that the Universe extends beyond the observable Universe. There is no doubt that the Universe is much larger than what we can observe, and that it is many orders of magnitude larger (In fact this type of theory is so well accepted that many physicists do not consider these models to be valid multiverses, but just the nature of our singular Universe). The question is only, what is beyond the limits of the observable Universe?


Level II: The Inflationary Multiverse

The second level is the inflationary multiverse. Our standard models of cosmology at present claim that the Universe began with a Big Bang, and then underwent a period of rapid inflation during which it rapidly expanded. The standard Big Bang cosmology also includes a phase known as electroweak breaking, in which bubbles form and expand, and in each one the strength of the electromagnetic and weak nuclear forces take on random values, leading to different laws of physics in different bubbles.

However there are even more complicated models of inflation, known as chaotic eternal inflation. In these models, the Universe has existed for much longer than the age of our own observable Universe. These theories claim that bubbles form in an older Universe, and then rapidly expand into complete Universes such as our own. What we know as the Big Bang might have just been the formation of a single bubble in a much older Universe. It is also possible that each of these inflationary bubble universes could have different laws of physics, and almost certainly different values of the physical constants.

In this case we have a multiverse comprised of a possibly infinite number of Universes of various ages. Although these Universes are connected to a single spacetime, they are essentially separate and unique Universes.


Level IIb: The Braneworld Multiverse

There is also a variant of the inflationary multiverse theories that warrant a separate classification, known as the braneworld multiverses. In these models, there exists some form of higher dimensional spacetime known as the multiverse. Within this spacetime, there exist a number of lower dimensional membranes that are floating around, and on each of them is a complete Universe such as our own.

These models have some interesting physical properties that have inspired extensive research over the last twenty years. Among these are the explanation for the fact that gravity is much weaker than the other three forces (caused by dilution of gravitational fields into higher dimensions), the fact that branes that collide would have the same properties of a Big Bang Universe, as well as some other, more technical properties. These are the models that are the most commonly discussed, and are at the heart of both superstring theories and many of the popular models of higher dimensions. It is also this type of multiverse that is most often referenced in the popular culture.


Level III: The Many-Worlds Multiverse

The third level of multiverse theory is the many-worlds multiverse. These models arise from the laws of quantum mechanics, in which some theoretical physicists who specialize in the foundations of quantum mechanics have argued that the best way of explaining the properties of quantum theory is for the Universe to constantly be branching off into parallel Universes.

This is called the Many Worlds Interpretation of quantum mechanics. In this theory, every time that an event occurs, the universe splits into multiple versions, each representing one of the different outcomes. This level of the multiverse is highly speculative and controversial, as it relies on interpretations of quantum mechanics that are not universally accepted. However it is also the first category of multiverse in which each Universe is completely disconnected from all others.


Level IV: The Ultimate Multiverse

The fourth and final level of Tegmark's taxonomy is the ultimate multiverse, and it is far more complex and philosophical than can be easily explained here. The basic idea though is that everything that can possibly exist, does exist in a form of purely mathematical Universe. Many of these Universes have no physical reality that we can understand from our existing laws of physics.

However this is a very philosophical type of theory, and beyond what most would consider to be a realistic physical theory of the Universe.

In conclusion, multiverse theories provide an interesting topic for theoretical physics research, and one that has found its way into popular culture. This categorization provides us with a useful framework for understanding the different levels of complexity and speculation involved in this fascinating concept. While some levels, such as the quilted and inflationary multiverses, have strong evidence supporting them, others, such as the many-worlds and ultimate multiverses, remain highly speculative and controversial. Regardless, the concept of the multiverse continues to fascinate and inspire researchers.