Parallel universes used to be the stuff of science fiction, but cutting edge physics theories have put them front and center in modern-day cosmology. The human imagination is fascinated by the idea of parallel worlds. Few things are more interesting than imagining another version of yourself living a different life in a far off universe. Unfortunately, many of these theories are now untestable. Some seem so far-fetched that they are impossible to believe. But these are ten of the leading theories on parallel universes and the crazy implications that they can have.
10. Quilted Multiverse
Is space finite or infinite? Physicists have been debating this question for years, with limited success. If space is finite, then you can measure how big it is, but if it is infinite, our observable “universe” may not be the only one out there, rather a patch of spacetime in a larger expanse called the multiverse.
While this seems outlandish, it is important to remember that our cosmic horizon has a finite distance 42 billion light years across.  Even though our universe has been around for almost 14 billion years, space itself is expanding faster than the speed of light (which it can do, the limit on faster than light travel only exists within a co-moving patch of space). This means that we can see distances farther away then expected. However, even with a cosmic horizon of 42 billion light years, there is the possibility that cosmic structures exist beyond our horizon. We would just never be able to see them.
If space ends up being infinite, parallel universes become a logical outcome. Consider the following example: Imagine that you had 1,000 pairs of pants, 1,000 t-shirts and 1,000 pairs of shoes. Wanting to try on all possible combinations, you decided that every day for the rest of your life, you would try a different combination of these three clothing types. Unfortunately, your human life is too short to test all possibilities, death would come before you could try them all. However, if you lived for an infinite amount of time, you would be able to try all combinations and would eventually repeat a combination.
This is the same idea as a quilted multiverse. Our universe can be considered as just a specific arrangement of subatomic particles. The combination of subatomic particles exists within a certain area, our universe. However, if the multiverse itself is infinitely big, the combination of subatomic particles will eventually repeat itself. In fact, it will repeat itself an infinite amount of times.  This means that not only is our universe not unique, but it is not even the center of the overall multiverse.  In an infinite multiverse, there are infinite repetitions of our arrangements of subatomic particles. But, you will never be able to visit a parallel you. Travelling outside of our cosmic horizon is extremely difficult, and some would say impossible. All the universes are patches in the multiverse, like a big cosmic quilt.
9. Many Worlds Interpretation
Quantum mechanics is the study of very small subatomic properties and how they interact with the world. In order to describe the happenings on a subatomic scale, physicists have developed the mathematical formalism of quantum mechanics. In quantum mechanics, particles are described as a waveform, or more accurately the probability of a quantum event occurring.
Before a quantum experiment is performed, all possible events exist in superposition of each other. This means that they are all mathematically real until a quantum event occurs. Think of the classic thought experiment of Schrodinger’s cat. The cat is placed in a box with a gun pointed at it. There is a 50/50 chance that the gun will go off, but until we measure the cat by looking at it, the possible outcomes exist in superposition. Once we measure the cat, the wave function collapses and we are left with an outcome that is mathematically predicted by equations governing the wave state evolution. 
Back in the 1970s, it was postulated that all the possible quantum states did exist after the experiment was performed. They would not be existing in superposition, but once a quantum experiment takes place, all the possible outcomes occur and exist in different, parallel universes. Although this may seem outlandish, it is important to note that quantum mechanics already accounts for multiple worlds before the wave function is collapsed. The MWI simply states that these outcomes do not suddenly stop existing, but go on in a different universe.
This is weird stuff, mostly because we are applying quantum effects on a macroscopic scale. While mathematically this theory holds up, it does come with a variety of philosophical questions. First of all, what is a “world”? When we say that a parallel universe springs into existence due to a quantum experiment, what are the characteristics that define that parallel universe? What makes it unique? Secondly, it is critical to consider who we are as individuals. If a quantum parallel universe is formed, how do we define which version of ourselves is the true one, or are the all the true version of ourselves? These are fascinating questions that come from the MWI. The interpretation itself is not completely accepted among the scientific community, with many physicists sticking with the Copenhagen interpretation. However, enough physicists take MWI seriously enough that we need to consider the implications if it is true.
8. Brane Cosmology
To understand this theory of parallel universes, we first need to understand what a “brane” is. Most people have heard of string theory, which simply put, views particles as being composed of 1-dimensional “strings” of energy that vibrate, with each vibration pattern creating a different particle. Originally, theorists believed that only 1-dimensional strings existed. Try to add more dimensions to them, and the mathematics just did not work. However, after working with various calculation tools, it was found that higher dimensional strings could in fact exist. The 2-dimensional string got called a “membrane” from which the word brane was derived. Researchers study branes with up to 10-dimensions, and this work kicked off a revolution in string theory.
String theorists have developed the idea that branes permeate space as we know it. The universe itself resides in and on a 3-brane (a 3-dimension string) that holds all the “stuff” in our universe. String theorist Brian Greene provides an example of this idea in 2 dimensions, which helps us begin to see how other universes can exist. According to Greene, we can imagine our universe as an infinitely thin slice of bread, so thin that it only exists in two dimensions. All of our stars, planets, and galaxies exist on that two-dimensional slice of bread. But there are other slices in the big cosmic loaf.
Now continuing with this metaphor, we can see where parallel universes would lie. If another two-dimensional slice of bread existed along with ours, it would be shifted slightly away from our slice, but in the third dimension. Since we exist only in two dimensions in the bread universe, we would never be able to travel to the other slice, because in our “universe” we only exist in two dimensions. You can not travel in a dimension that you do not exist in, and we are tied to 2-dimensions by nature of our bread-slice universe.
There is good reason to believe that the same thing is happening in our multiverse. We exist on a 3-brane, but there is nothing in physics that excludes higher dimensions. Other 3-brane universes could exist, and they could exist all around us, but (and this gets pretty mind twisting) they would be shifted away from our universe in a 4th spacial dimension. Parallel universes could be all around us, but we would never be able to reach them because we can not travel in a 4th spacial dimension. We would never be able to leave our 3-brane. This may seem pretty strange, but experiments occurring in the Large Hadron Collider are looking for signatures of these extra dimensions, and are looking promising.
7. String Theory Landscape
Let’s stay in string theory land. Instead of talking about branes this time, we are going to look at the idea of a false vacua and a Calabi-Yau manifold. Both are essential to understanding a key theory about the multiverse. First Calabi-Yau manifolds. When string theorists began postulating the idea of vibrating strings, they quickly learned that the string would have to vibrate in higher dimensional space-time. Instead of these higher dimensions being spread out like mentioned above, the dimensions that a string vibrates in would need to be small and compact, since the string itself is absurdly small. After much investigation, theorists discovered that the topological shape of these dimensions would be a Calabi-Yau manifold, a shape that had already been discovered and was then applied to string theory. Determining the exact shape of these dimensions would be a huge step forward in string theory. There is only one problem. Up to 10^100 possible versions of the Calabi-Yau manifold exist, and only one of them can describe our universe, but they are too small to directly look at.
Keep that idea in mind, it will come back in a second. Before we return to the manifolds the idea of false vacua needs to be discussed. In physics, a vacuum is considered the default state of a system. For example, if you have a particle, the vacuum for that particle will be how you would expect to normally find it in an experimental situation. This is not always the lowest energy state, but it tends to be. A false vacuum occurs when a particle appears to be in a vacuum, but is actually not, and will need to get to that state through quantum tunneling or other non-classical effects.
In string theory, the various possible configurations of the Calabi-Yau manifold count as a vacuum. They are the default state for a vibrating string. Only one shape exists for our universe, and this type of Calabi-Yau manifold is the only one that our universe can have. But this creates a problem. If there are as many as 10^100 versions, how did our universe happen to settle on one of them during expansion?
The string theory landscape tries to settle that issue. Instead of trying to figure out some mechanism by which a universe “chooses” its default manifold shape, theorists such as Leonard Susskind has proposed that the possible vacua in string theory do occur, with other universes spawning off of them. This creates a landscapes of universes that may have different manifolds and different physical constants determined by string theory. If this theory holds true, it would remove the problematic questions of why our universe ended up with the exactly fine tuned parameters for life. Instead of it being a freak occurrence, this would be one of many outcomes that has occurred in our multiverse landscape.
6. Cyclic Universes
One of the biggest questions in modern physics is what happened before the Big Bang. Many of these theories discussed have something to say on the issue, but since the 1930s, the most well-known theories to describe what happened before the Big Bang is cyclic universes. The most popular early proponent of a cyclic universe was Albert Einstein. In his theories, Einstein proposed that the universe started out with a Big Bang and is slowly being pulled back together to create a “Big Crunch”. Once all the matter compressed again, the universe starts over with a Bang, restarting the cycle. Although the theory seemed attractive, it is known to be incorrect for two major reasons.
The first is the 2nd Law of Thermodynamics, which states that the entropy of a system increases over time. Einstein’s cyclic universe did not work if entropy was factored into the model. When entropy was worked in, theorists realized that the cycles of the universe would be getting gradually longer in the future and would have to be gradually shorter in the past. Eventually in the past the cycles of the universe would have to be infinitely small. That does not make sense at all, and the theory was dropped. Nowadays we have a second reason for this theory being incorrect. We know that the universe is expanding faster and faster due to dark energy, so we do not expecting everything to come back together again.
With the advent of string theory, physicists Paul Steinhardt and Neil Turok proposed a new cyclic theory, this time based off of the idea of a brane multiverse. Remember from above that branes could exist right next to each other, separated by a small distance in an extra dimension. Steinhardt and Turok asked the simple question. What would happen if the two branes collided? As you might expect, near total annihilation.
When two branes collide, all the stuff in their universes gets either destroyed or mixed together. This collision and release of energy would be a new Big Bang, and from the perspective of the new brane universe, would be the beginning. This theory circumvents the two problems with earlier cyclic universes. First, it takes into account modern inflationary cosmology, and does not need a localized Big Crunch in the universe. Secondly, it sidesteps the entropy problem. In the Steinhardt-Turok model, the branes continue to expand even as they are colliding and forming a new brane universe. As the branes expand, their entropy still increases in accordance to the 2nd law, but the entropy density decreases, allowing the cycle to repeat forever. Although this is a hard idea to experimentally test, it is taken seriously enough to get the idea of a cyclic multiverse on the table.
TO BE CONTINUED