Parallel universes are a staple of science fiction television shows, like Fringe, for example. In these universes, most things are the same except for a few relatively minor differences. In our universe, we can only perceive four dimensions, but according to different theories in particle and theoretical physics, there could be more dimensions.
Parallel universes do exist, and scientists have the proof…
The Multiple Worlds Interpretation is a theory which postulates that everything that has happened or could have happened in history has happened in an alternate timeline or dimension. These dimensions sprout from each other like branches on a bush, infinitely. The core of the MWI is that the equations that model the evolution of systems over time function the same whether or not there are embedded observers in the system. So what does that mean? To understand, you first have to know what the Schrodinger’s Cat thought experiment is.
In this experiment, there is a cat in a sealed and opaque box with a radioactive element and a Geiger counter hooked up to some sort of poison. The radioactive element has a 50/50 chance of decaying, and therefore, a 50/50 chance of setting off the Geiger counter, which will release the poison and kill the cat. But, until you open up the box to see if the cat is alive or dead, the cat exists in a quantum super-positional state where it is both alive and dead at the same time.
Every time you make a decision for a microsecond, before you make that decision, you are in a super-positional state where you have made both choices. The MWI timeline splits into different universes where you have made the same choice in different ways. So, these universes theoretically continue to split and multiply with every decision by every person, animal, or particle, for that matter.
Now, String Theory tries to explain all of this by attempting to unify Quantum Mechanics and Gravity. No easy feat considering no one has been able to successfully do it yet. The problem is that our ability to measure gravity breaks down at the atomic and subatomic level. Gravity is one of the four fundamental forces of the universe, but it is also very weak. Weak? Really? Yes, it takes the entire mass of our planet to hold you to the ground. But, what about super massive black holes? Yes, they are very powerful, but require an astronomical amount of mass to achieve that power, and when compared to the other forces of the universe, it is still the weakest. So, what does all of this have to do with multiple universes?
Well, in order for String Theory to function, it needs extra “pocket” dimensions added to ours in order to make the math work – at least ten extra dimensions, in fact. These extra dimensions have to be “compactified” to a point where they are too small to measure. These extra dimensions can also be compactified in different ways. Picture the number 10 with 500 zeroes after it; that’s how many different ways we’re talking about here. Each of these compactifications will result in a separate universe with different physical laws. Unfortunately, string theory is useless in the search for which of these many universes we live in and where the rest of the universes reside.
Back to the beginning of our universe. After the Big Bang, there was a period of inflation where space/time itself was rushing outwards faster than the speed of light. Inflation theory was used to explain the even temperature of the observational universe. But, it also predicted there would be fluctuations in the background temperature, which were confirmed by multiple spacecraft, including the PLANCK and Cosmic Background Explorer. What this means is that our universe is eternally inflating, and some pieces of this universe may “break off” into other universes ad infinitum. When you combine this with string theory, you get an infinite number of universes with an infinite number of physical laws.
The universes predicted by String Theory and Inflation would occupy the same physical space, uno,like Quantum Mechanics, which currently is purely math driven. If they occupy the same physical space, then they may overlap or collide from time to time, and if they do, then we should be able to find a way to detect that collision or overlap. These collisions would leave evidence in the night sky that we could search for. Depending on the model used, they could be hot or cold spots in the cosmic background or voids in the distribution of galaxies within the universe.
While scientists are searching for these clues, others are looking for more indirect indications. Gravity waves were predicted by Einstein, and until recently, undiscovered. The LIGO (Laser Interferometer Gravitational-Wave Observatory) recently detected gravity waves from a pair of colliding black holes 1.3 billion light years away. This strengthens the case for inflation, and by extension, the idea of multiple universes.