Our universe is immense. Earth is one of eight planets that revolve around our sun, which is a star, and there are billions or maybe even trillions of stars in the Milky Way galaxy. With existing technology, we can see well beyond our own galaxy into the observable universe, which has a diameter of 92 billion light-years and contains perhaps two trillion galaxies. All available evidence indicates the universe is much, much bigger than what we can observe — and might be infinitely large.

Parallel universe theory explores the possibility that the universe contains planets and galaxies similar to our own or even that an infinite number of separate universes may form a grand multiverse. While the idea of a parallel universe has long been a popular plot line in movies, TV shows and books, it’s now supported by compelling scientific theories that help explain observations about the known universe.

The concept of a multiverse arises from inflation theory, string theory and quantum mechanics. Let’s explore these theories and break down the different levels of parallel universes that may exist to find out if another “you” is out there somewhere.

The Start of Our Universe: Inflation Theory and the Big Bang Theory

Our universe most likely started with a Big Bang, a theory that was first proposed in 1931 and is now widely accepted by the scientific community thanks to strong evidence that’s since been discovered supporting it. However, there were some aspects of the Big Bang that could not be explained until inflation theory was introduced in the 1980s, according to NASA.

Inflation theory explains what happened in the tiny fraction of a second before the Big Bang. More than 13.8 billion years ago, everything that we know was a tiny, subatomic speck. Then, the speck inflated faster than the speed of light in all directions. As inflation slowed, a flood of matter and radiation appeared, creating the Big Bang fireball that allowed for the formation of atoms and molecules and, eventually, stars and galaxies pulled together by gravity.

During cosmic inflation, the universe was filled with energy inherent to space itself, as Big Think explains. This allowed the universe to expand at an exponential rate, with more distant locations moving ever more quickly away. Our observations of the observable universe are consistent with this theory of inflation.

The end of inflation is triggered by quantum processes that occur at different times in different places. The regions where inflation ends are thought to pinch off to form a bubble universe. The bubble universe experiences a similar Big Bang and may form a universe like ours. It could also form a universe with entirely different particles and different laws of physics.

All bubble universes — including our own — continue to grow and are thought to remain separated from other bubble universes because of the space created by continuing inflation. This means no two bubble universes will ever interact or collide, and it’s possible that inflation is still continuing somewhere in the multiverse.

How Does String Theory Fit In?

Another key aspect of parallel universe theories comes from string theory, which is currently the best option for smoothing out the mathematical inconsistencies that arise when Albert Einstein’s theory of relativity (to explain big things) is combined with quantum mechanics (to explain small things). String theory attempts to explain the entire universe — from subatomic particles to the laws of speed and gravity — in terms of really small strings that vibrate in 10 or 11 dimensions.

String theory allows different bubble universes to have vastly different physical properties, including different types of matter and different laws of physics. When combined with inflation theory, which allows for an unlimited number of bubbles, it provides an explanation for why our universe is so exquisitely well-suited for life. Essentially, we got lucky and ended up in the right universe out of a near-infinite number of possibilities.

Breaking Down the Four Types of Parallel Universe Theories

Physicists are considering four basic types of parallel universes, as Professor Max Tegmark of MIT explains:

• Level 1: This type of parallel universe assumes that space is infinite and that the laws of physics always remain the same. Somewhere in this infinite space, there are other planets, solar systems and even galaxies similar to ours because there is a limited number of ways the same building blocks can arrange themselves. Our “universe” is defined as what humans can currently observe, while any parallel universe would statistically be a great distance away. This parallel universe theory is largely uncontroversial among scientists since many observations and well-supported theories suggest a universe that is immensely large, and perhaps truly infinite.
• Level 2: This type of parallel universe would allow for regions of space where the basic laws of physics are the same but where there may be different physical constants, different particles and even a different number of dimensions. These universes would arise when cosmic inflation produces different bubble universes.
• Level 3: This type of parallel universe is commonly seen in movies and books, where an alternate universe arises from a change that is small (e.g., stopping to pet a dog) or large (e.g., taking a job in a different state). The underlying scientific theory arises from an interpretation of quantum mechanics, which is a mathematical description of subatomic particles. Specifically, the “many worlds” interpretation of quantum mechanics states that, for every possible quantum option, there is a universe where that option is realized (e.g., leading to one world where you stop to pet a dog and another world where you do not).
• Level 4: This type of parallel universe is the most extreme of all, where other universes are governed by completely different physical laws that would be described by completely different mathematical equations. It combines the bubble universes predicted by inflation theory with the vastly different physical properties allowed by string theory.

What Are the Chances of Another You?

The observable universe contains an estimated 10⁹⁰ particles, including protons and neutrinos. The interactions among these 10⁹⁰ particles over the past 13.8 billion years (since the Big Bang) have led to you and all of your life experiences.

The odds of there being another “you” in a parallel universe have been calculated by determining the likelihood of 10⁹⁰ particles having the exact same interactions over the course of 13.8 billion years. The estimated number of possible outcomes was (10⁹⁰)!, which works out to a 2,477-digit number. In contrast, the number of universes created through exponential inflation over the same 13.8 billion years was estimated to be 10⁵⁰, which is only a 51-digit number.

In essence, this means that, since the number of possible outcomes from interacting particles increases faster than the number of possible universes arising from inflation, there is likely no parallel universe identical to our own.

While it can be entertaining to imagine what would have happened if you had taken the other job or if you’d caught a different flight, for example, it’s perhaps more important to remember that we will face similar decisions today and every day in the future. Let’s make the most of this exceptional universe.

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