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The sciences of physics and astrophysics explore a great many interesting ideas about the universe. One of the most intriguing is the concept of multiple universes. It's also referred to as "parallel universe theory." This is the idea that our universe is not the only one in existence. Most people have heard of the possibility of more than one universe from science fiction stories and movies. Far from being an imaginary idea, multiple universes can exist, according to modern physics. However, it's one thing to devise a theory about their existence, but quite another to actually detect them. This is something that modern physics is wrestling with, using observations of distant light signals from the Big Bang as data.

Just as our universe, with all its stars, galaxies, planets, and other structures exists and can be studied, physicists suspect that other universes filled with matter and space exist in parallel with ours. They may or may not be exactly like ours. Chances are that they're not. They might have different laws of physics than we do, for example. They don't necessarily intersect with ours, but they may collide with it. Some theorists go so far as to explain that each person has a twin or mirror in the other universes. This is one interpretation of the multiple-universe theory called "the many-worlds" approach. It says that there are many universes out there. Star Trek fans, for example, will recognize this from such episodes as "Mirror Mirror" in the original series, "Parallels" in Next Generation, and others.

There's another interpretation of multiple universes that gets quite complex and is an outgrowth of quantum physics, which is the physics of the very small. It deals with interactions at the level of atoms and subatomic particles (which make up atoms). Basically, quantum physics says that small interactions — called quantum interactions — happen. When they do, they have far-reaching consequences and set up endless possibilities with endless outcomings from those interactions.

As an example, imagine that in our universe a person takes a wrong turn on the way to a meeting. They miss the meeting and lose a chance to work on a new project. If they hadn't missed the turn, they would have gone to the meeting and gotten the project. Or, they missed the turn, and the meeting, but met someone else who offered them a better project. there are endless possibilities, and each one (if it happens) spurs endless consequences. In parallel universes, ALL of those actions and reactions and consequences take place, one to each universe.

This implies that there are parallel universes where all possible outcomes are happening simultaneously. Yet, we only observe the action in our own universe. All the other actions, we don't observe, but they are happening in parallel, elsewhere. We don't observe them, but they happen, at least theoretically.

The argument in favor of multiple universes involves many interesting thought experiments. One delves into cosmology (which is the study of the origin and evolution of the universe) and something called the fine-tuning problem. This says that as we grow to understand the way our universe is constructed, our existence in it grows more precarious. As physicists have examined the way the universe has changed over time since the Big Bang, they suspect that had the early conditions of the universe been just a bit different, our universe could have evolved to be inhospitable to life.

In fact, if a universe spontaneously came into existence, physicists would expect it to spontaneously collapse or possibly to expand so rapidly that particles never really interact with each other. British physicist, Sir Martin Reese wrote extensively about this idea in his classic book Just Six Numbers: The Deep Forces That Shape the Universe.

Using this idea of "finely-tuned" properties in the universe, some argue for the need of a creator. Tgbe existence of such a being (for which there is no proof), doesn't explain properties of the universe. Physicists would like to understand those properties without invoking a deity of any kind.

The easiest solution would just be to say, "Well, that's how it is." However, that's not really an explanation. It just represents a remarkable lucky break that a single universe would come into being and that universe would just happen to have the very precise properties needed to develop life. Most physical properties would result in a universe that collapses into nothingness instantly. Or, it continues to exist and expands into a vast sea of nothingness. It isn't just a matter of trying to explain human beings as we happen to exist, but of explaining the very existence of any sort of universe.

Another idea, which fits well with quantum physics, says that there is, indeed, a vast number of universes, which with different properties. Within that multiverse of universes, some subset of them (including our own) would contain properties that allow them to exist for relatively long periods of time. That means a subset (including our own universe) would have the properties that allow them to form complex chemicals and, ultimately, life. Others would not. And, that would be okay, since quantum physics tells us that all possibilities can exist.

String theory (which states that all the different fundamental particles of matter are manifestations of a basic object called a "string") has recently begun to support this idea. This is because there is a vast number of possible solutions to string theory. In other words, if string theory is correct then there are still many different ways to construct the universe.

String theory presents the idea of extra dimensions at the same that it includes a structure to think about where these other universes could be located. Our universe, which includes four dimensions of spacetime, seems to exist in a universe that may contain as many as 11 total dimensions. That multi-dimensional "region" is often called the bulk by string theorists. There's no reason to think that the bulk couldn't contain other universes in addition to our own. So, it is sort of a universe of universes.  

The question of a multiverse's existence is secondary to being able to detect other universes. So far no one has found solid evidence for another universe. That doesn't mean they aren't out there. The evidence may be something we haven't yet recognized. Or our detectors aren't sensitive enough. Eventually, physicists will find a way using solid data to find parallel universes and measure at least some of their properties. That could be a long way away, however.