In the realm of “big” things there is the world, the solar system, the universe, and then the…? Some scientists, primarily physicists, now believe in the existence of a multiverse. The idea of a multiverse has not been proven, but there is substantial evidence toward the theory. Some of the main points include:
1.) The observable universe goes on for as long as light has had the opportunity to get in the 13.7 billion years since the proposed Big Bang. Beyond the visible universe there can be other universes lasting to infinity. This is because space and time are thought to go on to infinity.
2.) Other universes could arise from something called “eternal inflation.” Inflation refers to the universe expanding very quickly after the Big Bang. Some believe that certain pockets stopped inflating, while others never stopped. As a result, separate “bubble universes” were created. In our universe inflation has ceased, and galaxies and stars formed as a result. Other bubbles may still be inflating.
3.) It is possible that more dimensions exist to our world than the three of space and one of time that we know. The idea of parallel universes that remain just out of reach of our own universe is a theory. In a higher-dimension space other three-dimensional universes could exist.
4.) Quantum mechanics support the existence of multiple universes. Quantum mechanics deal with probabilities and the idea that all outcomes of a particular situation occur somewhere in separate universes. In one universe you may chose to go left at a fork in the road, and in another universe you may go right.
5.) Some believe that math is a “fundamental reality,” and our perceptions of the universe are "imperfect observations" of the mathematical nature of reality. Hence, many mathematical structures exist. The mathematical structure that makes up our universe may differ from mathematical structures in other universes.
It’s hard to imagine that other universes may make up a multiverse; but, it is also hard to imagine that other universes don’t exist. Who is to say that we are part of the only universe? People may never really know or fully prove that we are part of a grand multiverse, but the topic will continue to intrigue scientists of all kinds.