Since ancient times, the term "universe" has meant "everything." In 1543, Copernicus introduced the heliocentric theory in his work "On the Revolutions of the Celestial Spheres," challenging the Earth-centered cosmology that had prevailed for thousands of years. After Copernicus, scientific geniuses such as Tycho Brahe, Kepler, Galileo, and Newton not only completely overturned the geocentric theory but also ushered in a new era of cosmology. This era evolved from a Sun-centered view to a Milky Way-centered understanding of the universe.

Our astronomers today can observe up to 42 billion light-years away, which is the farthest distance we can see, also known as the visual horizon. This represents the furthest light has traveled since the Big Bang and indicates the size the universe has expanded to since then. However, we have no reason to believe the universe ends there; it could very well be infinite.

In recent years, some cosmologists have proposed the concept of a "multiverse," where different universes exist without any causal influence on each other in space and time. In other words, the universe we perceive may be just one part of a larger, more magnificent, and broader panorama of existence, known as the multiverse.

First, we need to understand the origins of the multiverse theory. This theory is not a product of science fiction but is grounded in mathematics and physics. In the early 20th century, the development of quantum mechanics revealed the randomness and uncertainty in the behavior of microscopic particles. To explain this phenomenon, scientists proposed the many-worlds interpretation: whenever a quantum event has multiple possible outcomes, the universe splits into multiple versions, each corresponding to one of those outcomes.

For example, when you are faced with a choice—whether to go or not to go—according to the many-worlds interpretation, both choices are realized, but in different universes. The version of you that decides to go continues to exist in this universe, while the version of you that decides not to go exists in another universe.

Brian Greene, a renowned contemporary theoretical physicist, wrote a book interpreting the multiverse from a professional perspective. He summarized the theories related to the multiverse in modern physics and cosmology into eight categories, proposing that there may be nine kinds of multiverses: the Quilted Multiverse, the Inflationary Multiverse, the Brane Multiverse, the Cyclic Multiverse, the Landscape Multiverse, the Quantum Multiverse, the Holographic Multiverse, the Simulated Multiverse, and the Ultimate Multiverse.

Despite its theoretical appeal, the multiverse theory is not currently supported by experimental evidence, as we cannot directly observe other universes. This makes the multiverse theory a highly controversial topic. However, there's no denying that the multiverse theory offers us a whole new perspective on our place in the universe. If parallel universes do exist, then we may not be unique. In another universe, there could be an individual similar to you, but with a life and choices that are very different from yours.