Most cosmologists believe that at the moment the universe was formed, its size was not particularly large. But in an instant, the universe suddenly expanded significantly, and its volume increased by countless times.
It is very difficult for us to say how large the universe is, as this concept is largely speculative, but it can be said that it “is much larger than you think.” The expanding universe model suggests that the actual size of the universe is at least 1.052 times larger than the observable part. Since the observable universe has a diameter of 90 billion light-years, this means that the true extent of the universe is far beyond our comprehension.
In cosmology, the expanding universe or universal inflation refers to the expansion of the universe from its initial state at a speed faster than light.
The inflation theory addresses many issues in the standard Big Bang cosmology. For instance, different regions of the universe may be far apart, yet their temperatures are nearly the same. According to the inflation theory, these areas were once closely connected until they were separated by the expansion of the universe.
Another possible outcome of the expanding universe is that the expansion of the universe may not yet be over. In fact, this process may never end. This theory is known as “eternal inflation theory”, which believes that the universe will continue to expand on a grand scale forever, and only a small region of it can maintain a normal and peaceful existence, much like our universe. Each universe is like an “island” separated by infinite space, moving away from each other at speeds greater than the speed of light (as the expansion of the universe is indeed faster than the speed of light).
These “islands” within the “multiverse” will never meet, and will never be able to communicate with each other. In fact, we may not even be able to find direct evidence of their existence.
The multiverse is a group of hypotheses comprising multiple universes. Together, these universes encompass everything that exists: all space, time, matter, energy, information, and the laws and physical constants that describe them. The different universes within the multiverse are referred to as “parallel universes,” “alternate universes,” “alternative universes,” or “many worlds.”
Although there is no direct evidence, can we still reasonably speculate about whether the multiverse exists?
First, we must verify the expansion of the universe. Although scientists have not yet reached a conclusion, there is indeed evidence suggesting that something akin to inflation occurred in the early universe. If cosmic inflation did occur, the laws we observe from the fluctuations of cosmic microwave background radiation actually align with it. Aside from the inflation theory, no other theory regarding the early universe can accomplish this.
However, the expansion of the universe is not a separate theory; it resembles a “category” theory. Different models will assume different physical laws, different driving factors, different causes, and different effects for this event. Since all these theories are based on hypothetical models of the early universe, it is still too early to determine which of them is correct.
Physicists speculate that eternal inflation may be a more common scenario, and most models of cosmic expansion may lead to this outcome until the end of their developmental processes. Continuing with this speculation, if the inflation theory of the universe is correct, then the eternal inflation theory may also be correct, which implies that the multiverse could indeed exist.
Early recorded examples of the idea of infinite worlds existing can be found in ancient Greek atomic philosophy, which posited that infinite parallel worlds formed from the collision of atoms. In the third century BC, philosopher Chrysippus suggested that worlds would exist and be reborn eternally, effectively hinting at the existence of multiple universes across time. The concept of the multiverse was more clearly defined during the Middle Ages. American philosopher and psychologist William James used the term “multiverse” in 1895, but in a different context.
However, the existence of the multiverse is a truly difficult issue for scientists to “digest” for a long time. If the eternal inflation theory is correct, then there is not just one universe or multiple universes, but an infinite number of universes. Each universe may have its own physical laws and arrangements of particles. Therefore, if the distribution of matter and energy is limited (after all, there are many ways to construct a universe), there will always be recurring physical scenarios across countless multiverses.
But we can only say that if eternal inflation is indeed common (meaning, it applies to most models of cosmic expansion), then the multiverse may exist. A group of physicists recently published this viewpoint in a paper.
First, they discovered that eternal inflation is not as common as they initially thought. They believe that the reason cosmologists think eternal inflation is common is due to the limited number of models they study.
However, researchers also found that because we still do not know much about inflation models and their mechanisms, it is difficult for us to consider how “common” eternal inflation really is. They believe it is impossible to succinctly answer this question, and that more research on the expansion of the universe is needed. Therefore, the truth about the multiverse remains merely speculation and has yet to be scientifically validated and deeply researched.
