The multiverse theory encompasses a range of speculative ideas, and different versions of the theory propose various explanations for why we only observe one universe. Here are a few key concepts within the multiverse framework:
Level I Multiverse: In the simplest form of the multiverse theory, known as Level I, the idea is that our universe is just one of many regions within a larger spacetime. These regions may be disconnected from each other and have different properties. In this case, we only observe our universe because we are confined to it, and we cannot directly observe or interact with other regions.
Level II Multiverse: The Level II multiverse hypothesis suggests that universes with different properties, including physical constants and laws of physics, exist in parallel to our own. These universes might have originated from different initial conditions or experienced different events during their evolution. According to this view, we only observe one universe because we are limited by the physical laws and conditions that apply to our particular universe.
Inflationary Multiverse: The inflationary multiverse theory proposes that our universe is just one bubble within a larger "inflationary" space. During a rapid expansion phase called cosmic inflation, regions of space can undergo "bubble nucleation," leading to the formation of multiple universes. These universes, separated by vast expanses of inflating space, would be causally disconnected from each other, making direct observation or interaction impossible.
Many-Worlds Interpretation: The Many-Worlds Interpretation (MWI) is a specific interpretation of quantum mechanics that suggests every quantum event gives rise to multiple branches of reality. According to MWI, every possible outcome of a quantum event actually occurs, but in separate branches or parallel universes. In this view, we only observe one universe because our conscious experience is limited to a specific branch of reality.
Overall, the multiverse theory offers potential explanations for why we observe only one universe by positing the existence of other universes or regions beyond our direct observation. These speculative ideas are motivated by attempts to explain phenomena such as the fine-tuning of physical constants, the nature of quantum events, and the structure of the cosmos. However, it's important to note that the multiverse theory remains a subject of ongoing scientific investigation, and concrete empirical evidence for the existence of multiple universes is currently lacking.