The concept of what lies beyond the observable universe is still a topic of active scientific investigation and speculation. The observable universe refers to the portion of the universe that we can currently observe from Earth, given the finite speed of light and the age of the universe. It extends approximately 93 billion light-years in radius.
One possibility is that the universe is infinite in size, meaning that it continues indefinitely beyond the bounds of our observable universe. In this case, there would be no true "boundary" to the universe. However, due to the limitation of the speed of light, we can only observe a finite portion of it.
Another possibility is that the universe is finite and has some kind of curvature or shape that would make it possible to travel beyond the observable universe and eventually return to our starting point, analogous to traveling on the surface of a sphere. This notion is supported by some cosmological models, such as the concept of a closed universe.
Regarding other universes or dimensions, there are several speculative theories and hypotheses in physics that suggest the existence of additional realms beyond our own. One such idea is the concept of a multiverse, which proposes that our universe is just one of many universes that exist in a larger "multiverse." These universes could have different physical laws, constants, and properties, making them potentially inaccessible or undetectable from our own universe.
Other theories, such as string theory and its variants, propose the existence of additional spatial dimensions beyond the familiar three dimensions of space (length, width, and height) and one dimension of time. These extra dimensions, if they exist, might be compactified or "curled up" at incredibly tiny scales, making them imperceptible to us at our macroscopic level of observation.
However, it's important to note that these ideas are still speculative and largely theoretical. Currently, there is no direct empirical evidence to confirm or refute the existence of other universes or dimensions. Scientists continue to explore these concepts through mathematical models, theoretical physics, and experimental investigations such as particle accelerator experiments, hoping to gain a deeper understanding of the nature of our universe and its potential connections to other realms.