"Two Spheres of Reality: Observable vs. Physical Universe"

The physical universe refers to the entirety of space, time, matter, and energy that exists, including regions beyond human detection or observation. It potentially extends infinitely or at least far beyond our observational limits, and may include phenomena or regions fundamentally unreachable by current or future technology.

The observable universe, by contrast, is a smaller, measurable subset: it encompasses everything we can see, detect, or infer using current technologies, bounded by the finite speed of light. It is roughly 93 billion light-years across. Beyond this observational boundary, called the "cosmic horizon," the expansion of space moves objects away from us faster than their light can reach us, making these regions permanently inaccessible to direct observation.