The Universe Can Only Be Observed from Within -- Causal Connectivity and the Definition of the Universe --

This paper examines the relationship between causal connectivity, observability, and the definition of the universe. Modern cosmology distinguishes between the observable universe and the universe as a whole. This paper does not reject that distinction, but asks a more fundamental question: if two regions can never exchange information or participate in any causal interaction, what physical basis is there for treating them as parts of the same universe? The paper distinguishes mathematical inclusion within a common spacetime model from physical belonging to the same universe. It also considers what this distinction implies for the concepts of cosmic expansion and the age of the universe. Rather than arguing that causally disconnected regions do not exist, the paper asks what it physically means to include them within the same universe.