Expanding Horizons as Natural Quantum Erasers: Information Loss and Decoherence Across Cosmological Metric Expansion

The accelerating expansion of the universe, described by the Friedmann–Lemaˆıtre–Robertson– Walker (FLRW) metric, establishes a cosmological event horizon beyond which emitted pho-tons recede at effective velocities exceeding the speed of light (c). While general relativity treats this phenomenon as a geometric boundary resulting in causal disconnection, quantuminformation theory requires an account of the physical fate of quantum information crossing this horizon. In this paper, we propose a theoretical framework wherein cosmic metric ex-pansion acts as a Natural Quantum Eraser. We model the recession of quantum states across the cosmological horizon as a continuous, universe-scale decoherence channel that ir-reversibly erases path information (which-way information) for observers within the local causal patch. By synthesizing the FLRW spacetime metric with the Von Neumann entropy formulation and decoherence theory, we demonstrate that cosmic expansion enforces local-ized wave-function collapse without violating the No-Cloning Theorem or unitary quantum evolution. Furthermore, we show that the information ”erased” by the expanding horizondirectly contributes to the local thermodynamic entropy of the observable universe, offeringa novel quantum-informational foundation for the thermodynamic arrow of time.