Bell Visibility over Astronomical Baselines: How Much Fundamental Decoherence a Loophole-Free Test Can Tolerate

This paper calculates the visibility threshold for a Clauser–Horne–Shimony–Holt violation and converts it into the largest fundamental decoherence rate compatible with a violation over astronomical baselines. Violation requires visibility above 0.7071. Over a million-light-year baseline the decoherence rate must lie below 1.098e-14 per second; over a billion-light-year baseline below 1.098e-17 per second. These are demanding but purely kinematic tolerances: any observed violation at such baselines would bound decoherence at these levels, while a null result could not distinguish fundamental decoherence from ordinary loss without a full noise budget. The study is simulated and computed from stated assumptions; it is not a measurement.