A Boundary Closure Invariant Connecting Stellar Photospheres and the Friedmann Equation

We identify a radiative–gravitational boundary combination constructed from independently measured surface observables. Using a complete, unbiased sample of 190 detached eclipsing binary components, we show empirically that stellar photospheres satisfy a strict closure of this combination to a constant value. The ratio clusters tightly around unity, with a mean value of 0.996585 and a dispersion of 0.001293. Standard error propagation demonstrates that this narrow dispersion, despite the fourth-power dependence on radius and temperature, cannot be attributed to systematic cancellation of measurement uncertainties. We then apply the same boundary-closure condition to the cosmological horizon, treated as a radiative boundary. Imposing the appropriate boundary conditions yields the Friedmann matter equation as a direct consequence of boundary closure. These results suggest that both stellar photospheres and cosmological expansion are governed by a common radiative–gravitational boundary condition.