Visibility of White Dwarfs Older than the Standard Cosmic Age in the Rubin Survey: A Public-Grid Occupancy Calculation

The Rubin survey is expected to extend the halo white-dwarf luminosity function into a regime where current data neither establish nor exclude remnants with cooling ages beyond the standard cosmic age. Public Montreal thick-hydrogen DA tracks give dwell times in the window M_bol = 17–20 from 4.84 Gyr at 0.5 M_☉ to 0.70 Gyr at 1.2 M_☉. Under an illustrative uniform cooling-age density on [T₀, 2T₀], with T₀ = 13–14 Gyr, the Montreal-only occupancy is 0.10–0.19 at 0.6 M_☉, at most 0.21 on the grid, and zero at 0.95 M_☉ and above. BaSTI-calibrated bright-edge shifts widen the 0.6 M_☉ range to 0.10–0.27; no analyzed channel exceeds 0.3720. A robust-within-set candidate whose conservative cooling-age lower bound exceeds the standard ceiling under the youngest age assignment in the declared model set is a chronology anomaly under that set, regardless of abundance. A null remains parametric: the population inputs are underived, and the frozen two-stage Rubin operator has not been executed on Rubin observations. Supplementary Material 1 regenerates the calculations and figure.