Controlled Ensemble Null Tests of Dynamical Dark Energy Hints: Single-Probe Recovery of CDM with BAO, Compressed-CMB, and SNe Mocks

Recent combined cosmological analyses have reported mild apparent departures from the cosmological-constant equation of state. We perform controlled ensemble null tests using observable-level Gaussian BAO-like, compressed-CMB-like, and Type Ia supernova-like mock datasets generated from a common fiducial flat Λ CDM cosmology. The aim is to determine whether each constituent single-probe likelihood can produce an apparent dynamical-dark-energy (DDE)-like marginalized-posterior displacement, or spuriously exclude the fiducial model, through its own degeneracy structure and calibration freedom. We adopt probe-adapted parameterizations for the BAO scale, CMB physical matter density, and supernova normalization degeneracy, together with explicit convergence diagnostics, posterior-coverage tests, and realization-level pull statistics. None of the three single-probe analyses spuriously excludes the fiducial input at the 95% level. All completed BAO and SNe chains enter their final ensembles after adaptive extension of initially rejected runs; for compressed CMB, a mock-data-space audit finds no detectable selection dependence within the sensitivity of the 208-mock ensemble. The compressed-CMB-only likelihood nevertheless exhibits stable displaced marginalized medians together with broad late-time posterior intervals, illustrating that posterior location and model exclusion are distinct diagnostics. These results show that a displaced marginalized posterior in a single realization is not, by itself, evidence for dynamical dark energy. Such shifts should be interpreted through mock-ensemble recovery, convergence diagnostics, and the probe-dependent geometry of the likelihood.