A Planck 2018 Consistency Test of Late-Time Dark Energy Conditioning

This paper presents a controlled Planck 2018 CMB analysis testing whether a theory-conditioned restriction of the dark-energy equation-of-state parameter (w₀ > −1) is numerically compatible with early-universe constraints. Using the full Planck 2018 likelihood set (TTTEEE_lite, low-ℓ temperature and polarization, and CMB lensing), two neutral w₀CDM control runs are compared against a restricted-support run under identical numerical, observational, and algorithmic conditions. Standard cosmological parameters remain statistically stable, while the observed shift in w₀ is consistent with projection along the existing Planck (w₀, H₀) degeneracy at a modest Δχ² cost. No statistical preference over ΛCDM is claimed. The result establishes a reproducible, publication-grade baseline for theory-conditioned late-time extensions to be tested with genuinely late-time cosmological probes.