Hierarchical Gaussian-process reconstruction of late-time expansion with DESI BAO and compressed Planck priors

DESI DR2 BAO combined with CMB and Type Ia supernovae in the Chevallier--Polarski--Linder (CPL) parameterisation prefers evolving dark energy over CDM at roughly 2.8 -- 4.2 sigma. We reconstruct the same late-time expansion history with a hierarchical Gaussian process (GP) that co-samples the kernel hyperparameters (sigma f,l) together with (H 0, m, k, omega b h 2), coupling BAO, compressed Planck distance priors, and Pantheon+ through a Monte-Carlo effective likelihood conditioned on 37 cosmic-chronometer H(z) points. The baseline posterior gives w(z 0)=-0.80+0.26-0.23 (68% C.L.), about 0.8 sigma from w=-1, with l=3.79+0.81-1.12. A CPL fit on the identical compressed Planck pipeline improves nested CDM by only chi 2 about 1 (about 1 sigma). Ablations that fix (sigma f,l), drop SN or LRG1/2 BAO, replace the radial-basis kernel by Mat'ern- 5/2, or tighten the prior on l leave the median w(0) within 1 sigma of-1. The mild preference reported here is therefore specific to this compressed-CMB analysis and does not address DESI's full Planck-likelihood result.