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Abstract

The {\it Dark Energy Spectroscopic Instrument} (DESI) provides a comprehensive survey of {\it Baryon Acoustic Oscillation} (BAO) in the {\it Large Scale Structure} (LSS), {in stratified data covering a finite redshift range.} Extracting cosmological parameters in a joint analysis of LSS-CMB data is hereby inherently a nonlinear problem. In particular, this nonlinearity may concern the unknown equation of state of dark energy $w(a)$, defined in the general $w(a)$CDM framework. Nevertheless, a common approach is the linearized approximation hereto notably $w_0w_a$CDM, also applied by DESI. Here, we consider a potential source of a systematic uncertainty in this linearization due to non-commutativity between $w_0w_a$CDM and {\it a posteriori} linearization of $w(a)$CDM, identified with a symmetry in the latter, which is violated in the former. We shall refer to these as early and late linearization, respectively. We demonstrate this in the analysis of the Hubble expansion in the Local Distance Ladder (LDL). Strikingly, opposite results are found for the evolution of dark energy by early versus late linearization, indicating a thawing or respectively, increasing dark energy. It is unlikely that the DESI pipeline is immune to the same contradiction. Our results show rather than thawing, claimed by DESI, dark energy may in fact be increasing. Further confirmation is expected from {\it Euclid}.