Evidence for increasing dark energy in the Late Universe

Context. A comprehensive survey of Baryon Acoustic Oscillation (BAO) in the Large Scale Structure (LSS), in stratified data covering a finite redshift range is provided by the Dark Energy Spectroscopic Instrument (DESI). Extracting cosmological parameters in a joint analysis of LSS-CMB data is hereby inherently a nonlinear problem. Aims. 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 w0waCDM, also applied by DESI. Here, we consider a potential source of a systematic uncertainty in this linearization due to non-commutativity between w0waCDM and a posteriori linearization of w(a)CDM, identified with an intrinsic symmetry in the latter, which is violated in the former. We shall refer to these as early and late linearization, respectively. Methods. Observational consequences of symmetry violation is inherent to early linearization regardless of choice of data, here elucidated in the analysis of the Hubble expansion in the Local Distance Ladder (LDL) using cosmic chronometer data. Results. Strikingly, opposite results are found for the evolution of dark energy by early versus late linearization, indicating a thawing or respectively, increasing dark energy. This is further confirmed by mock data experiments. Accordingly, it is unlikely that the DESI pipeline is immune to the same contradiction. Conclusions. Our results show rather than thawing, claimed by DESI, dark energy may in fact be increasing upon preserving the underlying symmetry. Further confirmation is expected from Euclid.