A proposal to improve the accuracy of cosmological observables and address the Hubble tension problem

Abstract

(abridged)Cosmological observational programs often compare their data not only with $\Lambda$CDM, but also with extensions applying dynamical models of dark energy (DDE), with a time-dependent equation of state (EoS) parameter $w$. We found a degeneracy in the customary computational procedure for the expansion history, once DDE models are applied. This degeneracy provides an infinite number of cosmological models reproducing the Planck-measured CMB spectrum. Moreover, this degeneracy biases the comparison of $\Lambda$CDM with DDE extensions. We present a complementary computational approach, which breaks this degeneracy: the ``fixed early densities (EDs) approach'' evolves cosmological models from the early Universe to the present, in contrast to the customary ``fixed $H_0$ approach'' which evolves cosmological models in reverse order. We find the EDs are accurately approximated by the $\Lambda$CDM model. We implement a refined procedure, appyling both approaches, in an amended version of the code CLASS, where we focus on representative DDE models using the CPL parametrization. Our results reveal that the CPL model $w(a)=-0.9 + 0.1(1-a)$ could provide a resolution to the Hubble tension problem. Moreover, we find that combining both approaches, while requesting to yield consistent results and being in agreement with observations across cosmic time, can serve as a kind of consistency check for cosmological models. Moreover, our proposed consistency check applied within current data analysis methods will increase the accuracy of inferred cosmological parameters significantly, in particular for $\Lambda$CDM extensions. We find characteristic signatures in the late expansion histories of cosmological models, allowing a phenomenological discrimination of DE candidates and a possible resolution of the Hubble tension, by ongoing and future observational programs.