Dynamical dark energy in models with evolution close to $\Lambda$CDM

TL;DR

This paper investigates whether an almost $ ext{Lambda}$CDM expansion history implies an almost $ ext{Lambda}$CDM model and finds that it more likely corresponds to an almost unified dark fluid, raising questions about the standard model.

Contribution

It demonstrates that near $ ext{Lambda}$CDM expansion histories do not necessarily equate to $ ext{Lambda}$CDM models, suggesting a closer link to unified dark fluid models.

Findings

Almost $ ext{Lambda}$CDM expansion history corresponds to an almost unified dark fluid.

Exact $ ext{Lambda}$CDM condition ($j(z)=1$) is not obtained from cosmographic data.

Questions raised about the $ ext{Lambda}$CDM model as the best cosmological standard.

Abstract

In this communication we address whether or not there is an equivalence between the kinematical and dynamical descriptions of the spatially flat $\Lambda$CDM model. We address this by investigating whether an almost $\Lambda$CDM expansion history ($j(z)\approx1$) corresponds to an almost $\Lambda$CDM model ($w_{\rm DE}(z)\approx-1$) by considering two particular explicit examples. At least for the cases considered, this turns out not to be the case. Instead, what we find is that an almost $\Lambda$CDM cosmic evolution rather corresponds to an \emph{almost unified dark fluid model}. Considering that one never gets the exact condition $j(z)=1$ from any cosmographic data sets, this raises further questions on whether the $\Lambda$CDM model is the best candidate for the standard model of the evolution of the universe.