Expansion-history preferences of DESI DR2 and external data

Abstract: We explore the origin of the preference of DESI DR2 baryon acoustic oscillation (BAO) measurements and external data from cosmic microwave background (CMB) and type Ia supernovae (SNIa) that dark energy behavior departs from that expected in the standard cosmological model with vacuum energy ($\Lambda$CDM). In our analysis, we allow a flexible scaling of the expansion rate with redshift that nevertheless allows reasonably tight constraints on the quantities of interest, and adopt and validate a simple yet accurate compression of the CMB data that allows us to constrain our phenomenological model of the expansion history. We find that data consistently show a preference for a 3-4 % increase in the expansion rate at $z\simeq 0.7$ relative to that predicted by the standard $\Lambda$CDM model, in excellent agreement with results from the less flexible $(w_0, w_a)$ parameterization which was used in previous analyses. Even though our model allows a departure from the best-fit $\Lambda$CDM model at zero redshift, we find no evidence for such a signal. We also find no evidence (at greater than 1$\sigma$ significance) for a departure of the expansion rate from the $\Lambda$CDM predictions at higher redshifts for any of the data combinations that we consider. Overall, our results strengthen the robustness of the findings using the combination of DESI, CMB, and SNIa data to dark-energy modeling assumptions.