We explore the origin of the preference of Dark Energy Spectroscopic Instrument (DESI) Data Release 2 (DR2) baryon acoustic oscillation 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 ($\mathrm{\Lambda }\mathrm{CDM}$). 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 $\mathrm{\Lambda }\mathrm{CDM}$ model, in excellent agreement with results from the less flexible $(w_0,w_a)$ parametrization which was used in previous analyses. Even though our model allows a departure from the best-fit $\mathrm{\Lambda }\mathrm{CDM}$ 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 $\mathrm{\Lambda }\mathrm{CDM}$ 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.

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• Cosmological parameters
• Dark energy
• Large scale structure of the Universe