In this study, we use a flexible parametrization of the equation of state of dark energy to explore its possible evolution with datasets from the Dark Energy Spectroscopic Instrument (DESI), Planck cosmic microwave background (CMB), and either the 5-year Dark Energy Survey (DES) or the $\mathrm{Pantheon}+$ (PP) supernova (SN) compilation. This parametrization, called transitional dark energy (TDE), allows for rapid changes in the equation of state but also changes like that in the Chevallier-Polarski-Linder (CPL) $w_0–w_a$ parametrization. We find a $3.8\sigma$ preference for evolving dark energy over $\mathrm{\Lambda }\mathrm{CDM}$ with the DES SN dataset and a weaker $2.4\sigma$ preference when using the PP dataset. This corroborates the finding of the DESI Collaboration, who found that their baryon acoustic oscillation data preferred evolving dark energy when fit with the CPL parametrization of the equation of state. Our analysis reveals no significant outliers in the DESI data around the TDE best-fit, while the data is asymmetrically distributed around the $\mathrm{\Lambda }\mathrm{CDM}$ best-fit model such that the measured distances are on average smaller. The DESI and SN data both prefer an expansion history that implies a higher dark energy density around $z=0.5$ than in the Planck-$\mathrm{\Lambda }\mathrm{CDM}$ model, with the inferred equation of state being greater than $-1$ around $z=0$ and close to or below $-1$ at $z>0.5$. We show that when the expansion rate is greater than that in the Planck-$\mathrm{\Lambda }\mathrm{CDM}$ model (around $z=0.5$), the growth rate calculated assuming General Relativity is suppressed relative to the Planck-$\mathrm{\Lambda }\mathrm{CDM}$ model, and it rebounds as the expansion rate differences between the models become smaller closer to the present time. We derive an approximate analytic expression for the growth rate differences to understand this behavior quantitatively. The resulting flattening of the $f{\sigma }_8(z)$ curve compared to the $\mathrm{\Lambda }\mathrm{CDM}$ model could be an independent signature of the temporal evolution of dark energy.

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• Baryon acoustic oscillations
• Cosmic microwave background
• Cosmological parameters
• Dark energy
• Large scale structure of the Universe
• Distances, redshifts, & velocities