Can future CMB data discriminate between a cosmological constant and dynamical dark energy?

Recent cosmological analyses combining Planck CMB data with baryon acoustic oscillation (BAO) measurements from the DESI collaboration have revealed a statistically significant preference for evolving dark energy (DE) models over the standard cosmological constant. In particular, fits using the Chevallier–Polarski–Linder (CPL) parameterization indicate a ∼4𝜎 deviation from a constant equation of state, suggesting a possible departure from the ΛCDM paradigm. In this work, we investigate whether forthcoming cosmic microwave background (CMB) experiments such as CMB-S4 and Simons Observatory (SO) can independently discriminate between ΛCDM and these dynamical dark energy scenarios without relying on external low-redshift data. We perform a detailed Fisher matrix forecast and mock likelihood analysis using fiducial cosmologies consistent with current DESI+Planck-preferred CPL models. Our results show that, under realistic experimental assumptions, next-generation CMB data can place meaningful constraints on DE evolution parameters 𝑤0 and 𝑤𝑎, and potentially detect departures from a cosmological constant at the ∼2–3𝜎 level. We discuss the implications for model selection and the robustness of CMB-based probes in testing extensions to the standard cosmological model.