Cosmological constraints on Λ sCDM scenario in a type II minimally modified gravity

The idea of a rapid sign-switching cosmological constant (mirror AdS-dS transition) in the late universe at z∼1.7, known as the ΛsCDM model, has significantly improved the fit to observational data and provides a promising scenario for alleviating major cosmological tensions, such as the H0 and S8 tensions. However, in the absence of a fully predictive model, implementing this fit required conjecturing that the dynamics of the linear perturbations are governed by general relativity. Recent work embedding the ΛsCDM model with the Lagrangian of a type II minimally modified gravity known as VCDM has propelled ΛsCDM to a fully predictive model, removing the uncertainty related to the aforementioned assumption; we call this new model ΛsVCDM. In this work, we demonstrate that not only does ΛsCDM fit the data better than the standard ΛCDM model, but the new model, ΛsVCDM, performs even better in alleviating cosmological tensions while also providing a better fit to the data, including cosmic microwave background, baryon acoustic oscillations, type Ia supernovae, and cosmic shear measurements. Our findings highlight the ΛsCDM framework, particularly the ΛsVCDM model, as a compelling alternative to the standard ΛCDM model, especially by successfully alleviating the H0 tension. Additionally, these models predict higher values for σ8, indicating enhanced structuring, albeit with lower present-day matter density parameter values and consequently reduced S8 values, alleviating the S8 tension as well. This demonstrates that the data are well fit by a combination of background and linear perturbations, both having dynamics differing from those of ΛCDM. This paves the way for further exploration of new ways for embedding the sign-switching cosmological constant into other models.