Cosmological constraints on ΛₛCDM 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 CMB, BAO, SNe Ia, 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.