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 similar to 1.7, known as the Lambda sCDM model, has significantly improved the fit to observational data and provides a promising scenario for alleviating major cosmological tensions, such as the H-0 and S-8 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 Lambda sCDM model with the Lagrangian of a type II minimally modified gravity known as VCDM has propelled Lambda sCDM to a fully predictive model, removing the uncertainty related to the aforementioned assumption; we call this new model Lambda sVCDM. In this work, we demonstrate that not only does Lambda sCDM fit the data better than the standard Lambda CDM model, but the new model, Lambda sVCDM, 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 Lambda sCDM framework, particularly the Lambda sVCDM model, as a compelling alternative to the standard Lambda CDM model, especially by successfully alleviating the H-0 tension. Additionally, these models predict higher values for sigma(8), indicating enhanced structuring, albeit with lower present-day matter density parameter values and consequently reduced S-8 values, alleviating the S-8 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 Lambda CDM. This paves the way for further exploration of new ways for embedding the sign-switching cosmological constant into other models.