Background dynamics and observational constraints of flat and non-flat $Λ(t)$CDM models from $H(z)$ and DESI DR2 BAO measurements

Abstract

In this work, we present comprehensive observational constraints on the time-varying vacuum Lambda(t)CDM cosmology using the latest baryon acoustic oscillation (BAO) data from Dark Energy Spectroscopic Instrument (DESI) Data Release 2 measurements in combination with cosmic chronometer H(z) data. We explicitly quantify the impact of vacuum dynamics on the expansion history, the total effective equation of state parameter of the unified cosmic fluid, the effective dark energy equation of state parameter, and the deceleration parameter in the spatially flat Lambda(t)CDM model. We perform a full Markov Chain Monte Carlo (MCMC) analysis and statistical model comparison, providing a consistent assessment of the Lambda(t)CDM model relative to the standard LambdaCDM framework. Our results demonstrate that H(z) and BAO observations strongly constrain deviations from the LambdaCDM model, driving the vacuum dynamics parameter alpha toward zero, while significantly reducing parameter degeneracies and alleviating the Hubble tension.