Constraining scalar-tensor theories in the late universe

Abstract

This work investigates the minimally coupled quintessence scalar field, ϕ, as a dynamic alternative to the cosmological constant, Λ, in explaining the observed late-time accelerated expansion of the Universe. While the ΛCDM model remains the prevailing cosmological framework, it is confronted by several unresolved issues. Among the most notable is the persistent tension in measurements of the Hubble constant, H0. Quintessence models, characterised by a time-evolving scalar field, offer a potential resolution to this obstacle by replacing the fixed dark energy density, Λ, with one that evolves over cosmic time. A comprehensive theoretical framework was developed to analyse four types of quintessence potentials, V(ϕ), which govern the evolution of a dynamical scalar field, namely, the Inverse Power-Law Potential, the Simple Exponential Potential, the Double Exponential Potential and the Near-Inverse Power-Law Potential. Using Markov Chain Monte Carlo (MCMC) algorithms, these models were constrained against current observational data, including Cosmic Chronometers (CC), Pantheon+&SH0ES (SN), and Baryon Acoustic Oscillations (BAO). Each dataset holds unique sensitivities, where CC provides direct measurements of the Hubble parameter, H(z), PN+&SH0ES constrains the luminosity distance at low redshifts using data from Type Ia supernovae, and BAO serves as a standard ruler to establish intermediate-redshift expansion of the Universe. Model comparison is performed using two different model selection criteria, namely the Akaike Information Criterion (AIC) and the Bayesian Information Criterion (BIC), allowing for an assessment of the viability of each quintessence model relative to the standard ΛCDM paradigm. The results of this work suggest that quintessence models can replicate the background dynamics of ΛCDM and provide compatible values for the Hubble constant, H0 and the matter energy density parameter, Ωm,0.