Scalar-tensor theories in the regime of cosmological observations

Abstract

In light of cosmic tensions, most notable that in H0, this work investigated the behaviour of quintessence, a form of dynamical dark energy which is modelled as a minimally coupled canonical scalar field. In this context, the dynamics are solely contained in the potential term. This differs from other scalar field implementations such as k-essence, in which the kinetic term results in cosmic expansion, or those which additionally consider couplings to other components. The five potentials considered in this work naturally lead to accelerated scenarios for a variety of initial conditions. In particular, the Albrecht-Skordis potential is considered, whose early time influence is known to potentially alleviate the H0 tension. The viability of each model was tested using MCMC techniques. Constraints were obtained against the 4th release of Planck CMB data, transversal baryon acoustic oscillations, and type 1a Supernovae. Akaike and Bayesian information criterion (AIC/BIC) where used to quantify the goodness of fits of the models. Furthermore, the Gaussian tension between the H0 of the model and that of the 2021 SH0ES determination was calculated as a metric of how well the tension is addressed by the model. The minimum χ found when constraining against a data set which included SH0ES calibration and that found excluding it was also determined for this purpose. In general, results show that the quintessence models considered are mostly competitive when under AIC evaluations which do not show strong preference for ΛCDM. In fact, two models obtained negative ∆AIC values, indicating a better fit to the data than ΛCDM. The first of these was the inverse power law model which obtained ∆AIC= −0.6 when considering PR4+BAO+SN data. The second was the oscillatory tracker model, which obtained ∆AICs of −0.2 and −0.4 when considering PR4+SH0ES and PR4+BAO+SH0ES data combinations, respectively. As for the H0 tension, this was not seen to be alleviated for four of the models considered. The Albrecht-Skordis model stands in contrast. The AIC/BIC criterion show favour for ΛCDM. However, H0 is constrained to be 69.04 ± 0.49 km s−1 Mpc−1 and 69.75+0.44−0.49 km s−1 Mpc−1 when considering PR4+BAO and PR4+BAO+SN data, respectively. These inferences are the largest obtained for each given data combination, and alleviate the tension by an amount of 0.4σ.