Accurate phenomenology of interacting dark energy

Abstract

The latest results from The Dark Energy Spectroscopic Instrument (DESI) indicate that the equation of state parameter w crosses the phantom divide, corresponding to a value of −1. This implies the need for high-accuracy studies of dark energy models when crossing the divide. In this dissertation, we improve the accuracy of the Parametrized Post-Friedmann (PPF) approach for the dark scattering model by modifying the source term for dark energy perturbations and taking into account the drag term associated with the dark scattering that appears in the Euler equation for dark energy. We implement the derivation results in CLASS. Our results demonstrate a significant improvement in the accuracy of the PPF approach for the dark scattering model, reducing the error from approximately 1.3% to only 0.1%. By being able to test different cases of varying w(a) confidently, we find that the commonly used scale independence approximation around 10−2 < k might not be true and that we need to be careful and consider this when analyzing real data. We also apply the latest constraints from DESI and use the best-fit values for w0 and wa from the three different data sets, DESI, CMB, and SN Ia, to the dark scattering model, which in all three showed an enhancement of the power spectrum. These results of the dark scattering model in the case of varying w(a) are possible due to our modification of the PPF approach for the dark scattering model.