Speaker: Alkistis Pourtsidou

Abstract:

The standard cosmological model, consisting of dark energy in the form of a cosmological constant together with cold dark matter fits all the available datasets (CMB, H0, SNIa, BAOs) extremely well, but it also suffers from two fundamental problems, namely the fine-tuning and coincidence problems. Given that the precise nature of the two dark sectors is at present unknown, it may be that dark matter and dark energy have non-zero couplings to each other – this would help to alleviate the coincidence problem. In this work we answer the question “What is the most general phenomenological model of coupled dark matter to dark energy one can construct?” and present three distinct classes of interacting dark energy theories in the form of a scalar field which is explicitly coupled to dark matter. Our construction draws from the pull-back formalism for fluids and generalises the fluid action to involve couplings to the scalar field. We investigate the cosmology of each class of model both at the background and linearly perturbed level. We also present the most general parameterisation of such models using the Parameterised Post-Friedmannian approach. Finally, we investigate observational signatures via the coupling’s effects on the CMB and matter power spectra.