Orateur
Description
Extensions of LCDM with interactions between dark matter and dark energy are a popular route to address the ongoing cosmic tensions. However, many recent models are phenomenological and do not admit a Lagrangian description, leaving their theoretical validity uncertain. One example is phenomenological dark energy fluid models with a phantom equation of state, where the perturbative structure is often ignored; such models are therefore difficult to test for instabilities and pathologies, and harder still to reconcile with fundamental physics. After discussing these challenges, I will introduce several classes of interacting dark sector models derived from Lagrangians. I will cover the two main formalisms for studying relativistic fluids in cosmology: the Brown/Schutz formulations, and the effective field theory approach. I outline the theoretical viability of new and existing models and their distinctive observational signatures. In relation to the S8 tension, I focus on pure-momentum transfer and dark elastic scattering mechanisms. To conclude, I review the current status and future prospects for interacting dark sector models.