Erik Jensko (U. College London); Host: Aneta Wojnar

Extended cosmological models are widely studied as alternatives to the standard Lambda-CDM framework, ranging from phenomenological prescriptions to full microphysical models. Somewhere in between the two are thermodynamic descriptions of cosmological fluids, which use effective coarse-grained fluid quantities (such as the energy density and pressure) but can be consistently derived from Lagrangian principles. In this talk, I will focus on extended cosmological fluids with intrinsic entropy perturbations, focusing on their stability and effective cosmological dynamics. Drawing on a recent example with couplings between dark matter entropy and scalar field dark energy, I will show that such models can exhibit desirable cosmological phenomenology, such as a late-time suppression of structure growth, while remaining compatible with background observations and primary CMB constraints. I conclude by outlining the ongoing theoretical and observational directions within this framework.