Interacting dark sector from Horndeski theories and beyond: Mapping fields and fluids

Understand

In Cosmology, when dissipative effects are minimal, the energy content of the Universe can be effectively described as a sum of perfect fluids.

• Perfect fluid descriptions ensure thermal equilibrium since they equilibrate immediately.
• However, interactions among different energy content of the Universe might prevent such rapid equilibration.
• This limitation calls for a more fundamental framework that incorporates these interactions directly at the level of the action.