Description
In this talk the implications of a restricted symmetry breaking in the gravitational sector, where standard diffeomorphism invariance (Diff) is reduced to transverse diffeomorphisms (TDiff) through the matter action, will be analysed. The idea of restoring the full diffeomorphism invariance by the introduction of additional fields will be also explored and reformulations with the full symmetry will be presented. Furthermore, interesting applications of this framework to the dark sector of the Universe will be discussed. On the one hand, multi-field dynamics naturally induces an effective interaction and energy exchange between fields. Thus, multi-field scenarios provide a robust, symmetry-principled foundation for interacting dark fluids. On the other hand, the silent unified dark fluid will be presented. This is a novel class of single scalar field-theory models that provide a unified description of the dark sector as a single perfect fluid with a vanishing speed of sound.