Interacting dark sector from intrinsic entropy couplings

TL;DR

This paper proposes a new class of dark sector models where dark matter's intrinsic entropy couples to dark energy, leading to scale-dependent effects on structure formation without altering the background expansion, consistent with current observations.

Contribution

It introduces covariant actions for entropy-based dark sector interactions, revealing their effects on perturbations and structure growth, a novel approach in dark sector modeling.

Findings

Interactions cause scale-dependent modifications to structure growth

Models are compatible with current CMB observations

Distinct signatures in large-scale structure emerge

Abstract

We introduce a new class of interacting dark sector models that couple the intrinsic entropy of dark matter to scalar field dark energy. Using the Lagrangian formulation for relativistic perfect fluids, we construct consistent covariant actions that incorporate algebraic and derivative entropy couplings. These interactions leave the expansion history unchanged, rendering the background cosmology indistinguishable from $\Lambda$CDM or uncoupled quintessence. At the level of cosmological perturbations, the entropy couplings generate scale-dependent modifications to the dark matter Euler equation, while the continuity equation remains unaltered at linear order. The resulting interactions correspond to a pure-momentum exchange within the dark sector. We show that intrinsic entropy perturbations can carry primordial scale dependence, and non-minimal couplings can lead to a scale-dependent suppression or enhancement of structure growth. Finally, we demonstrate that these models are generically compatible with current Cosmic Microwave Background observations, while inducing distinctive signatures in large-scale structure. The framework provides a theoretically well-motivated and observationally viable extension to the standard cosmological model, opening new directions to explore novel interactions in the dark sector.