Disformal interactions in the dark sector: From driving early dark energy to confronting cosmological tensions

The $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ model faces significant challenges, including an incomplete understanding of the dark sector and persistent tensions in the Hubble constant and the clustering amplitude. To address these issues, we propose a general disformal coupling between dark energy (DE) and dark matter from a field-theoretic action which can generate a rich variety of interactions, including conformal and pure-momentum coupling scenarios. Our analysis reveals that a pure disformal coupling naturally produces a unique interacting early dark sector, wherein the interactions with dark matter suppress the Hubble friction on the DE scalar field leading to a kinetic-driven cosmological-constant-like behavior at early times followed by its dilution as ${a}^{\ensuremath{-}6}$ and eventually leading to a potential-driven epoch characteristic of late-time dark energy. In contrast to existing early dark energy (EDE) models that rely on finely tuned potentials, the EDE-like behavior, in our framework, is purely a consequence of the disformal coupling paired with the dilution of dark matter, offering a more fundamental and less ad hoc solution to cosmological tensions. This framework also predicts a suppression of power in the CMB temperature spectrum on large angular scales, offering a potential physical explanation for the observed low-$\ensuremath{\ell}$ anomaly. By deriving these effects from a fundamental action, our work provides a unified, testable alternative to $\mathrm{\ensuremath{\Lambda}}\mathrm{CDM}$ that can be constrained by next-generation cosmological surveys and gravitational wave observations.