Coupled Early Dark Energy

Abstract: Early dark energy has emerged as one of the more promising approaches to address the Hubble tension - the statistically significant disparity between measurements of the Hubble constant made using data from different epochs in cosmic history. However, the idea is not without its own set of challenges, both from the data, in the effects it has on other measurements, such as the large-scale structure tension, and from theoretical concerns such as technical naturalness and the introduction of a new coincidence problem in cosmology. In this brief note, delivered as an invited plenary lecture at the {\it 15th Frontiers of Fundamental Physics conference}, I discuss how some of the fine-tuning problems of early dark energy can be ameliorated by using couplings to other fields already present in cosmology, and for which the epoch of matter-radiation equality is already a special one. The resulting models - neutrino assisted early dark energy, and chameleon early dark energy - provide testable, theoretically robust implementations of this general idea. I will discuss the formulation and the cosmology of such approaches, including some constraints arising from both observational and theoretical considerations.