Resolving the Hubble Tension with Early Dark Energy

Early Dark Energy (EDE), a new form of dark energy active before recombination, has emerged as a promising way to explain the Hubble tension. This chapter summarizes constraints on EDE (paying particular attention to the axion-like model) in light of recent cosmological data, including constraints from the Planck cosmic microwave background (CMB) data, baryonic acoustic oscillations and Type Ia supernovae, and the more recent hints driven by CMB observations using the Atacama Cosmology Telescope. We describe the many consequences of EDE beyond these data, including the impact of EDE on the matter power spectrum; the decrease in the age of the universe; the impact for models of inflation; fine-tuning issues raised in the axion-like model. We also discuss model-building efforts towards resolving some of the challenges that EDE is facing. We stress that the next-generation of ground-based CMB experiments will firmly establish whether EDE is the mechanism responsible for the Hubble tension.