The Hubble Tension and Early Dark Energy

Understand

Over the past decade, the disparity between the value of the cosmic expansion rate directly determined from measurements of distance and redshift or instead from the standard $\Lambda$CDM cosmological model calibrated by measurements from the early Universe, has grown to a level of significance requiring a solution.

• Proposed systematic errors are not supported by the breadth of available data (and "unknown errors" untestable by lack of definition).
• Simple theoretical explanations for this "Hubble tension" that are consistent with the majority of the data have been surprisingly hard to come by, but in recent years, attention has focused increasingly on models that alter the early or pre-recombination physics of $\Lambda$CDM as the most feasible.
• Here, we describe the nature of this tension, emphasizing recent developments on the observational side.