The Hubble Tension and Early Dark Energy

TL;DR

The paper reviews the Hubble tension, highlighting recent observational developments and discussing early dark energy as a promising solution that alters early Universe physics, with potential for future testing.

Contribution

It provides a comprehensive overview of the Hubble tension, emphasizing early-Universe solutions like early dark energy and their observational constraints.

Findings

Early dark energy can potentially resolve the Hubble tension.

Observational data increasingly favor models altering early Universe physics.

Future measurements can test early dark energy models.

Abstract

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. We then explain why early-Universe solutions are currently favored and the constraints that any such model must satisfy. We discuss one workable example, early dark energy, and describe how it can be tested with future measurements. Given an assortment of more extended recent reviews on specific aspects of the problem, the discussion is intended to be fairly general and understandable to a broad audience.