  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.
