  For 100 years since galaxies were found to be flying apart from each other,
astronomers have been trying to determine how fast. The expansion,
characterized by the Hubble constant, H0, is confused locally by peculiar
velocities caused by gravitational interactions, so observers must obtain
accurate distances at significant redshifts. Very nearby in our Galaxy,
accurate distances can be determined through stellar parallaxes. There is no
good method for obtaining galaxy distances that is applicable from the near
domain of stellar parallaxes to the far domain free from velocity anomalies.
The recourse is the distance ladder involving multiple methods with overlapping
domains. Good progress is being made on this project, with satisfactory
procedures and linkages identified and tested across the necessary distance
range. Best values of H0 from the distance ladder lie in the range 73 - 75
km/s/Mpc. On the other hand, from detailed information available from the power
spectrum of fluctuations in the cosmic microwave background, coupled with
constraints favoring the existence of dark energy from distant supernova
measurements, there is the precise prediction that H0 = 67.4 to 1%. If it is
conclusively determined that the Hubble constant is well above 70 km/s/Mpc as
indicated by distance ladder results then the current preferred LambdaCDM
cosmological model based on the Standard Model of particle physics may be
incomplete. There is reason for optimism that the value of the Hubble constant
from distance ladder observations will be rigorously defined with 1% accuracy
in the near future.
