Brent Tully

Inconsistencies have arisen with theory based values of the Hubble constant, which fundamentally is a relation between galaxy velocities and distances.  Unfortunately, there is no single way of measuring distances over the range accessed by stellar parallaxes and the regime of cosmic expansion unconfused by peculiar motions.  Instead, there is recourse to the distance ladder.  A variety of methodologies can be laced together, each accurate within a limited domain but with substantial overlaps between techniques.   The construction of the ladder has a long and tenuous history but is reaching a fruitful conclusion.