The expansion of the universe was suggested in 1927 by Georges Lemaître and confirmed with improved data by Edwin Hubble in 1929 and 1931. Hubble paired the Henrietta Swan Leavitt Law (see right figure) of Cepheids with Vesto Slipher's redshift measurements to present an empirical relation of increasing recessional velocity with increasing distance.

Henrietta Swan Leavitt discovered the Cepheid Period-Luminosity relation, enabling Astronomers to measure distances to galaxies beyond our own Milky Way and to discover the expanding universe.

Under the assumptions of the cosmological principle, this relation describes the uniform expansion of the universe and its slope is named Hubble's constant, or Hubble-Lemaître constant.

The Hubble constant sets the absolute scale of the universe, thus providing a powerful predictive test for our models of the universe -- the current standard is referred to as Lambda-Cold Dark Matter (LCDM)

Because of its significance, the value of the Hubble constant has been frequently debated over the decades. The debate always hinges upon the age-old problem of astrophysics: systematic errors. Systematics, as we call them, are additional errors or biases that simply do not exist in our present framework of understanding.

In today's era of "precision cosmology", the sizes of debated systematic errors have decreased in size from factors of several in the early 1900s to <10% thanks to the pioneering work associated with the Hubble Space Telescope (HST) Key Project and its member scientists.

Now, we regularly debate 1-2% effects in the distance scale as we continue to hunt down the true value of the Hubble constant, representing a stunning achievement of human engineering (e.g., the Hubble Space Telescope, its servicing missions, and instrumentation) and the scientific method.