Most galaxies are moving away from us, and their receding speed (redshift) is proportional to the distance of the galaxies  This proportional coefficient, now known as the Hubble constant, represents the current rate of expansion of the universe  One of the core tasks of observational cosmology is to measure the relationship between distance and redshift  it describes the history of cosmic expansion, answers basic questions about the age, geometry and composition of the universe, and even tests many new physical predictions
In astronomical observations, the redshift of distant celestial bodies can be accurately obtained, but the accurate measurement of distance has always been the biggest problem for astronomers
Difficulties encountered in the measurement of distances.