**Hubble’s Law**At present this relationship between distance and velocity is referred to as Hubble’s Law and it is the foundation for the cosmological model of an expanding Universe. There is no doubt about its validity, although the actual value of Hubble’s constant (the ratio between velocity and distance) has been under discussion ever since 1929. Finding the present value of the Hubble constant has been the result of decades of work by many astronomers, both in analysing the measurements of galaxy red shift and in calibrating the steps of the Cosmic Distance Ladder. During these decades, the value of Hubble’s constant has been the subject of many debates among astronomers. Hubble’s original estimate was 500 km/s/Mpc, but we now know that the value is much smaller.

Original graph by Hubble.(Hubble, Proceedings of the National Academy of Sciences, 1929,15,168).His graph would fit in an almost invisible tiny cornerat the origin of the graph at the right (compare scale along the axes). |
Main results from the H_{0} Key project.Velocity - Distance diagram (top) Solutions for H _{0} (bottom) Credit CfA, Harvard. |

In cosmology, the Hubble parameter is a time-dependent quantity that basically describes the rate of expansion over time. The Hubble constant is its value at the present cosmological time. The Hubble H

_{0}Key Project concluded in 2001 on a value of the Hubble constant of H0 = 72 ± 8 km/sec/Mpc. The uncertainties in the determination of the Hubble constant with distance ladder techniques are still dominated by the uncertainty in the calibration of the Cepheid method of distance measurement.