In cosmology, the scale factor (or Robertson-Walker scale factor) is a function of time whose value is 1 for the current time, and at other times is the ratio of the size of space at that time with its current time. Friedmann models treat it as a parameter.
It is currently supposed to have grown from zero over the course of the universe's history, and it is assumed that it will continue to grow. The Hubble constant expresses the rate of change of the scale factor in this manner:
dS/dt H = ————— S
This implies that the function's value over time is revealed by the Hubble constant, but the actual function is subject observational evidence, so the value listed above as the "Hubble constant" might, itself, be a function over time. Evidence that the Hubble constant is growing inspired the concept of dark energy.