In cosmology, the scale factor (or Robertson-Walker scale factor, often expressed as a in equations) 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 are typically expressed with the scale factor as a parameter.
Current thought is that the scale factor has grown from zero over the course of the universe's history, and will continue to grow. The Hubble constant expresses the rate of change of the scale factor in this manner:
da/dt H = ————— a
This implies that the function's value over time is revealed by the Hubble constant, but the actual function is subject to observational evidence, and the value termed the "Hubble constant" might well be found to be a function of time. Evidence that the Hubble constant is growing inspired the concept of dark energy.