scale factor

In cosmology, the scale factor (or Robertson-Walker scale factor, often expressed as a in equations) is the ratio of the size of space at a given time with the size at present (so the current value is always "1"). It is the ratio of the distance between two objects (not otherwise moving or accelerating or held together or drawn to each other) at some other (e.g., past) time versus such distance at the present time. The scale factor is presumed to have grown from zero over the course of the universe's history (Hubble expansion), and it is assumed its growth will continue. Friedmann models are typically expressed with the scale factor as a parameter. The function of time yielding the scale factor characterizes the history of space's expansion. The Hubble parameter (H) expresses the rate of change of the scale factor in this manner:

    da/dt
H = —————
      a

• H - Hubble parameter.
• a - scale factor.
• t - time.