Scale factor

Summary

The scale factor is the function $a(t)$ of cosmic time that gives the size of the universe relative to a reference time. In a Friedmann–Lemaître–Robertson–Walker model, the physical distance between any two galaxies at rest in comoving coordinates is proportional to $a(t)$. Its logarithmic rate of change is the Hubble parameter. Redshifts, distances and horizons in cosmology are all computed from $a(t)$.