(* we'll denote the time relative to now (t - to) as x *)

Hostandard=70.
kmperMpc = 3.09×10^19
Ho=Hostandard/kmperMpc


a[x_]:= (1+(3/2)*Ho*x)^(2/3)


Plot[{a[x]},{x,-1/Ho,0.8×10^17},AxesLabel ->{"time (s)","scale factor, a"}]