The rate of expansion of the Universe:
The rate of expansion of the universe was higher in the past. So to calculate Ht back in time, we can use the standard "lambda CDM model" to retrospectively estimate the value of Hz in the past. For example, if z = 7 and assuming H0 = 70 km/s/Mpc ⇒ H7 = 870 km/s/Mpc > 12 × H0:
