Hubble Constant and Distance

Remember that the redshifts of the quasars shown in the left panel are certain because they are derived directly from the frequency shift of spectral lines. However, the distances and look-back times that the redshifts correspond to are computed quantities that depend on the value assumed for the Hubble constant and the history and geometry of the Universe. Since these are not known with certainty, the values quoted for distance and look-back time also are only approximate. However, it is likely that the uncertainty is 50 percent or less, so there is no doubt that the quasars are at great distances from us.