LYNDEN-BELL1 has argued that Hubble's constant H0 should be doubled, from its present notional value of 55 km s−1 Mpc−1 to 110 km s−1 Mpc−1. He claims that with this value of H0 the apparent superluminal expansions in compact radio sources can be explained in a more consistent manner than previously, by means of his model of the expansion based on the old ‘light echo’ theory used to explain the Nova Persei phenomenon of 1901. Lynden-Bell's conclusion is heavily dependent on his special interpretation of the expansion phenomena in the source 3C120. Here I revert to the interpretation previously presented by Cohen et al.2 and use a more general ‘energy echo’ model that allows the primary energy ejected from the source nucleus to be propagated with a speed v⩽c, with Lynden-Bell's case v = c as a limiting possibility. The primary energy may therefore be in the form of relativistic particles, plasmoids, shock waves or photons.