Flux variations in relativistically expanding spherical radio sources

The conclusion reached by Terrell regarding relativistic expansion of a spherical light source, namely, that such expansion does not lead to observed pulses which are short compared with the light travel time across the apparent source, is shown to be a model-dependent result which does not apply in general. This is illustrated in the case of synchrotron radiation sources with a thin spherical shell which evolves according to the standard model. It is also suggested, on the basis of this example, that some observed effects in compact radio sources could be attributed to relativistic expansion of the source