The Lemaitre Model and the Generalisation of the Cosmic Mass
  We consider the spherically symmetric metric with a comoving perfect fluid
and non-zero pressure -- the Lemaitre metric -- and present it in the form of a
calculational algorithm. We use it to review the definition of mass, and to
look at the apparent horizon relations on the observer's past null cone. We
show that the introduction of pressure makes it difficult to separate the mass
from other physical parameters in an invariant way. Under the usual mass
definition, the apparent horizon relation, that relates the diameter distance
to the cosmic mass, remains the same as in the Lemaitre-Tolman case.
