  We study the Maxwell field with a general gauge fixing (GF) term in the
radiation-dominant (RD) and matter-dominant (MD) stages of expanding Universe,
as a continuation to the previous work in de Sitter space. We derive the exact
solutions, perform the covariant canonical quantization and obtain the stress
tensor in the Gupta-Bleuler (GB) physical states, which is independent of the
GF constant and is also invariant under the quantum residual gauge
transformation. The transverse stress tensor is similar in all flat
Robertson-Walker spacetimes, and its vacuum part is $\propto k^4$
  and becomes zero after the 0th-order adiabatic regularization. The
longitudinal-temporal stress tensor, in both RD and MD stages, is zero due to a
cancelation between the longitudinal and temporal parts in the GB states, and
so is the particle part of the GF stress tensor. The vacuum GF stress tensor,
in the RD stage, contains $k^4,k^2$ divergences and becomes zero by the
2nd-order regularization, however, in the MD stage, contains $k^4, k^2, k^0$
divergences and becomes zero by the 4th-order regularization. So, the order of
adequate regularization depends not only upon the type of fields, but also upon
the background spacetimes. In summary, in both the RD and MD stages, like in de
Sitter space, the total regularized vacuum stress tensor is zero, only the
transverse photon part remains, and this holds independent of the GF term.
