  We study the U(1) Higgs model in spacetime-dependent background fields (a
background metric and a background scalar field).
  Particle creation can occur because of the time-dependence of these
background fields. In gauge theories, there is a unphysical sector and
consequently unphysical particles may be produced. However, it is shown that
produced unphysical particles have no contribution to backreaction to
background fields.
