Renormalizing an initial state
  The intricate machinery of perturbative quantum field theory has largely been
devoted to the 'dynamical' side of the theory: simple states are evolved in
complicated ways. This article begins to address this lopsided treatment.
Although it is rarely possible to solve for the eigenstates of an interacting
theory exactly, a general state and its evolution can nonetheless be
constructed perturbatively in terms of the propagators and structures defined
with respect to the free theory. The detailed form of the initial state in this
picture is fixed by imposing suitable `renormalization conditions' on the
Green's functions. This technique is illustrated with an example drawn from
inflation, where the presence of nonrenormalizable operators and where an
expansion that naturally couples early times with short distances make the
ability to start the theory at a finite initial time especially desirable.
