  The current understanding of structure formation in the early universe is
mainly built on a magnification of quantum fluctuations in an initial vacuum
state during an early phase of accelerated universe expansion. One usually
describes this process by solving equations for a quantum state of matter on a
given expanding background space-time, followed by decoherence arguments for
the emergence of classical inhomogeneities from the quantum fluctuations. Here,
we formulate the coupling of quantum matter fields to a dynamical gravitational
background in an effective framework which allows the inclusion of
back-reaction effects. It is shown how quantum fluctuations couple to classical
inhomogeneities and can thus manage to generate cosmic structure in an evolving
background. Several specific effects follow from a qualitative analysis of the
back-reaction, including a likely reduction of the overall amplitude of power
in the cosmic microwave background, the occurrence of small non-Gaussianities,
and a possible suppression of power for odd modes on large scales without
parity violation.
