  We study quantum effects on moduli dynamics arising from the production of
particles which are light at special points in moduli space. The resulting
forces trap the moduli at these points, which often exhibit enhanced symmetry.
Moduli trapping occurs in time-dependent quantum field theory, as well as in
systems of moving D-branes, where it leads the branes to combine into stacks.
Trapping also occurs in an expanding universe, though the range over which the
moduli can roll is limited by Hubble friction. We observe that a scalar field
trapped on a steep potential can induce a stage of acceleration of the
universe, which we call trapped inflation. Moduli trapping ameliorates the
cosmological moduli problem and may affect vacuum selection. In particular,
rolling moduli are most powerfully attracted to the points with the largest
number of light particles, which are often the points of greatest symmetry.
Given suitable assumptions about the dynamics of the very early universe, this
effect might help to explain why among the plethora of possible vacuum states
of string theory, we appear to live in one with a large number of light
particles and (spontaneously broken) symmetries. In other words, some of the
surprising properties of our world might arise not through pure chance or
miraculous cancellations, but through a natural selection mechanism during
dynamical evolution.
