  We study the nonlinear effects of minimally coupled, massless, cosmological
scalar fields on the cosmic microwave background (CMB). These fields can
exhibit post-recombination parametric resonance and subsequent nonlinear
evolution leading to novel contributions to the gravitational potential. We
compute the resulting contributions to the CMB temperature anisotropies through
the time-variation of the gravitational potential (i.e., the integrated
Sachs-Wolfe (ISW) effect). We find that fields that constitute 5% of the total
energy density and become dynamical at $z_c \simeq 10^{4}$ can produce
marginally observable ISW signals at multipoles $\ell \simeq 2000$. Fields that
become dynamical at earlier times and/or have initial displacements at a
flatter part of their potential, produce ISW contributions that are
significantly larger and at higher multipoles. We calculate these dynamics and
the resulting evolution of gravitational perturbations using analytic estimates
alongside detailed nonlinear lattice simulations, which couple scalar fields
and cosmological fluids to a perturbed metric. Finally, we discuss the
possibility of detecting these features with future high-resolution CMB
observations.
