  The discovery of cosmic acceleration has stimulated theorists to consider
dark energy or modifications to Einstein's General Relativity as possible
explanations. The last decade has seen advances in theories that go beyond
smooth dark energy -- modified gravity and interactions of dark energy. While
the theoretical terrain is being actively explored, the generic presence of
fifth forces and dark sector couplings suggests a set of distinct observational
signatures. This report focuses on observations that differ from the
conventional probes that map the expansion history or large-scale structure.
Examples of such novel probes are: detection of scalar fields via lab
experiments, tests of modified gravity using stars and galaxies in the nearby
universe, comparison of lensing and dynamical masses of galaxies and clusters,
and the measurements of fundamental constants at high redshift. The
observational expertise involved is very broad as it spans laboratory
experiments, high resolution astronomical imaging and spectroscopy and radio
observations. In the coming decade, searches for these effects have the
potential for discovering fundamental new physics. We discuss how the searches
can be carried out using experiments that are already under way or with modest
adaptations of existing telescopes or planned experiments. The accompanying
paper on the Growth of Cosmic Structure describes complementary tests of
gravity with observations of large-scale structure.
