  Gravity, and the puzzle regarding its energy, can be understood from a gauge
theory perspective. Gravity, i.e., dynamical spacetime geometry, can be
considered as a local gauge theory of the symmetry group of Minkowski
spacetime: the Poincare group. The dynamical potentials of the Poincare gauge
theory of gravity are the frame and the metric-compatible connection. The
spacetime geometry has in general both curvature and torsion. Einstein's
general relativity theory is a special case. Both local gauge freedom and
energy are clarified via the Hamiltonian formulation. We have developed a
covariant Hamiltonian formulation. The Hamiltonian boundary term gives
covariant expressions for the quasi-local energy, momentum and angular
momentum. A key feature is the necessity to choose on the boundary a
non-dynamic reference. With a best matched reference one gets good quasi-local
energy-momentum and angular momentum values.
