Types of quantum field thories
with values in the Poincare Lie algebra.
A graviton has spin , and is massless. We can see that it has spin from the fact that the source of gravity is , the energy-momentum tensor, which is a second-rank tensor. It can be shown that a massless spin- particle has to be a graviton. The basic concept behind this is that massless particles have to couple to conserved currents - the stress-energy tensor , the source of gravity.