Stress-energy tensors and the Lichnerowicz Laplacian

To a critical point of a variational problem, we associate a divergence-free symmetric 2-tensor, called the stress-energy tensor. We calculate the Laplacian of this object as defined by Lichnerowicz. This has the property that it commutes with the divergence provided the Ricci curvature is covariantly constant. We deduce relations between different stress-energy tensors, discuss growth formulae and harmonic maps between spheres.