Wess-Zumino terms for relativistic fluids, superfluids, solids, and supersolids.

We use the coset construction of low-energy effective actions to systematically derive Wess-Zumino (WZ) terms for fluid and isotropic solid systems in two, three, and four spacetime dimensions. We recover the known WZ term for fluids in two dimensions as well as the very recently found WZ term for fluids in three dimensions. We find two new WZ terms for supersolids that have not previously appeared in the literature. In addition, by relaxing certain assumptions about the symmetry group of fluids we find a number of new WZ terms for fluids with and without charge, in all dimensions. We find no WZ terms for solids and superfluids.

Resolving the ghost problem in nonlinear massive gravity.

We analyze the ghost issue in the recently proposed models of nonlinear massive gravity in the Arnowitt-Deser-Misner formalism. We show that, in the entire two-parameter family of actions, the Hamiltonian constraint is maintained at the complete nonlinear level and we argue for the existence of a nontrivial secondary constraint. This implies the absence of the pathological Boulware-Deser ghost to all orders. To our knowledge, this is the first demonstration of the existence of a consistent theory of massive gravity at the complete nonlinear level, in four dimensions.