Covariant diagonalization of the perfect fluid stress-energy tensor
  We introduce new tetrads that manifestly and covariantly diagonalize the stress-energy tensor for a perfect fluid with vorticity at every spacetime point. We discuss the origin of inertia in this special case from the standpoint of our new local tetrads. We believe that this new tetrad might bring about simplification in the analysis of astrophysical relativistic problems where vorticity is present, for instance through the Carter-Lichnerowicz equation. 
Alcides Garat
gr-qc astro-ph.HE hep-th