The equations of relativistic, perfect-fluid hydrodynamics are cast in Eulerian form using six scalar "velocity-potential" fields, each of which has an equation of evolution. These equations determine the motion of the fluid through the equation
Uʋ=µ-1 (ø,ʋ + αβ,ʋ + ƟS,ʋ).
Einstein's equations and the velocity-potential hydrodynamical equationsfollow from a variational principle whose action is
I = (R + 16π p) (-g)1/2 d4x,
where R is the scalar curvature of spacetime and p is the pressure of the fluid. These equations are also cast into Hamiltonian form, with Hamiltonian density –T00 (-goo)-1/2.
The second variation of the action is used as the Lagrangian governing the evolution of small perturbations of differentially rotating stellar models. In Newtonian gravity this leads to linear dynamical stability criteria already known. In general relativity it leads to a new sufficient condition for the stability of suchmodels against arbitrary perturbations.
By introducing three scalar fields defined by
ρ ᵴ = ∇λ + ∇x(xi + ∇xɣi)
(where ᵴ is the vector displacement of the perturbed fluid element, ρ is the mass-density, and i, is an arbitrary vector), the Newtonian stability criteria are greatly simplified for the purpose of practical applications. The relativistic stability criterion is not yet in a form that permits practical calculations, but ways to place it in such a form are discussed.
