The area of stationary solutions of the Einstein equations
coupled to field theoretic matter models has been active in
recent years as a consequence of the discovery by Bartnik and
McKinnon [16] of a discrete family of regular static spherically symmetric
solutions of the Einstein-Yang-Mills equations with gauge group
*SU*
(2). The equations to be solved are ordinary differential
equations and in [16] they were solved numerically by a shooting method. The first
existence proof for a solution of this kind is due to Smoller,
Wasserman, Yau and McLeod [158] and involves an arduous qualitative analysis of the
differential equations. The work on the Bartnik-McKinnon
solutions, including the existence theorems, has been extended in
many directions. Recently static solutions of the
Einstein-Yang-Mills equations which are not spherically symmetric
were discovered numerically [112]. It is a challenge to prove the existence of solutions of this
kind. Now the ordinary differential equations of the previously
known case are replaced by elliptic equations. Moreover, the
solutions appear to still be discrete, so that a simple
perturbation argument starting from the spherical case does not
seem feasible. In another development it was shown that a
linearized analysis indicates the existence of stationary
non-static solutions [30]. It would be desirable to study the question of linearization
stability in this case, which, if the answer were favourable,
would give an existence proof for solutions of this kind.

Now we return to phenomenological matter models, starting with the case of spherically symmetric static solutions. Basic existence theorems for this case have been proved for perfect fluids [150], collisionless matter [135], [130] and elastic bodies [125]. The last of these is the solution to an open problem posed in [149]. All these theorems demonstrate the existence of solutions which are everywhere smooth and exist globally as functions of area radius for a general class of constitutive relations. The physically significant question of the finiteness of the mass of these configurations was only answered in these papers under restricted circumstances. For instance, in the case of perfect fluids and collisionless matter, solutions were constructed by perturbing about the Newtonian case. Solutions for an elastic body were obtained by perturbing about the case of isotropic pressure, which is equivalent to a fluid. Further progress on the question of the finiteness of the mass of the solutions was made in the case of a fluid by Makino [119], who gave a rather general criterion on the equation of state ensuring the finiteness of the radius. Makino's criterion was generalized to kinetic theory in [133]. This resulted in existence proofs for various models which have been considered in galactic dynamics and which had previously been constructed numerically. (Cf. [24], [155] for an account of these models in the non-relativistic and relativistic cases respectively.)

In the case of self-gravitating Newtonian spherically symmetric configurations of collisionless matter, it can be proved that the phase space density of particles depends only on the energy of the particle and the modulus of its angular momentum [17]. This is known as Jeans' theorem. It was already shown in [130] that the naive generalization of this to the general relativistic case does not hold if a black hole is present. Recently counterexamples to the generalization of Jeans' theorem to the relativistic case which are not dependent on a black hole were constructed by Schaeffer [153]. It remains to be seen whether there might be a natural modification of the formulation which would lead to a true statement.

For a perfect fluid there are results stating that a static solution is necessarily spherically symmetric [115]. They still require a restriction on the equation of state which it would be desirable to remove. A similar result is not to be expected in the case of other matter models, although as yet no examples of non-spherical static solutions are available. In the Newtonian case examples have been constructed by Rein [128]. (In that case static solutions are defined to be those where the particle current vanishes.) For a fluid there is an existence theorem for solutions which are stationary but not static (models for rotating stars) [92]. At present there are no corresponding theorems for collisionless matter or elastic bodies. In [128] stationary, non-static configurations of collisionless matter were constructed in the Newtonian case.

For some remarks on the question of stability see section 4.1 .

Local and Global Existence Theorems for the Einstein
Equations
Alan D. Rendall
http://www.livingreviews.org/lrr-2000-1
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