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 [24] 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 [24] 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 [230] 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 that are not spherically symmetric
were discovered numerically [158]. 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 [50]. 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 [218], collisionless matter [195,
189], and elastic bodies [185]. The last of these is the solution to an open problem posed
in [215]. All these theorems demonstrate the existence of solutions that
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 [172], 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 [197]. This resulted in existence proofs for various models that have
been considered in galactic dynamics and which had previously
been constructed numerically (cf. [38,
227] for an account of these models in the non-relativistic and
relativistic cases, respectively). Most of the work uoted up to
now refers to solutions where the support of the density is a
ball. For matter with anisotropic pressure the support may also
be a shell,
*i.e.*
the region bounded by two concentric spheres. The existence of
static shells in the case of the Einstein-Vlasov equations was
proved in [193].

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 [25]. This is known as Jeans' theorem. It was already shown in [189] 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 [225]. It remains to be seen whether there might be a natural modification of the formulation that would lead to a true statement.

For a perfect fluid there are results stating that a static solution is necessarily spherically symmetric [167]. 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 [193]. (In that case static solutions are defined to be those in which the particle current vanishes.) For a fluid there is an existence theorem for solutions that are stationary but not static (models for rotating stars) [129]. At present there are no corresponding theorems for collisionless matter or elastic bodies. In [193], stationary, non-static configurations of collisionless matter were constructed in the Newtonian case.

Two obvious characteristics of a spherically symmetric static
solution of the Einstein-Euler equations that has a non-zero
density only in a bounded spatial region are its radius
*R*
and its total mass
*M*
. For a given equation of state there is a one-parameter family
of solutions. These trace out a curve in the (*M*,
*R*) plane. In the physics literature, pictures of this curve
indicate that it spirals in on a certain point in the limit of
large density. The occurrence of such a spiral and its precise
asymptotic form have been proved rigorously by Makino [173].

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

Theorems on Existence and Global Dynamics for the
Einstein Equations
Alan D. Rendall
http://www.livingreviews.org/lrr-2002-6
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