Consider now the Einstein equations coupled to a perfect fluid
with the radiation equation of state
. Then it has been shown [124,
62] that solutions with an isotropic singularity are determined
uniquely by certain free data given at the singularity. The data
which can be given is, roughly speaking, half as large as in the
case of a regular Cauchy hypersurface. The method of proof is to
derive an existence and uniqueness theorem for a suitable class
of singular hyperbolic equations. In [7] this was extended to the equation of state
for any
satisfying
.
What happens to this theory when the fluid is replaced by a different matter model? The study of the case of a collisionless gas of massless particles was initiated in [8]. The equations were put into a form similar to that which was so useful in the fluid case and therefore likely to be conducive to proving existence theorems. Then theorems of this kind were proved in the homogeneous special case. These were extended to the general (i. e. inhomogeneous) case in [5]. The picture obtained for collisionless matter is very different from that for a perfect fluid. Much more data can be given freely at the singularity in the collisionless case.
These results mean that the problem of isotropic singularities has largely been solved. There do, however, remain a couple of open questions. What happens if the massless particles are replaced by massive ones? What happens if the matter is described by the Boltzmann equation with non-trivial collision term? Does the result in that case look more like the Vlasov case or more like the Euler case?
![]() |
Local and Global Existence Theorems for the Einstein
Equations
Alan D. Rendall http://www.livingreviews.org/lrr-2000-1 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |