### 6.1 Isotropic singularities

The existence and uniqueness results discussed in this section are motivated by Penrose’s Weyl curvature
hypothesis. Penrose suggests that the initial singularity in a cosmological model should be such that the
Weyl tensor tends to zero or at least remains bounded. There is some difficulty in capturing this by a
geometric condition, and it was suggested in [160] that a clearly formulated geometric condition (which, on
an intuitive level, is closely related to the original condition) is that the conformal structure should
remain regular at the singularity. Singularities of this type are known as conformal or isotropic
singularities.
Consider now the Einstein equations coupled to a perfect fluid with the radiation equation of state
. Then, it has been shown [257, 258, 118] that solutions with an isotropic singularity are
determined uniquely by certain free data given at the singularity. The data that can be given are, roughly
speaking, half as much 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 [24] 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 [25]. 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 [23]. 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?
A formal power series analysis of this last question was given in [342]. It was found that the
asymptotic behaviour depends very much on the growth of the collision kernel for large values of the
momenta.