The kinds of regularity properties which can be dealt with in the Cauchy problem depends of course on the mathematical techniques available. When solving the Cauchy problem for the Einstein equations it is necessary to deal at least with nonlinear systems of hyperbolic equations. (There may be other types of equations involved, but they will be ignored here.) For general nonlinear systems of hyperbolic equations there is essentially only one technique known, the method of energy estimates. This method is closely connected with Sobolev spaces, which will now be discussed briefly.
Let u be a real-valued function on . Let:
The space of functions for which this quantity is finite is the Sobolev space . Here denotes the sum of the squares of all partial derivatives of u of order i . Thus the Sobolev space is the space of functions, all of whose partial derivatives up to order s are square integrable. Similar spaces can be defined for vector valued functions by taking a sum of contributions from the separate components in the integral. It is also possible to define Sobolev spaces on any Riemannian manifold, using covariant derivatives. General information on this can be found in . Consider now a solution u of the wave equation in Minkowski space. Let u (t) be the restriction of this function to a time slice. Then it is easy to compute that, provided u is smooth and u (t) has compact support for each t, the quantity is time independent for each s . For s =0 this is just the energy of a solution of the wave equation. For a general nonlinear hyperbolic system, the Sobolev norms are no longer time-independent. The constancy in time is replaced by certain inequalities. Due to the similarity to the energy for the wave equation, these are called energy estimates. They constitute the foundation of the theory of hyperbolic equations. It is because of these estimates that Sobolev spaces are natural spaces of initial data in the Cauchy problem for hyperbolic equations. Due to the locality properties of hyperbolic equations (existence of a finite domain of dependence), it is useful to introduce the spaces which are defined by the condition that whenever the domain of integration is restricted to a compact set the integral defining the space is finite.
In the end the solution of the Cauchy problem should be a function which is differentiable enough in order that all derivatives which occur in the equation exist in the usual (pointwise) sense. A square integrable function is in general defined only almost everywhere and the derivatives in the above formula must be interpreted as distributional derivatives. For this reason a connection between Sobolev spaces and functions whose derivatives exist pointwise is required. This is provided by the Sobolev embedding theorem. This says that if a function u on belongs to the Sobolev space and if k < s - n /2 then there is a k times continuously differentiable function which agrees with u except on a set of measure zero.
In the existence and uniqueness theorems stated in Section (2.2), the assumptions on the initial data for the vacuum Einstein equations can be weakened to say that should belong to and to . Then, provided s is large enough, a solution is obtained which belongs to . In fact its restriction to any spacelike hypersurface also belongs to , a property which is a priori stronger. The details of how large s must be would be out of place here, since they involve examining the detailed structure of the energy estimates. However there is a simple rule for computing the required value of s . The value of s needed to obtain an existence theorem for the Einstein equations is that for which the Sobolev embedding theorem, applied to spatial slices, just ensures that the metric is continuously differentiable. Thus the requirement is that s > n /2+1=5/2, since n =3. It follows that the smallest possible integer s is three. Strangely enough, uniqueness up to diffeomorphisms is only known to hold for . The reason is that in proving the uniqueness theorem a diffeomorphism must be carried out, which need not be smooth. This apparently leads to a loss of one derivative. It would be desirable to show that uniqueness holds for s =3 and to close this gap, which has existed for many years. There exists a definition of Sobolev spaces for an arbitrary real number s, and hyperbolic equations can also be solved in the spaces with s not an integer . Presumably these techniques could be applied to prove local existence for the Einstein equations with s any real number greater than 5/2. However this has apparently not been done explicitly in the literature.
Consider now initial data. Corresponding to these data there is a development of class for each s . It could conceivably be the case that the size of these developments shrinks with increasing s . In that case their intersection might contain no open neighbourhood of the initial hypersurface, and no smooth development would be obtained. Fortunately it is known that the developments cannot shrink with increasing s, and so the existence of a solution is obtained for data. It appears that the spaces with s >5/2 are the only spaces containing the space of smooth functions for which it has been proved that the Einstein equations are locally solvable.
What is the motivation for considering regularity conditions other than the apparently very natural condition? One motivation concerns matter fields and will be discussed in the Section (2.4). Another is the idea that assuming the existence of many derivatives which have no direct physical significance seems like an admission that the problem has not been fully understood. A further reason for considering low regularity solutions is connected to the possibility of extending a local existence result to a global one. If the proof of a local existence theorem is examined closely it is generally possible to give a continuation criterion. This is a statement that if a local solution is such that a certain quantity constructed from the solution is bounded, then the solution can be extended further. If it can be shown that the relevant quantity is bounded on any region where a local solution exists, then global existence follows. It suffices to consider the maximal region on which a solution is defined, and obtain a contradiction if no global solution exists. This description is a little vague, but contains the essence of a type of argument which is often used in global existence proofs. The problem in putting it into practise is that often the quantity whose boundedness has to be checked contains many derivatives, and is therefore difficult to control. If the continuation criterion can be improved by reducing the number of derivatives required, then this can be a significant step towards a global result. Reducing the number of derivatives in the continuation criterion is closely related to reducing the number of derivatives of the data required for a local existence proof.
A striking example is provided by the work of Klainerman and Machedon  on the Yang-Mills equations in Minkowski space. Global existence in this case was first proved by Eardley and Moncrief , assuming initial data of sufficiently high differentiability. Klainerman and Machedon gave a new proof of this which, though technically complicated, is based on a conceptually simple idea. They prove a local existence theorem for data of finite energy. Since energy is conserved this immediately proves global existence. In this case finite energy corresponds to the Sobolev space for the gauge potential. Of course a result of this kind cannot be expected for the Einstein equations, since spacetime singularities do sometimes develop from regular initial data. However, some weaker analogue of the result could exist.
|Local and global existence theorems for the Einstein
Alan D. Rendall
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