3.4 Hyperboloidal initial data3 The Regular Conformal Field 3.2 The reduction process for

3.3 Initial value problems 

From the physical point of view the most interesting scenario is the following one: A gravitating material system (like e.g. several extended bodies) evolves from a given initial state, possibly interacting with incoming gravitational radiation and emitting outgoing gravitational radiation until it reaches a final state. This situation is sketched in Figure  6 .


Click on thumbnail to view image

Figure 6: The physical scenario: The figure describes the geometry of an isolated system. Initial data are prescribed on the blue parts, i.e. on a hyperboloidal hypersurface and the part of tex2html_wrap_inline3849 which is in its future. Note that the two cones tex2html_wrap_inline3847 and tex2html_wrap_inline3849 are separated to indicate the non-trivial transition between them.

In more mathematical terms, this requires the solution of an initial value problem: We provide appropriate initial data, describing the initial configuration of the matter and the geometry, on a hyperboloidal hypersurface tex2html_wrap_inline4659, and appropriate boundary data, describing the incoming gravitational radiation, on the piece of tex2html_wrap_inline3849 which is in the future of tex2html_wrap_inline4659 . Then we have to show that there is a unique solution of the conformal field equations coupled to the matter equations which exists for some time. If the situation is ``close enough'' to a Newtonian situation, i.e., the gravitational waves are weak and the matter itself is rather ``tame'', then one would expect that there is a solution, i.e. a space-time, which is regular on arbitrary hyperboloidal hypersurfaces intersecting tex2html_wrap_inline3847 . In general, however, we cannot expect to have a regular point tex2html_wrap_inline4134 representing time-like infinity.

So far, results of this kind are out of reach. The reason is not so much the incorporation of matter into the conformal field equations but a more fundamental one. Space-like infinity tex2html_wrap_inline3683 is a singularity for the conformal structure of any space-time, which has a non-vanishing ADM-mass. Without the proper understanding of tex2html_wrap_inline3683 there will be no way to bridge the gap between past and future null-infinity because tex2html_wrap_inline3683 provides the link between the incoming and the outgoing radiation fields.

The results obtained so far are concerned only with the pure radiation problem, i.e. the vacuum case. In [29] Christodoulou and Klainerman prove the global non-linear stability of Minkowski space, i.e. the existence of global solutions of the Einstein vacuum equations for ``small enough'' Cauchy data which satisfy certain fall-off conditions at space-like infinity. Their result qualitatively confirms the expectations based on the concept of asymptotic flatness. However, they do not recover the peeling property for the Weyl tensor but a weaker fall-off, which implies that in this class of solutions the conformal compactification would not be as smooth as it was expected to be. This raises the question whether their results are sharp, i.e., whether there are solutions in this class which indeed have their fall-off behaviour. In that case, one would probably have to strengthen the fall-off conditions of the initial data at space-like infinity in order to establish the correct peeling of the Weyl tensor. Then an interesting question arises as to what the physical meaning of these stronger fall-off conditions is. An indication that maybe more restrictive conditions are needed is provided by the analysis of the initial data on hyperboloidal hypersurfaces (see below).

The first result [46] obtained with the conformal field equations is concerned with the asymptotic characteristic initial value problem (see Figure  7) in the analytic case. It was later generalized to the tex2html_wrap_inline4675 case.


Click on thumbnail to view image

Figure 7: The geometry of the asymptotic characteristic initial value problem: Characteristic data are given on the blue parts, i.e. an ingoing null surface and the part of tex2html_wrap_inline3849 which is in its future. Note that the ingoing surface may develop self-intersections and caustics.

In this kind of initial value problem, one specifies data on an ingoing null hypersurface tex2html_wrap_inline4679 and that part of tex2html_wrap_inline3905 which is in the future of tex2html_wrap_inline4679 . The data which have to be prescribed are essentially the so-called null data on tex2html_wrap_inline4679 and tex2html_wrap_inline3905, i.e. those parts of the rescaled Weyl tensor which are entirely intrinsic to the respective null hypersurfaces. In the case of tex2html_wrap_inline3905 the null datum is exactly the radiation field.

Theorem 3:  [Kánnár [88]]  For given smooth null data on an ingoing null hypersurface tex2html_wrap_inline4679 and a smooth radiation field on the part tex2html_wrap_inline4693 of tex2html_wrap_inline3849 which is to the future of the intersection S of tex2html_wrap_inline4679 with tex2html_wrap_inline3849 and certain data on S, there exists a smooth solution of Einstein's vacuum equations in the future of tex2html_wrap_inline4705 which implies the given data on tex2html_wrap_inline4705 .  

The result is in complete agreement with Sachs' earlier analysis of the asymptotic characteristic initial value problem based on formal expansion methods [128].

Another case is concerned with the existence of solutions representing pure radiation. These are vacuum solutions characterized by the fact that they are smoothly extensible through past time-like infinity, i.e. by the regularity of the point tex2html_wrap_inline4132 . This case has been treated in [49, 51]. A solution of this kind is uniquely characterized by its radiation field, i.e. the intrinsic components of the rescaled Weyl tensor on tex2html_wrap_inline3849 . In the analytic case, a formal expansion of the solution at tex2html_wrap_inline4132 can be derived, and growth conditions on the coefficients can be given to ensure convergence of the formal expansion near tex2html_wrap_inline4132 . Furthermore, there exists a surprising relation between this type of solutions and static solutions, summarized in

Theorem 4:  [Friedrich]  With each asymptotically flat static solution of Einstein's vacuum field equations can be associated another solution of these equations which has a smooth conformal boundary tex2html_wrap_inline3849 and for which the point tex2html_wrap_inline4132 is regular.  

This result establishes the existence of a large class of purely radiative solutions.

For applications, however, the most important type of initial value problem so far, in the sense that the asymptotic behaviour can be controlled, has been the hyperboloidal initial value problem where data are prescribed on a hyperboloidal hypersurface. This is a space-like hypersurface whose induced physical metric behaves asymptotically like a surface of constant negative curvature (see Section  2.4). In the conformal picture, a hyperboloidal hypersurface is characterized simply by the geometric fact that it intersects tex2html_wrap_inline3905 transversely in a two-dimensional space-like surface. Prototypes of such hypersurfaces are the space-like hyperboloids in Minkowski space-time. In the Minkowski picture they can be seen to become asymptotic to null cones which suggests that they reach null-infinity. However, the picture is deceiving: The conformal structure is such that the hyperboloids always remain space-like, the null-cones and the hyperboloids never become tangent. The intersection is a two-dimensional surface S, a ``cut'' of tex2html_wrap_inline3905 . The data implied by the conformal fields on such a hypersurface are called hyperboloidal initial data. The first result obtained for the hyperboloidal initial value problem states that if the space-time admits a hypersurface which extends smoothly across tex2html_wrap_inline3847 with certain smooth data given on it, then the smoothness of tex2html_wrap_inline3847 will be guaranteed at least for some time into the future. This is contained in

Theorem 5:  [Friedrich [47]]  Smooth hyperboloidal initial data on a hyperboloidal hypersurface tex2html_wrap_inline4731 determine a unique solution of Einstein's vacuum field equations which admits a smooth conformal boundary at null-infinity in the future of tex2html_wrap_inline4731 .  

There exists also a stability result which states that there are solutions which behave exactly like Minkowski space near future time-like infinity:

Theorem 6:  [Friedrich [50]]  If the hyperboloidal initial data are in a sense sufficiently close to Minkowskian hyperboloidal data, then there exists a conformal extension of the corresponding solution which contains a point tex2html_wrap_inline4134 such that tex2html_wrap_inline3847 is the past null cone of that point.  

It should be emphasized that this result implies that the physical metric of the corresponding solution is regular for all future times . Thus, the theorem constitutes a (semi-)global existence result for the Einstein vacuum equations.


image Conformal Infinity
Jörg Frauendiener
© Max-Planck-Gesellschaft. ISSN 1433-8351
Problems/Comments to livrev@aei-potsdam.mpg.de