10

PAUL E. EHRLICH AND KEVIN L. EASLEY

A celebrated early example of ( 4.3) is the Topological Sphere Theorem of Rie-

mannian geometry of the 1950s and 1960s: suppose that a complete, simply con-

nected Riemannian manifold admits a metric whose sectional curvatures vary be-

tween 1/4 and 1, but are always strictly greater than 1/4. Then the manifold M

must be homeomorphic to the standard round sphere of the same dimension as the

given manifold.

In contrast with the Riemannian situation, we have already remarked in Sec-

tion 1 that many standard examples of space-times fail to be nonspacelike geodesi-

cally complete. As we thought about ( 4.3) and certain of the singularity theorems

already published in General Relativity from

the viewpoint of differential geometry,

the following contrasting pattern (4.4) emerged:

(4.4) curvature inequality (gravitation is attractive)

and

physical or geometric assumption

implies

the existence of an incomplete timelike or nonspacelike geodesic.

As we were working on the concept of causal disconnection, we also had as a

motivation that some cosmological and physical models are not globally hyperbolic,

but are only strongly causal. Thus we wanted to establish a formalism in the more

general strongly causal setting for which (a) the space-time distance function may

be less tractable than for globally hyperbolic space-times, and also (b) the tool

of maximal nonspacelike geodesic connectability is not available. (Later, in the

context of the Lorentzian splitting theorem in the timelike geodesically complete

case, Newman

[76]

and Galloway and Horta

[57]

would find the use of almost max-

imizers essential.) We were also motivated by the theory of the end structure for

Riemannian or topological manifolds in establishing this concept. Strong causality

did at least have the virtue that convergence in the limit curve sense and conver-

gence in the

C

0 topology on curves were closely related, as well as the fact that the

upper semi-continuity of arc length in the

C

0

topology fits in well with the lower

semi-continuity of the space-time distance function

[cf.

Beem, Ehrlich and Easley

[24],

Section 3.3].

As the concept was stated in

[24],

p. 283:

DEFINITION

4.1. A space-time (M,g) is said to be causally disconnected by

a compact set K if there exist two infinite sequences {Pn} and { qn} diverging to

infinity such that for each n, Pn

~

qn, Pn

of.

qn, and all future directed nonspacelike

curves from Pn to qn meet K.

This definition as finally formulated had the virtue that all Lorentzian metrics

in the conformal class C(M,g) are causally disconnected if any one of them is,

unlike the original formulation in Beem and Ehrlich

[16]

which also assumed the

finite distance condition

(4.5) 0 d(pn, qn) +oo for all n.

This finiteness assumption made it possible to use a simpler limit curve con-

struction procedure, but unfortunately this earlier version of causal disconnection

was only conformally invariant in the case that ( M,

g)

was globally hyperbolic, so