As a consequence of the Lorentzian version of Liouville’s Theorem, everyconformally flat space-time of dimension 3 is a (Ein1,n,O0(2, n + 1))-manifold. The Einstein’s space-time Ein1,n is the space Sn × S1 with theconformal class of the metric d2−dt2, where d2 and dt2 are the canonicalRiemannian metrics of Sn and R. The group O0(2, n+1) is the group of theconformal diffeomorphisms of Ein1,n whose action preserve the orientationand the time-orientation of Ein1,n. A space-time M is globally hyperbolicif it contains a spacelike hypersurface which intersects every inextensiblecausal curve of M exactly in one point. As a consequence M is not compact.The hypersurface is called a Cauchy hypersurface of M. Geroch’s Theorem([?]) say that if M is globally hyperbolic, then M is homeomorphic to×R. There is a naturally defined partial order on the set of globally hyperbolicspace-times (up to conformal diffeomorphism) : M M0 if does existsa conformal embedding f : M ,! M0 which sends Cauchy hypersurfaces ofM to Cauchy hypersurfaces of M0 (f is called a Cauchy-embedding ). Wecall C-maximal space-times the maximal elements for this partial order onthe set of globally hyperbolic space-times. We can restrict the partial orderto the subset of conformally flat space-times : in this case we call themaximal elements C0-maximal space-times. The first result of the thesis isa generalization of a Theorem proved by Choquet-Bruhat and Geroch in[?] : let M be a globally hyperbolic conformally flat space-time. Then thereis a globally hyperbolic conformally flat C0-maximal space-time N and aCauchy-embedding f : M ,! N. The space-time N is unique up to conformaldiffeomorphisms.The uniqueness of the C0-maximal extension imply that every globally hyperbolicconformally flat simply connected C0-maximal space-time (of dimension3) with a compact Cauchy hypersurface is conformally diffeomorphicto gEin1,n.In the second part of the thesis we study the injectivity of the developingmap of a globally hyperbolic conformally flat space-time M looking at theshape of its the causal boundary.We say that two points p, q are conjugatedin a space-time M if there are two different lightlike geodesics and whichstart at p and meet at q, such that and don’t intersect between p and q.The most remarkable result of this part is : let M a globally hyperbolicconformally flat C0-maximal space-time. If fM has two conjugated pointsthen fM ' gEin1,n. In particular M is a finite quotient of gEin1,n.As a consequence of this result we obtain that the developing map of Mrestricted to the chronological past and future of every point is injective.In the last part of the thesis we give an abstract construction of the Cmaximalextension for a given conformally flat globally hyperbolic spacetime.The idea is that a globally hyperbolic space-time is completely determinedby one of his Cauchy hypersurfaces. This result helps to understandhow to relate the different notions of maximality. In particular we provethat every conformally flat globally hyperbolic space-time M which is C0-maximal is also C-maximal.
