Let (M , g) be a time-and space-oriented Lorentzian spin manifold, and let M be a compact spacelike hypersurface of M with induced Riemannian metric g and second fundamental form K. If (M , g) satisfies the dominant energy condition in a strict sense, then the Dirac-Witten operator of M ⊆ M is an invertible, self-adjoint Fredholm operator. This allows us to use index theoretical methods in order to detect non-trivial homotopy groups in the space of initial on M satisfying the dominant energy condition in a strict sense. The central tool will be a Lorentzian analogue of Hitchin's α-invariant. In case that the dominant energy condition only holds in a weak sense, the Dirac-Witten operator may be non-invertible, and we will study the kernel of this operator in this case. We will show that the kernel may only be non-trivial if π 1 (M ) is virtually solvable of derived length at most 2. This allows to extend the index theoretical methods to spaces of initial data, satisfying the dominant energy condition in the weak sense. We will show further that a spinor ϕ is in the kernel of the Dirac-Witten operator on (M, g, K) if and only if (M, g, K, ϕ) admits an extension to a Lorentzian manifold (N , h) with parallel spinor φ such that M is a Cauchy hypersurface of (N , h), such that g and K are the induced metric and second fundamental form of M , respectively, and ϕ is the restriction of φ to M .
DOMINANT ENERGY CONDITION AND SPINORS ONLORENTZIAN MANIFOLDS 1 4.1. The hypersurface spinor bundle 10 4.2. Dirac currents 11 4.3. The Cauchy problem for parallel spinors 12 5. The kernel of the Dirac-Witten operator 15 5.1. Dirac-Witten operators and Schrödinger-Lichnerowicz formula 15 5.2. From the kernel of the Dirac-Witten operator to initial data triples 16 5.3. Examples of lightlike (generalized) initial data triples 20 5.4. Lightlike initial data manifolds and other notation 22 5.5. Lightlike generalized initial data triples with compact leaves 24 5.6. More results for all lightlike generalized initial data triples 25 5.7. Further results in the case of non-compact leaves 29 5.8. Conclusions 32 6. Homotopy groups of I > (M ) and I ≥ (M ) 32 6.1. Initial data sets and positive scalar curvature 33 6.2. The α-index and index difference for psc metrics 35 6.3. The index difference for initial data sets strictly satisfying DEC 37 6.4. Application to general relativity 40 Concluding remark 42 Appendix A. The Taylor development map for Ricci-flat metrics 42 Appendix B. Proof of Lemma 5.17 45 References 46