Let $(overline M,overline g)$ be a time- and space-oriented Lorentzian spin manifold, and let $M$ be a compact spacelike hypersurface of $overline M$ with induced Riemannian metric $g$ and second fundamental form $K$. If $(overline M,overline g)$ satisfies the dominant energy condition in a strict sense, then the Dirac--Witten operator of $Msubseteq overline M$ 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 Hitchins $alpha$-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 $pi_1(M)$ 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 $phi$ is in the kernel of the Dirac--Witten operator on $(M,g,K)$ if and only if $(M,g,K,phi)$ admits an extension to a Lorentzian manifold $(overline N,overline h)$ with parallel spinor $barphi$ such that $M$ is a Cauchy hypersurface of $(overline N,overline h)$, such that $g$ and $K$ are the induced metric and second fundamental form of $M$, respectively, and $phi$ is the restriction of $barphi$ to $M$.
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