arXiv subjects

Oskar Schiller

Publications and source records attributed to Oskar Schiller.

3 recordsLinked to original sources

arXiv2026

The Initial Value Problem for the Generalised Einstein Equations

We discuss the initial value problem for the Einstein equations in Hitchin's generalised geometry for the case of closed divergence (which correspond to the equations of motion in the bosonic part of the NS-NS sector in type II ten-dimensional supergravity) and establish the existence of a maximal globally hyperbolic development (MGHD). The dynamical fields, defined on a manifold of dimension $n+1$, are the space-time metric, a scalar field known as the dilaton function, and a two-form known as the $B$-field. We develop a generalisation of the Lorenz gauge which, applied to the $B$-field (and combined with a suitable gauge condition breaking diffeomorphism invariance), renders the system a wave equation with principal symbol given by the (dynamical) metric. Given initial data, we construct a development satisfying the gauge conditions. We show that all other developments are (in the appropriate sense) related to this development by a diffeomorphism, establishing geometric uniqueness. The existence of the MGHD follows then by a famous result by Choquet-Bruhat and Geroch. In showing existence and geometric uniqueness of developments, we follow an approach developed in detail by Ringström for the Einstein equations coupled to a scalar field. In a preliminary section, we present a formulation which is disentangled from the specific assumptions made on the matter, so that adaptation to other systems is straightforward.

math.DG↗

arXiv2025

The canonical generalised Levi-Civita connection and its curvature

Given a (semi-Riemannian) generalised metric $\mathcal G$ and a divergence operator $\mathrm{div}$ on an exact Courant algebroid $E$, we geometrically construct a canonical generalised Levi-Civita connection $D^{\mathcal G, \mathrm{div}}$ for these data. In this way we provide a resolution of the problem of non-uniqueness of generalised Levi-Civita connections. Since the generalised Riemann tensor of $D^{\mathcal G, \mathrm{div}}$ is an invariant of the pair $(\mathcal G, \mathrm{div})$, we no longer need to discard curvature components which depend on the choice of the generalised connection. As a main result we decompose the generalised Riemann curvature tensor of $D^{\mathcal G, \mathrm{div}}$ in terms of classical (non-generalised) geometric data. Based on this set of master formulas we derive a comprehensive curvature tool-kit for applications in generalised geometry. This includes decompositions for the full generalised Ricci tensor, the generalised Ricci tensor, and three generalised scalar-valued curvature invariants, two of which are new.

math.DG↗

arXiv2025

Exterior Generalised Geometry

It is the aim of this paper to transfer to generalised geometry tools employed in the study of semi-Riemannian immersions, specializing at times to semi-Riemannian hypersurfaces. Given an exact Courant algebroid $E \to M$ and an immersion $ι\colon N \hookrightarrow M$, there is a well-known construction of an exact Courant algebroid $ι^! E \to N$, the pullback of $E$. This paper explains the pullback of generalised metrics and divergence operators. Assuming $N$ is a hypersurface, it develops the notion of generalised exterior curvature, introducing the generalised second fundamental form and the generalised mean curvature. Generalised versions of the Gauß-Codazzi equations are obtained. As an application, the constraint equations for the initial value formulation of the generalised Einstein equations are established in the formalism of generalised geometry. Further applications include a generalised geometry version of the fundamental theorem for hypersurfaces and the result that generalised Kähler and hyper-Kähler structures restrict to submanifolds compatible with the generalised almost complex structure. In particular, we characterise exact semi-Riemannian Courant algebroids which are flat with respect to the canonical generalised connection. These play the role of the ambient space in the fundamental theorem mentioned above.

math.DG↗