$L^2$-Gamma-Fredholmness for spacetimes

TL;DR

This paper proves the Fredholmness of a $ ext{L}^2$-$ extGamma$-invariant Lorentzian Dirac operator on certain globally hyperbolic spacetimes with boundary, extending previous results to more general boundary conditions.

Contribution

It establishes the $ ext{L}^2$-$ extGamma$-Fredholmness of Lorentzian Dirac operators under generalized Atiyah-Patodi-Singer boundary conditions, building on prior work.

Findings

Fredholmness of the Lorentzian Dirac operator established

Extension to generalized Atiyah-Patodi-Singer boundary conditions

Results applicable to Galois coverings of globally hyperbolic manifolds

Abstract

Let $M$ be a temporal compact globally hyperbolic manifold with Cauchy hypersurface $\Sigma$ which is a Galois covering with respect to a discrete group $\Gamma$ of automorphisms such that the quotient $\Sigma/\Gamma$ is compact without boundary. We will show Fredholmness of a (spatial) $\Gamma$-invariant Lorentzian Dirac operator under (anti) Atiyah-Patodi-Singer boundary conditions in the von Neumann sense, known as ($L^2$-)$\Gamma$-Fredholmness. The results already have been published for a simpler setting in arXiv:2107.08532. This version is only focused on the Fredholmness part, based on previous results from arXiv:2409.17344, and also generalised (anti) Atiyah-Patodi-Singer boundary conditions are going to be considered as well.