Instability of Spacelike and Null Orbifold Singularities

TL;DR

The paper demonstrates that time-dependent Lorentzian orbifolds with spacelike and null singularities are inherently unstable: a single localized particle can trigger a global gravitational collapse to a Big Crunch through backreaction, as shown first via general-relativistic covering-space analyses and shock-wave physics, and then via string-theory considerations (eikonal scattering and backreaction). It shows that perturbation theory breaks down long before horizon formation in these setups, implying that such orbifolds do not provide a simple safe backdrop for cosmology. Generalizations with shifts (null branes) can delay or modify horizon formation in some cases, but the underlying instability persists as the deformation parameters are reduced. Overall, the work cautions against relying on these orbifolds to realize a cosmological bounce and emphasizes the need to confront strong-curvature dynamics in string theory.

Abstract

Time dependent orbifolds with spacelike or null singularities have recently been studied as simple models of cosmological singularities. We show that their apparent simplicity is an illusion: the introduction of a single particle causes the spacetime to collapse to a strong curvature singularity (a Big Crunch), even in regions arbitrarily far from the particle.