Desingularization of the Milne Universe

TL;DR

The paper tackles the Milne universe singularity by embedding it in a flat 2+1D higher spin gravity framework and using a Grassmann contraction to import AdS techniques. It shows that a flat-space higher-spin Chern-Simons setup can desingularize the Milne geometry via a carefully chosen gauge transformation that preserves the original holonomies. The authors construct an explicit ansatz with higher-spin fields that preserves Milne symmetries and yields a finite Ricci scalar, producing a smooth bounce and regular spin-3 fields. They argue this gauge-based desingularization aligns with the tensionless (alpha' -> infinity) limit of string theory and does not rely on non-perturbative string effects, suggesting a novel mechanism for resolving cosmological singularities in a gauge-theoretic setting.

Abstract

Resolution of cosmological singularities is an important problem in any full theory of quantum gravity. The Milne orbifold is a cosmology with a big-bang/big-crunch singularity, but being a quotient of flat space it holds potential for resolution in string theory. It is known however, that some perturbative string amplitudes diverge in the Milne geometry. Here we show that flat space higher spin theories can effect a simple resolution of the Milne singularity when one embeds the latter in 2+1 dimensions. We explain how to reconcile this with the expectation that non-perturbative string effects are required for resolving Milne. Along the way, we introduce a Grassmann realization of the İnönü-Wigner contraction to export much of the AdS technology to our flat space computation.