Weakly regular T2 symmetric spacetimes. The future causal geometry of Gowdy spaces

TL;DR

This paper studies the future causal structure of weakly regular Gowdy spacetimes on T3, proving they are future causally geodesically complete and extending previous results to lower regularity settings.

Contribution

It establishes future causal geodesic completeness for Gowdy spacetimes with weak regularity, using novel energy decay and geodesic analysis methods.

Findings

Proves weakly regular Gowdy spacetimes are future causally geodesically complete.

Extends Ringström's theorem to lower regularity metrics.

Provides an alternative, shorter proof of energy decay and geodesic completeness.

Abstract

We investigate the future asymptotic behavior of Gowdy spacetimes on T3, when the metric satisfies weak regularity conditions, so that the metric coefficients (in suitable coordinates) are only in the Sobolev space H1 or have even weaker regularity. The authors recently introduced this class of spacetimes in the broader context of T2 symmetric spacetimes and established the existence of a global foliation by spacelike hypersurfaces when the time function is chosen to be the area of the surfaces of symmetry. In the present paper, we identify the global causal geometry of these spacetimes and, in particular, establish that weakly regular Gowdy spacetimes are future causally geodesically complete. This result extends a theorem by Ringstr\"om for metrics with sufficiently high regularity. We emphasize that our proof of the energy decay is based on an energy functional inspired by the Gowdy-to-Ernst transformation. In order to establish the geodesic completeness property, we prove a higher regularity property concerning the metric coefficients along timelike curves and we provide a novel analysis of the geodesic equation for Gowdy spacetimes, which does not require high-order regularity estimates. Even when sufficient regularity is assumed, our proof provides an alternative and shorter proof of the energy decay and of the geodesic completeness property for Gowdy spacetimes.