Regularity of Cauchy horizons in S2xS1 Gowdy spacetimes

We study general S2xS1 Gowdy models with a regular past Cauchy horizon and prove that a second (future) Cauchy horizon exists, provided that a particular conserved quantity $J$ is not zero. We derive an explicit expression for the metric form on the future Cauchy horizon in terms of the initial data on the past horizon and conclude the universal relation $A\text{p}A\text{f}=(8\pi J{)}^2$ where $A\text{p}$ and $A\text{f}$ are the areas of past and future Cauchy horizon respectively.