On the Canonical Contact Structure of the Space of Null Geodesics of a Spacetime

Abstract: The space of null geodesics of a spacetime carries a canonical contact structure which has proved to be key in the discussion of causality in spacetimes. However, not much progress has been made on its nature and not many explicit calculations for specific spacetimes can be found over the literature. We compute the spaces of null geodesics and their canonical contact structures for the manifold $\mathbb{S}^2\times\mathbb{S}^1$ equipped with the family of metrics ${g_c = g_\circ-\frac{1}{c^2}dt^2 }_{c\in\mathbb{N}^+}$. We obtain that these are the lens spaces $L(2c,1)$ and that the contact structures are the pushforward of the canonical contact structure on $ST\mathbb{S}^2\cong L(2,1)$ under the projection map. We also study the applicability of Engel geometry on the discussion of three-dimensional spacetimes. We show that, for a particular type of three-dimensional spacetimes, one can obtain the space of null geodesics and its contact structure solely from the information of the Lorentz prolongation of the spacetime. We present an approach that makes use of this result to recover the spacetime from its space of null geodesics and skies.