Isometric Splitting of Metrics Without Conjugate Points on Σ × S^1

We study the geometry of Riemannian metrics without conjugate points on manifolds which are diffeomorphic to \(M = \Sigma \times S^1\), where \(\Sigma\) is a compact orientable surface of genus \(g \geq 2\). This addresses a question related to the generalized Hopf conjecture: whether such metrics must necessarily exhibit a product structure on the universal cover, despite the negatively curved nature of \(\Sigma\). We prove that any such metric \(g\) forces the universal cover \((\tilde{M}, \tilde{g})\) to split isometrically as a Riemannian product \((\mathbb{H}^2, g_0) \times (\mathbb{R}, c^2du^2)\), where \((\mathbb{H}^2, g_0)\) is the hyperbolic plane equipped with a complete \(\pi_1(\Sigma)\)-invariant metric and \(c > 0\) is a constant. This affirmatively resolves the question and extends rigidity theorems known for flat tori and manifolds of non-positive curvature. We present two proofs: the main proof relies on the analysis of Busemann functions associated with the lifted \(S^1\)-action, while an alternative proof utilizes Jacobi field analysis along the flow lines of the corresponding Killing field. Both approaches show that the absence of conjugate points compels the horizontal distribution orthogonal to the Killing field flow to be parallel and integrable, leading to a global isometric splitting via the de Rham theorem. Several geometric and dynamical consequences follow from this rigid structure.