Geodesic flow on a compact manifold is defined for all time

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How can I prove that on a compact manifold, the geodesic flow is defined for all time? Is this as simple as citing the Hopf-Rinow theorem?

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Besides Hopf-Rinow you can use a general fact. Namely that the flow of a vector field defined on a compact manifold is complete i.e. defined for all time. Then you should know that the geodesic flow is defined by a vector field (some times called spray) defined on the unit tangent bundle of the compact manifold. Such unit tangent bundle is also compact hence the geodesic flow is complete.