Can every curve on a Riemannian manifold be interpreted as a geodesic of a given metric?

Solution 1:

I'm not sure what happens for general curves, but I think I can prove the following:

Let $\gamma:[0,1]\rightarrow M$ be any injective curve segement. Then there is a Riemannian metric for which $\gamma$ is a geodesic. If instead $\gamma$ is a simple closed curve and $\gamma'(0) = \gamma'(1)$, the conclusion still holds.

I'm not sure what happens in the other cases.

Here's the idea of the proof in the (slightly harder) second case:

Pick a background Riemannian metric once and for all. The normal bundle of $\gamma$ embeds into $M$ via the exponential map (for a suitably short time). Call the image of this embedding $W$. Choose an open set $V$ with the property that $V\subseteq \overline{V}\subseteq W$ and let $U = M-\overline{V}$. Notice that $W\cup U = M$, so we can find partition of unity $\{\lambda_U,\lambda_W\}$ subordinate to $\{U,W\}$.

Now, the classification of vector bundles over circles is easy: There are precisely 2 of any rank - the trivial bundle of rank $k$ and the Möbius bundle + trivial bundle of rank $k-1$. The point is that both of these have (flat) metrics where the $0$ section ($\gamma$) is a geodesic.

Since $W$ is diffeomorphic to a vector bundle over the circle, we can assume it has a metric $g_W$ for which $\gamma$ is a geodesic. Now, pick any Riemannian metric $g_U$ on $U$. Finally, define the metric $g_M$ on $M$ by $\lambda_W g_W + \lambda_U g_U$. This is a convex sum of metrics, and hence is a metric. Near $\gamma$, $\lambda_U \equiv 0$ and $\lambda_W\equiv 1$, so the metric near $\gamma$ looks just like $g_W$, so $\gamma$ is a geodesic in $M$.

Solution 2:

I expect that if $M$ is a (connected!) differentiable manifold and $\gamma_1, \gamma_2: S^1 \rightarrow M$ are any two smooth embeddings, there is a diffeomorphism $\Phi: M \rightarrow M$ such that $\gamma_2 = \Phi \circ \gamma_1$. If so, this gives a positive answer to your question restricted to smoothly embedded loops. And something similar should work for smooth embeddings of $\mathbb{R}$ with closed image.

Added: The above is certainly not generally valid: I seem to have forgotten about the fundamental group. It seems like it might still have a chance to hold in the simply connected case. (Also, in the case of surfaces, if you take a metric of constant curvature, I seem to recall that every homotopy class has a unique geodesic representative, so this obstruction is not a problem at least in that case.)

As for the second question: of course there are going to be many Riemannian metrics than geodesic curves: changing the metric in an open set bounded away from the geodesic will certainly not disturb that curve's being a geodesic. As for changes of metric which preserve all geodesic curves rather than just a given one, that's a more interesting question, but at least you can uniformly rescale the metric without affecting any of the geodesics.