Which way does light travel in a homogeneous medium?
See what Einstein said in his 1920 Leyden Address: "This space-time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that 'empty space' in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gμν), has, I think, finally disposed of the view that space is physically empty".
Einstein was saying space was a medium, an "aether", and that a gravitational field was a place where space was neither homogeneous nor isotropic. Light curves downwards in a gravitational field, where space is inhomogeneous. If space is homogeneous, light doesn't curve at all. It goes straight.
You might think this aether is archaic, but it isn't. Julian Schwinger wrote a paper in 1949 called quantum electrodynamics II : vacuum polarization and self-energy. He was saying space is a polarizable medium. You can also find modern authors saying much the same thing. See for example the 2008 paper Inhomogeneous vacuum: an alternative interpretation of curved spacetime. Also see the Wikipedia aether theories article and note the 2005 quote by Robert B Laughlin: "it is ironic that Einstein's most creative work, the general theory of relativity, should boil down to conceptualizing space as a medium when his original premise [in special relativity] was that no such medium existed". Laughlin also said space is more like a piece of window glass than ideal Newtonian emptiness. He finished up saying this: "the modern concept of the vacuum of space, confirmed every day by experiment, is a relativistic ether. But we do not call it this because it is taboo".