If you use toroidal boundary conditions then you could remain in an inertial reference frame and return back to the same pont. See https://en.m.wikipedia.org/wiki/Periodic_boundary_conditions
In that case you would be traveling a curved path so there would be acceleration even if your speed remained constant.
I think the question you are asking is what if the universe is toroidal. What if you traveled so far you came back around.
If you use toroidal boundary conditions then you could remain in an inertial reference frame and return back to the same pont. See https://en.m.wikipedia.org/wiki/Periodic_boundary_conditions
In that case you would be traveling a curved path so there would be acceleration even if your speed remained constant.
I think the question you are asking is what if the universe is toroidal. What if you traveled so far you came back around.