# Galaxies moving in fixed background or fixed in expanding background

Question: what is the most direct experimental evidence to suggest that the galaxies in the universe are fixed in an expanding spacetime, as opposed to moving in a fixed spacetime?

• It has been claimed elsewhere on this site$^1$ that there is no physical difference between the two scenarios. This seems wrong to me. An expanding background has non-zero Riemann tensor, and in principle this should be physically detectable through measurements of geodesic deviation and geometric quantities such as angle defect.
$^1$This almost identical question doesn't have answers which satisfy me. For instance, with regards the top response, whilst it may be that the observed motion of galaxies becomes somewhat contrived in the case that spacetime is fixed, this doesn't rule it out categorically.