Coordinate confusion in conformal cosmology

Abstract

Abstract A straightforward interpretation of standard Friedmann–Lemaître–Robertson–Walker (FLRW) cosmologies is that objects move apart because of the expansion of space, and that sufficiently distant galaxies must be receding at velocities exceeding the speed of light. Recently, however, it has been suggested that a simple transformation into conformal coordinates can remove superluminal recession velocities, and hence the concept of the expansion of space should be abandoned. This work demonstrates that such conformal transformations do not eliminate superluminal recession velocities for open or flat matter-only FLRW cosmologies, and all possess superluminal expansion. Hence the attack on the concept of the expansion of space based on this is poorly founded. This work concludes by emphasizing that the expansion of space is perfectly valid in the general relativistic framework; however, asking the question of whether space really expands is a futile exercise.