# Inertial mass increase of the farthest receding galaxies

If the farthest galaxies that we can see are receding at near the speed of light, shouldn't this increase their inertial mass as well relative to our galaxy?

• – user4552 Apr 17 '18 at 22:08

But the main issue here is that general relativity doesn't have any unambiguous way of defining the motion of one object relative to another when those objects are at cosmological distances from one another. Another way of putting it is that GR doesn't have global frames of reference, only local ones. For this reason, we can't really say whether distant galaxies are receding from us at close to the speed of light. Depending on what coordinates you pick, you can say that they are not moving at all (space is just expanding in between) or that they are receding from us faster than $c$, while still being observable.