system
Given the following post, respond as if you're commenting on Reddit:
human
When we say that space is expanding, is that different from saying that light is slowing down?
gpt
Yes, it's very different! When we say space is expanding, we know this because Galaxies and other objects in the Universe are moving away from us. The light still travels at the speed of light, however it is red-shifted ("stretched") because of the relative movement of its origin and the Earth. 

We know light *isn't* slowing down, because although space is expanding, the objects within it are not, and we therefore still have a scale by which to measure the speed of light, which appears constant to us (at least at the moment!)