It's perfectly possible to say when things happened at Alpha Centauri. Relativity doesn't really start messing with the notion of simultaneity until you're in a

It's perfectly possible to say when things happened at Alpha Centauri. Relativity doesn't really start messing with the notion of simultaneity until you're in a different reference frame, i.e. moving at a different speed from the observer you're comparing yourself to. But if the only difference is distance, and you know the distance, you really can just run the speed of light backward and get a meaningful time in your own past for a distant event you're just now getting the light from. Think of it this way: since you're moving the same speed, there's no time dilation between your clocks, so you can safely line up ticks between your clock and a hypothetical clock far away on Alpha Centauri or whatever. I assume things get a little more interesting at big redshifts, because you really are moving at relativistic speeds wrt what you're observing. I don't know off hand how they handle that. I will say I'm pretty sure the reason cosmologists tend to talk in redshifts is that redshifts can be pretty much directly measured and actual distance is... tricky. If you haven't already looked into the "cosmic distance ladder" you're likely to find it interesting. Source: we covered this stuff in a college class. My math is very rusty, but I did do the math.