When light from an object takes longer to reach us, how do we know that it’s spacetime that’s “moving”, and not the object itself?

Question

I guess that it’s easier to tell when the shift is happening faster than the speed of light, but what about when it’s not that fast?

In: Physics

Answer 1

With a single object, there’s no real way to know. However, there is a trend that points in this direction when you look at all of the objects in the sky.

As a general rule, everything is moving away from us. Additionally, the farther something is from us, the faster it is moving, and the speed with which things are moving away from us is directly correlated with the distance.

This leaves us with two options. Either Earth is literally the center of the entire universe and we are super duper extra special, or we aren’t and something else is going on. And if you take the position that we are average and not literally the center of the universe and then try to model what is going on, it turns out that what we see in the sky maps very accurately onto the metric expansion of space.