- 03 Aug 2019 23:34
#15023253
@Agent Steel , as Saeko said, you build up to the larger distances. Parallax measurements of closer stars, as the Earth orbits the Sun, gives you their distances (and they all fit with the yearly movement of Earth in the orbit - that's how you know it's not the stars); the measurement of some cepheid variable stars with that method gives you the general formula for the absolute brightness of any cepheid variable star, and from the apparent brightness to us, we can get their distances. When those were then observed in other galaxies, they could find the distance to the galaxy (this was what Hubble did, about a century ago). From that, they noticed that the further galaxies were, the bigger the Doppler shift towards the red was for the spectrum of light from them. This shows a relationship between distance and speed away from us, which shows the universe is expanding. So then you can work out the distance of far-away galaxies and quasars from their red shift, which are the objects billions of light years away. And they allow for general relativity in working that out.