Please forgive a probably naïve question from a lay person.
I understand that galaxies are all moving away from us in every direction we look. And for any given distance the speed is the same. And the greater the distance, the greater the speed.
From which it is inferred that the speed is not constant but accelerating.
Now the further away an object is from the observer, the longer ago it emitted its light. But also. the further away it is the older it is - or rather was. We have no way of predicting what happened to it in the intervening (for example) 8 billion years.
My naïve question is how it is known that the correlation between distance/age and speed does not have the opposite implication? Everything nearer to us is moving away more slowly than everything further away.
I found a short article by Hao-Ran Yu of Cornell, (https://arxiv.org/abs/1311.2363). describing (back in 2013) the measurement of acceleration as “indirect”. It seems to involve radio telecopy in some way.
- Is there a lay way of explaining how it is done?
- how reliable is this method?