How do they calculate light-years?

Question

For example, how do they know that the Eistein Ring is “12 billion light-years” away? Why not 10 or 20? How do they arrive at that number?

In: 2

Answer 1

I just want to add a quick answer to “why not 20 billion light years” – because the universe, as far as we know today, is only 13.8 billion years old, so light can not possibly have travelled 20 billion light years. You know, “light years” is not just a measurement of distance, but also tells us how long the light we’re seeing has travelled, and thus how old it is…