How can astronomers tell how ‘old’ light is? In other words, how can they tell if they are looking at an object 10 light years away or 1009 light years?

Question

How can astronomers tell how ‘old’ light is? In other words, how can they tell if they are looking at an object 10 light years away or 1009 light years?

In: Physics

Answer 1

As the universe expands, light expands with it, redshifting the light. That’s the essence of Hubble’s Law. The older the light, the more its waves will be expanded (and its frequencies redshifted) by its journey through the expanding universe. So astronomers can look at a galaxy’s spectrum, find the emission and absorption peaks of different elements, see how much they’ve shifted, and then apply Hubble’s Law to that shift to figure out how far away that galaxy is.

For close objects, light “age” is known from a distance measurement, as Charyou-Tree said.