For some parts relative to us, only a billion years would have passed, for others maybe 20?
In: Physics
First of all, ask yourself the question: why is the universe so dark? If the universe is infinite and contains an infinite number of stars, shouldn’t it be brighter? A lot of smart people asked this question too and it’s know as [Olber’s paradox](https://en.wikipedia.org/wiki/Olbers%27_paradox).
Next, convince yourself that the further you look in the universe, the further you look in time. If you don’t understand this, let me try to explain it with a simple analog.
Suppose you are giving a small party. One of your friends lives 2km away and the another one 15km. they both travel by bike, because they care about the environment, with a constant velocity of 20km/h and arrive at the same time at your place. which one of them has left his house the earliest?
The same reasoning can be done for a [photon](https://en.wikipedia.org/wiki/Photon) (fancy name for light particles, “why is light a particle?”, that’s a whole other story). A photon that comes from a distant star will have traveled a lot longer than a photon from the sun and thus has to leave the star at an earlier time than the photon from the sun.
Now back to Olber’s paradox. The paradox can be solved by stating that the universe has a finite age (13.8 billion year). This means that we can only see photons that have traveled less than 13.8 billion years. Or in other words, and this is the clue, we can only see photons from stars that are less than 13.8 billion light years away (a light year is the distance a photon can travel in one year). Remember: the further you look in the universe the further you look in time. We don’t see every star in the universe and thus the universe is a lot darker (less stars ==> less light). Saying that the universe is 13.8 billion years, is thus a way the explain why the universe is so dark.
I now, it’s a lot to read, but I need to explain just one more thing before I can answer your question, so bear with me 😉
The universe is not completely dark. If we look at a dark spot on the hemisphere, we can still detect some radiation. This is known as the [cosmic microwave background](https://en.wikipedia.org/wiki/Cosmic_microwave_background). Smart people have stated that this is the light from the [Big Bang](https://en.wikipedia.org/wiki/Big_Bang) (the beginning of the universe) that has “dimmed” over time. This background light is almost the same at every place in the universe.
Aha! so now we can understand why we say the universe is 13.8 billion years old. The finite age of the universe depends on how dark the universe is and because the universe is equally dark at every point, we can find that the universe is 13.8 billion years old no matter where we look.
I know that this isn’t a direct answer to your question, but in order to give you that, I think that we need to talk about special relativity, redshift, [Hubble’s law](https://en.wikipedia.org/wiki/Hubble%27s_law), … Stuff that makes your head spin, or at least mine, and I don’t think I can do that in a reddit comment.
Just keep in mind that a lot of smart people are looking at stars and that they figured out how to account for a lot of stuff that can change a picture of the night sky, like relative motion, and after all those calculations, they find the same cosmic microwave background.
Quick note: I put some links to wikipedia in this explanation to be complete, but be aware that it can melt your brain.
Sorry for the long post, hopefully someone can explain it in less words 😉
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