# If we know how old the universe is based on the fact that objects furthest away from us we can see are ~14 billion light years away, how do we know that there was light right from the beginning of the Big Bang? Couldn’t there, in theory, have been darkness for X billions of years before light?

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If we know how old the universe is based on the fact that objects furthest away from us we can see are ~14 billion light years away, how do we know that there was light right from the beginning of the Big Bang? Couldn’t there, in theory, have been darkness for X billions of years before light?

In: Physics

Explosions tend to emit light.

Also, it took about 200 million years after the big bang for the first stars to form.

https://starchild.gsfc.nasa.gov/docs/StarChild/questions/question55.html

It’s not just about how far away these objects are, it’s about how far away they are *and* how fast they’re moving away from us. Think of 2 cars on a road driving away from each other at some speed. If you know their speeds and the current distance between them, you can calculate when they were both at the same place. We can also check this with another method which is to measure the age of certain star clusters (we can do this because there’s a relationship between the luminosity, spectral classification, and age of a star). It turns out these 2 methods are pretty much in agreement with each other.

We also know that the universe was, for the first ~379,000 years of it’s existence, opaque. That means we can’t see anything of the universe for the first ~379,000 years. This is because the universe back then was so hot that atoms couldn’t form, and blocked photons from traveling freely.

> Couldn’t there, in theory, have been darkness for X billions of years before light?

Yes, it could. The 14 billions years thing is for the visible universe. If there was something before or beyond the visible universe we dont know, we cant see or measure it.