ELi5: Is the entire Universe (unobservable + observable) 13.8 billion years old or just the observable Universe 13.8 billion years old?
In: Physics
So, we know the universe is expanding, right? And we know a lot about what is causing that expansion and how it has unfolded over time, and we have some clues such as the cosmic microwave background that tell us what was going on at specific points in time. Well, if you put all of this information together and trace the expansion backwards, you get to a point around 13.8 billion years ago at which the universe very rapidly expanded and cooled, going through various stages in which it was dominated by different forms of matter. e.g. at one point it was too hot for atoms to stay together so the entire universe was made up of a plasma of free protons, electrons, etc., like you get in stars today. Well, the further back in time you go, the more unclear things get, as the increasingly hot and dense matter becomes increasingly unlike anything we can see today.
So we don’t know for sure that nothing happened prior to 13.8 billion years ago, we just know that our universe went through a lot of extreme changes at that point and that it’s impossible to probe any earlier in time with current knowledge and technology. And the big sticking point isn’t the speed of light or the edge of the observable universe or anything to do with that, it’s our lack of knowledge about how extremely extremely hot matter behaves.
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