Eli5 How did people figure out how old the universe is?

Question

I know it’s definitely very complicated but I’m very curious

In: 3

Answer 1

It’s actually not all that complicated. Basically, we observed that everything in the universe is moving away from everything else, which means that the universe is expanding. We measured the current size of the universe and how fast the universe is expanding (and the rate that the expansion is accelerating) and then simply “ran the clock backwards” to find out when everything was in the same place.