After seeing the first few minutes of [Veritasium’s](https://www.youtube.com/watch?v=Kp_kqamkYpw) video on how half of the baryonic matter was yet unobserved until recently, he mentions how the ratio of deuterium and helium and other elements were all formed within the first 20-ish minutes after the big bang but also how with around 10 seconds the universe had cooled significantly.
How is that sort of timescale measured accurately when the big bang happened so long ago, especially when even some modern history events cant be traced down to hapening within an exact year sometimes?
In: Physics
Particle accelerators, like CERN. They allow us to run the film backwards, like watching a video of an explosion in reverse.
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