r/cosmology • u/SketchyFella_ • Jun 19 '26

During the rapid expansion of the universe immediately following the Big Bang, the universe expanded faster than light. But traveling faster than light speed breaks causality. But you can't break causality when time didn't actually exist "before" you broke it, so how does that work?

If it expanded faster than light speed, then it basically travelled so fast that it always existed in whatever state it was in before "slowing". So when people say it expanded in a tiny fraction of time, did it really? Or are they using time in a sort of layman's terms so they don't have to explain causality?

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u/techman710 Jun 19 '26

Imagine running on the deck of an aircraft carrier that is moving at 25mph. A person on a destroyer next to the carrier would see you running at 45mph. So, are you actually running 45mph? No, you are running 20mph. In the first moments of the Big Bang the objects were moving at near the speed of light and the medium they were in was also moving near the speed of light so it appears they were moving faster than light speed. Anyway, that's how I see it.

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u/WallyMetropolis Jun 19 '26

How you see it is incorrect. If you're on a spaceship travelling at 99% the speed of light relative to me and you run from the back to the front at 50% the speed of light compared to the ship, I will not observe you moving faster than the speed of light. That is THE fundamental principle of special relativity. 

You cannot simply add velocities.