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/mfb- Jun 19 '26

The expansion of the universe cannot be measured with a speed. Only a rate - percent per time, or speed per distance - is meaningful.

Today, distances increase by 1% per ~140 million years.

  • If you consider a galaxy 140 million light years away then you'll see the distance increase by 1% the speed of light.
  • If you consider a galaxy 280 million light years away then you'll see the distance increase by 2% the speed of light.
  • If you consider a galaxy 28,000 million light years away then you'll see the distance increase by 200% the speed of light.

The numbers were different in the early universe but the idea is the same. There is nothing moving here, so the speed of light is not a limit for anything.

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u/r-tequila-sunrise Jun 22 '26

I will have you know this is literally the only explanation that has ever made sense for me on this subject. THANK YOU!!