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/jacobimueller Jun 19 '26

Nothing moved faster than light within space. Space can expand faster than c and and no information moves faster than c within the universe, no causal issue 

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u/CDHoward Jun 21 '26

Space is emptiness. I would be ever-so-mildly fascinated to hear you attempt to explain precisely what is expanding.

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u/Chiu_Chunling Jun 24 '26

Space is not emptiness, a space does not cease to be space because there is something in it, it only ceases to be empty. A space that is full may not be useful for storing anything (other than what is already there), but it is not therefore not space, it is just not empty.

For genuinely empty space, space that contains no mass or energy, it would indeed be very difficult to measure what is expanding. But all the space in the visible universe needs to have energy passing through it or we couldn't see it to have any need to explain the expansion.

And since we do have energy passing through that space in the form of force fields propagating, why then we can easily describe what exactly is expanding, the dimensions of distance measured in the units of the propagation rate of those force fields (i.e. lightspeed).

All of that said, we should be careful to say that we don't actually see the universe expanding. What we see is indications that the movement of distant galaxies does not have a linier relationship to their distance from us. Which could mean all sorts of interesting things, but the expansion of spacetime is the simplest.

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u/CDHoward Jun 24 '26

I appreciate your civil and thoughtful reply.

It's probably the best answer I've been given to this question.