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Aseyhe t1_jamocwu wrote

Reply to comment by DocJanItor in Why do cosmologists say that gravity should "slow down" the expansion of the universe? by crazunggoy47

Repeating a response I made to a similar question elsewhere in the thread:

Relative velocities of distant objects aren't well defined in curved spacetimes. It's often said that distant objects are receding faster than light, and there are standard ways of writing down their distance such that the distance grows faster than the speed of light. However, there is no relativistically meaningful sense in which these objects are moving faster than light in relation to us. Also, the distance isn't uniquely defined either.

In intuitive terms, the relative velocity is the angle between two vectors in spacetime. Imagine drawing two arrows on a sheet. If those arrows are in the same place, you can measure the angle between them. If they are in different places, but the sheet is flat, you can also define the angle between them uniquely. However, if they are in different places and the sheet is not flat, the angle between the arrows is not uniquely defined.

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DocJanItor t1_jan4djg wrote

Doesn't matter about their current velocities. The universe is 14B years old. It's bigger than 28B light years wide. Therefore the universe had to expand faster than the speed of light.

Further, light goes through the universe. The universe expands outside of itself into who knows what. We have no idea if the speed of light applies outside of the universe. Same for thermodynamics.

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Ape_Togetha_Strong t1_jan9j7f wrote

It sounds like you're imagining "expansion" to be some outward expansion from a point in space, rather than spacetime expanding everywhere. There's no reason to tie the age of the universe to its size (other than just how its size changes proportionally, relative to itself with time). It could have been any size prior to inflation. The fact that the observable universe is larger in lightyears than the age of the universe in years is not surprising or particularly meaningful.

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