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Resaren t1_jam94cw wrote

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

This is the answer I’ve been looking for after years of having this concept explained only with vague references to expanding balloons, or allusions to the expansion of space being some sort of intrinsic property. Thank you! This quagmire of a concept reminds me a lot of the confusion around ”relativistic mass”, which has thankfully fallen out of usage.

A follow up question to this, does this mean there is some unique Center Of Momentum frame, or is this precluded by SR? And how is this related to the CMBR rest frame?

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Aseyhe t1_jamb3p2 wrote

The CMB rest frame is the frame of a comoving observer, that is, one who is at rest with respect to their (cosmologically) immediate surroundings. At different locations, the CMB rest frames are different. There's no global "center of momentum frame" if the universe is homogeneous, only local ones.

(I should also note that in curved spacetimes, reference frames only make sense locally. However, this is more minor consideration, in a certain technical sense. While the impact of the difference in the velocities of different comoving observers scales linearly with their separation, the impact of curvature scales as the square of the separation. So the latter only becomes important at very large separations.)

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