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canadave_nyc t1_j26yqlr wrote

Reply to comment by Aseyhe in How fast does the Milky Way spin? How far does Earth move through space in a year? by Sabre-Tooth-Monkey

Thank you. I'm finding it hard to understand how, if everything in the universe is moving away from everything else (aside from local gravitational interactions), using the classic "expanding balloon" analogy, how are we moving in an identifiable particular direction relative to the "largest reference frame in the universe". I would've thought that we'd be "stationary" with respect to the CMB if everything is moving away from everything else.

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DreamOfTheEndlessSky t1_j27w0f6 wrote

Suppose we were at exactly zero motion relative to the CMB. If we underwent any gravitational interaction with another body, each would undergo some acceleration. That would leave each body with a non-zero motion relative to the CMB. So any local gravitational interaction would take us away from zero-CMB-motion.

Barring some sort of speculative restorative effect that brings us back to zero-CMB-motion, you'll be left with non-zero motion relative to the CMB.

So local interactions later on would be enough to create some motion.

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nhammen t1_j29fv7h wrote

I have a slightly related question. Because we are basically seeing back in time, the net velocity of the CMB is basically the same as the net velocity of the big bang, right? However, by conservation of momentum, we know that the velocity of the center of mass of any system is conserved. So it should be conserved from the big bang up until now. Thus, shouldn't our velocity with respect to the CMB match our velocity with respect to the velocity of the observable universe's center of mass? Does it?

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DreamOfTheEndlessSky t1_j29pa23 wrote

Momentum is only conserved in aggregate when there is no external influence. Parts of the system can still transfer momentum between each other. It's quite permitted for us to change our momentum, as long as other things have a change in momentum that is equal and opposite to our change.

I don't have specific sources for "net momentum of the universe is zero in the CMB frame of reference", but it sounds like a good expectation. If we found a way to test that, it would provide either a confirmation or open new scientific questions. Unfortunately, as the observable universe is only a subset of the whole universe, I suspect that we cannot determine the net momentum of the whole universe.

The momentum of the Earth, or a vehicle, or the Sun, or our galaxy, could vary from the aggregate due to any number of interactions. For instance, the Earth orbiting the Sun must involve the Earth and Sun having different velocities, so they can't both match the CMB. As it happens, neither does.

Even pointing a flashlight into space would cause a tiny change of momentum for the Earth: the outgoing photons have momentum, so the flashlight (hence the Earth, indirectly) must experience a change in momentum in the opposite direction.

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silent_cat t1_j28didp wrote

I think the bit you're missing is that when you move in a particular direction, light from that direction becomes slightly bluer and light behind you becomes slightly red.

If you assume the "expanding balloon" is expanding everywhere at the same rate, by looking at the colour of the expanding balloon you can determine a speed relative to you where the balloon will have the same colour everywhere.

Yes, it would have been more logical if we'd have been stationary relative to the global frame, but it turns out we're not. Science is more interesting when it gives you answers you don't expect.

(It could be that the "bubble" is not expanding at a uniform rate everywhere, but we (currently) have no way of distinguishing that. And it seems the weaker assumption that we're simply moving than some fancy new physics.)

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