nhammen t1_j29fv7h wrote on December 30, 2022 at 4:42 PM

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

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?

DreamOfTheEndlessSky t1_j29pa23 wrote on December 30, 2022 at 5:42 PM

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.