Center of mass of the universe

[edpell]

Isn't the inertial frame defined by the center of mass of the universe somehow special? To get to any other frame an object must experience a force and be accelerated to a new velocity. In this case the CMB shows us the inertial frame of the CM of the universe. I understand that velocity is relative but there is a unique history in this case. One frame was first.

 

[DaveC426913]

Isn't the inertial frame defined by the center of mass of the universe somehow special? To get to any other frame an object must experience a force and be accelerated to a new velocity. In this case the CMB shows us the inertial frame of the CM of the universe. I understand that velocity is relative but there is a unique history in this case. One frame was first.

But how do you determine where the centre of mass of the universe is? No matter where you are in the universe, your observations will tell you you are at the centre.

 

[edpell]

Yes I worded that badly. I mean we can find the inertial frame of the CM. It is the one where the red shift of the CMB is the same in all directions.

 

[marcos54]

Redshifting due to CMB local streaming toward the center of the Virgo supercluster or any "Attractor" must be de minimis. That's sensible given the huge redshift to which the CMB is already subjected and minimal local spacetime warping caused by such streaming.
So, to the extent two observers found themselves in truly intergalactic space they should both see the CMB with essentially equal redshifts in all directions- subject only to any effect caused by streaming or large intervening anisotropies from lensing, voids, etc.- even if they're moving at constant speeds in opposite directions. It's the severe local warping of space-time, of which the Earth's and sun's constant radial accelerations are symptomatic, that results in the measurable doppler effects.

 

[edpell]

So, to the extent two observers found themselves in truly intergalactic space they should both see the CMB with essentially equal redshifts in all directions ... even if they're moving at constant speeds in opposite directions.

What happened to the Doppler effect?

 

[Dmitry67]

Isn't the inertial frame defined by the center of mass of the universe somehow special? To get to any other frame an object must experience a force and be accelerated to a new velocity. In this case the CMB shows us the inertial frame of the CM of the universe. I understand that velocity is relative but there is a unique history in this case. One frame was first.

There is in fact special frame which is in rest to CMB

*BUT*

In different places these frames are different! So different observers don't agree on ONE special frame. So this frame is special locally, but not globally.