Is the Inertial Frame Defined by the Universe's Center of Mass Unique?

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Discussion Overview

The discussion revolves around the uniqueness of the inertial frame defined by the center of mass (CM) of the universe, particularly in relation to the Cosmic Microwave Background (CMB). Participants explore the implications of this frame in terms of relativity, observations, and the effects of local gravitational influences.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants propose that the inertial frame defined by the universe's center of mass is special because transitioning to any other frame requires an object to experience a force and acceleration.
  • Others question how to determine the exact location of the universe's center of mass, suggesting that observations from any point in the universe could indicate that one is at the center.
  • A participant clarifies that the inertial frame of the CM can be identified as the frame where the redshift of the CMB is uniform in all directions.
  • Concerns are raised regarding the influence of local streaming effects, such as those from the Virgo supercluster, on the observed redshift of the CMB, suggesting that these effects should be minimal in intergalactic space.
  • Another participant highlights the apparent contradiction regarding the Doppler effect, questioning its role in the observations of the CMB redshift.
  • It is noted that while there may be a special frame at rest with respect to the CMB, this frame can vary depending on the observer's location, leading to a lack of agreement on a single global frame.

Areas of Agreement / Disagreement

Participants express differing views on the uniqueness and definition of the inertial frame related to the universe's center of mass. There is no consensus on whether a single global inertial frame exists, as some argue for local specialness while others emphasize the variability across different locations.

Contextual Notes

Limitations include the dependence on definitions of the center of mass and the effects of local gravitational influences, which may complicate the understanding of the CMB observations and the Doppler effect.

edpell
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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.
 
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edpell said:
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.
 
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.
 
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.
 
marcos54 said:
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?
 
edpell said:
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.
 

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