CMBR as an absolute reference frame

[Dorje]

I've been reading Singh's Big Bang book, and towards the end he mentions that the CMBR was used to compute the relative speed of the Milky Way Galaxy, roughly 1,000,000 mph. This seems to suggest that the CMBR can be used as an absolute reference frame for any observer in the universe. However, relativity rejects the notion of absolute motion. What's the resolution here?

 

[Garth]

A good question Dorje, one that exposes a possible inconssitency in GR.

GR is based on SR, which itself is based on the principle of relativity, there are no preferred frames of reference. From a GR pont of view the SR space-time is empty except for observer test particles of no mass. There are no preferred frames because there is nothing to 'hang your frame on' all frames are equivalent.

However GR considers space-time with a gravitational field, and a gravitational field requires the presence of either mass or energy. If there is mass in the space-time manifold then it is possible to define a special frame of reference - that of the Centre of Mass (CoM) of the system. The question is: "Is the special frame of reference preferred in the same that the laws of physics are different in it than in all other frames, can you identify this frame by performing a laboratory experiment?"

Before we answer this question it is pertinent to observe that in the standard GR theory there are such special frames. In a local gravitational field it is easy to identify the CoM of the system, in the cosmological solution it is easy to identify that frame in which the CMB is globally isotropic and as you point out we are traveling at about 0.1%c relative to the surface of last scattering. Furthermore, in The Cosmological Twin Paradox the topology of a closed universe does determine a special frame of reference.

There are modifications of GR, the Brans Dicke theory being the most famous, which include Mach's Principle.

 

[George Jones]

Furthermore, in The Cosmological Twin Paradox the topology of a closed universe does determine a special frame of reference.

I don't think I have a problem with this. I wish I had enough time to read in detail the papers referenced in the thread!

A caution: some readers (not Garth) may be under the impression that this has been ruled out for our universe, since a Friedmann-Robertson-Walker universe is closed if and only if it will eventually start contracting, and our universe, as modeled by FRW, expands forever. Not.

The presence of a cosmological constant/dark energy can falsify this statement, and, if our universe is an FRW universe, the simplest models for universe do falsify this statement, i.e., we cannot rule out by present observational evidence that our universe is closed. Observational evidence stongly favours a universe that expands forever, but is quite neutral about whether spatial sections of our universe are closed, flat, or open.

Regards,
George