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May21-09, 02:11 AM
P: 645
Quote Quote by Chalnoth View Post
Because we're talking about a later time. The universe has expanded in the interim.

Right, there is an arbitrariness in the time coordinate, but as I said, you can just set the time coordinate based upon the expansion of the universe, in the case measured via the CMB. In this case the time coordinate that we're using is the proper time of a clock at rest with respect to the CMB at that particular location, starting at some "equal-time slicing" of the universe where the universe was at equal density when the various clocks read the same value. Neglecting density fluctuations for the time being, such clocks will always read the same time value when they see the same density and temperature of the universe.
I should have read more closely, shouldn't I?

On a cosmological scale, are you saying that a particular rest colour of the CMB represents a moment of simultaneity throughout the universe? - where a "rest colour" is the spectrum of the CMB for an observer who is at rest with the CMB as previously discussed.

Is there anything preventing us from using this as our universal clock (assuming that we could overcome the inaccuracy of measurement)?

The scheme I am thinking of here is this: check that you are inertial (absence of forces), check the CMB in all directions, note the axis of your motion (point of maximum red-shift to point of maximum blue-shift) and then note the colour of the CMB from a direction perpendicular to the axis of motion, take into acount your motion (from the degree of red-/blue-shift) and then you can get the resting CMB colour. Then you know "when" you are in a pretty much absolute sense.

Wouldn't you?

I stress that it is not a question of accuracy here, I am more interested in whether you can talk about a universal "when" even if you can't pin it down to the precise year, or even millenium.