B How is the Age of the Universe Determined?

[Daleri Mc Rileda]

It means you don't see the CMB as isotropic; you see a higher temperature in the direction you are moving, and a lower temperature in the opposite direction. We actually observe this here on Earth; the usual term is "dipole anisotropy" in the CMB. But practically all published data on the CMB corrects for this by subtracting out the dipole in order to display what the CMB would look like to a "comoving" observer at our location.

So it appears the CMB would be relative to our frame of reference. What effect would another frame of reference have, say near a black hole?

 

[phinds]

So it appears the CMB would be relative to our frame of reference.

I think perhaps you still misunderstand what a frame of reference is. Or, perhaps that sentence doesn't quite reflect what you want to ask.

What effect would another frame of reference have, say near a black hole?

same comment.

To say "the CMB would be relative to our frame of reference" as a stand-alone statement just doesn't make sense. You can define a frame of reference in which the Earth is at rest, or at most is rotating, and you can define a frame of reference near a black hole and the computations on getting the age of the universe for a co-moving observer would be more complicated for the frame of reference near the black hole because you would have to account not only for the speed of the black hole relative to the CMB, you would also have to account for the fact that your space-time path is in a deep gravity well.

 

[PeterDonis]

So it appears the CMB would be relative to our frame of reference

I'm not sure what you mean by this, but it doesn't look correct to me.

 

[Chalnoth]

So it appears the CMB would be relative to our frame of reference. What effect would another frame of reference have, say near a black hole?

There's very little practical difference between CMB observations made by one observer and those made by another.

First, nobody can live close enough to a black hole for the time dilation to become significant (as there are no stable orbits very close to a black hole).

Second, velocities with respect to the CMB of astronomical objects just aren't large enough to have a significant impact. For example, most galaxies don't have velocities with respect to the CMB that are greater than 2,000km/s, a speed which results in time dilation of only 0.002%. Even a speed of 10,000km/s only results in time dilation of 0.05%.

Third, when we measure the age of the universe we don't actually look at any sort of physical clock, as nothing that could be used as a clock existed until long after the CMB was emitted. The idea of the age being different for a particular observer assumes a clock that has traveled along a particular worldline from the beginning of the universe to today, but no such clock exists. Instead the age is inferred based upon various observations of the expansion history.