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Pete, would you please explain "momentum density"?
Thanks Pete, it does help. I was quickly out of my depth with the link, but that is a feature of my major weakness - lack of maths.
Are you willing to help me by learning more math?
Think of it as the frame of reference in which the 3K background radiation is isotropic (i.e. has the same value no matterwhere you look). Our frame is very close to that. There is a very small bias towards one direction and that's accounted for by our motion in our galaxy which is moving relative to that frame.
According to the most current ideas, the universe is so many billion years old. But according to Einstein, time is relative to the observer. So the question arises: according to what observer is this so?