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A friend, after attending a talk recently discussing relativistic and other time-like notions, pondered on this question:

"Given the fact that much of the talks I attended discussed at length how there is no "universal now" and that, potentially, describing time as a line is functionally inaccurate - what does the quoted figure of 13.8 billion years (for the age of the universe) represent?"

This seems to me to be an extremely interesting question - he hypothesised that their figure was, in a sense, an upper-bound for all objects' world-lines, i.e. that there exists no object that has percieved the passing of more than 13.8 billion years.

How do we resolve the question? Is his hypothesis correct?

Remark: While there may have been similar questions before, I do not consider them duplicates as these issues - i.e. explicitly giving explanations as to the meaning behind the "13.8 billion" figure - were not discussed thoroughly or in any meaningful way, such as here, where almost nothing is discussed beyond the statement of an unexplained equation which almost certainly resolved nothing for the OP.

Isky Mathews
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    kinematics relativity is frame-invariant, but the universe has an incidental distinguished frame where the CMB radiation has no preferred spatial orientation. You can establish that frame as a basis for an universal clock standard – lurscher Jun 15 '18 at 19:51
  • @lurscher: Sorry, could you explain that in a bit more detail for those ignorant of much of your vocabulary, such as I? I understand what a frame of reference is, but what does "the universe has an incidental distinguished frame where the CMB radiation has no preferred spatial orientation" mean? – Isky Mathews Jun 15 '18 at 20:00
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    CMB (https://en.wikipedia.org/wiki/Cosmic_microwave_background) is radiation left after the inflationary phase of the big bang finished. That radiation looks mostly uniform across all directions of the sky, but there is some directional Doppler that is attributed to the velocity of the sun and the milky way against the special CMB frame – lurscher Jun 15 '18 at 20:04
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    the point is that you can define a special frame for all points of the universe as the frame where the CMB has no such directional Doppler shifts, and that frame can be used as an universal clock standard – lurscher Jun 15 '18 at 20:05
  • @lurscher It's not the same frame for all points, a different frame for each point. – safesphere Jun 16 '18 at 06:16
  • Yes, the hypothesis seems correct. Time can slow down, but cannot speed up. – safesphere Jun 16 '18 at 06:34
  • @Isky Mathews lurscher pointed out "that frame can be used as an universal clock standard". This means elaborated a little more that any observer who sees no dipole anisotropy is at rest with the CMB restframe and thus sees the universe isotropic. All clocks obeying this condition since the big bang would show the same cosmological time today. – timm Jun 16 '18 at 09:21

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