In a recent podcast, the philosopher William Lane Craig said that while Special Relativity taught us to doubt whether any given clock is, or whether it could be, an absolute measure of time, General Relativity restored the possibility and perhaps evidence for a single "parameter": "Cosmic Time", which is the same for all frames-of-reference.

[...] what I point out in my published work is that when you turn to General Relativity then Absolute Time reemerges on a cosmic scale that was denied in Special Theory. So the Special Theory has been superseded by General Theory of Relativity. In General Relativity, there emerges a cosmic time, Kevin, that measures the duration of the universe from its inception. And this cosmic time is the same for every observer in the universe, regardless of his state of motion. It is not relative to reference frames or motion. It is frame independent (and in that sense absolute) and it measures the absolute duration of the universe.

Is this true? Have physicists found in General Relativity some evidence that there is in fact a single time parameter that both of the twins in the twins paradox, together with us and the most distant astronomical object, on our side and the distant side of the cosmological horizon... everything "knows" the true age of the Universe?

  • Let's give it some time... I'll do that if this takes too long. Dec 4, 2018 at 21:31
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    I do understand "Cosmic Time" to be defined as time since the Big Bang, but what I don't know is whether the assertion is true that such an age (or a clock) is in fact universal to all energy and matter. Dec 4, 2018 at 22:16
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    Wikipedia gives a definition - I don't claim the understand it, but it seems to involve a method of choosing the appropriate frame of reference, rather than claiming that it is true for all frames of reference. Wikipedia isn't much of a source, and I don't have enough of an understanding to answer.
    – Oddthinking
    Dec 5, 2018 at 0:59
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    Seconding sumelic here, this is a Physics.SE question. And, Elliot, this is not the first time you've been notified that questions you post on Skeptics.SE should rather be asked in the relevant science stacks.
    – DevSolar
    Dec 5, 2018 at 8:37
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    Sounds like this is a construct synthesized by this particular philosopher, not physicists. Dec 5, 2018 at 15:43

1 Answer 1


No, cosmic time is only universal for observers moving just with the expansion of the universe.

As explained in M. Pettini: Introduction to Cosmology:

An observer at rest with respect to the substratum is a fundamental observer. If the substratum is in motion, we say that the class of fundamental observers are comoving with the substratum. (italics in original)


An immediate consequence of homogeneity is the existence of a universal cosmic time, t. Since all fundamental observers see the same sequence of events in the universe, they can synchronise their clocks by means of these events. (italics in original)

So the universe on a large scale is moving like a fluid,and more particularly in our actual universe it is expanding. Observers moving only with the expansion are fundamental observers experiencing cosmic time.

  • It almost sounds like Craig is making a circular argument - that, taken on a universal scale, relativity is unimportant because, other than, perhaps, other universes, there is nothing "relative" to the entire universe. But maybe I'm interpreting his argument wrong. Dec 5, 2018 at 15:43
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    You have a one character error, too small for me to correct. It should be cosmic time rather than comic time. Dec 5, 2018 at 16:30
  • So if an entire galaxy moves at same velocity and direction as the expansion of the universe then everything on that galaxy is at or rounds close to cosmic time? Sep 10, 2019 at 15:03

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