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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 So the Special Theory has been superseded by General Theory of Relativity.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?

elliot svensson
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    I think you'd get more attention from physics experts if you posted on the Physics SE site. – paradisi Dec 04 '18 at 21:12
  • Let's give it some time... I'll do that if this takes too long. – elliot svensson Dec 04 '18 at 21:31
  • The only similar thing I've seen is time elapsed since the Big Bang. That isn't for all frames of reference, only for locally stationary ones. Since the twin paradox deals with frames of reference differing while local, it wouldn't apply. Again, if you want a serious answer, ask on physics.se. I can't supply sources for this, or I'd put it as an answer. – David Thornley Dec 04 '18 at 22:08
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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. – elliot svensson Dec 04 '18 at 22:16
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    [Wikipedia](https://en.wikipedia.org/wiki/Cosmic_time) 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 05 '18 at 00:59
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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 05 '18 at 08:37
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    Sounds like this is a construct synthesized by this particular philosopher, not physicists. – PoloHoleSet Dec 05 '18 at 15:43
  • Darn it, I think I misunderstood Craig's claim. He says that the cosmic time is the same for every observer, but that's not the same as saying that such a clock may be found on each and every observer. Suppose a relativistically-affected observer had only experienced 14 million years since the Big Bang... that observer would still be able to do astronomy and cosmology that resulted in knowledge of the absolute duration of the universe, even though the observer's surroundings were 1000 times younger. – elliot svensson Dec 05 '18 at 22:09

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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)

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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.

DavePhD
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  • 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. – PoloHoleSet Dec 05 '18 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. – David Hammen Dec 05 '18 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? – Stefanos Zilellis Sep 10 '19 at 15:03