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Beyond a formal preference for background independence, what is stopping us from setting cosmological time as a de facto universal timeline, analogous to newtonian absolute time? General relativity doesn't entail it on its own, but neither does it forbid us if certain conditions hold (for a given model.)

Qmechanic
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RedDot
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  • Welcome to SE. Look maybe at answers and discussions here: https://physics.stackexchange.com/q/86686/ I think the crucial difference is still that Newton thought that there is an absolute time everyone agrees on in any frame. This is not true for cosmological time, although you could argue that it is the most convenient one... different observers will disagree on time (and also may disagree on the universe being isotropic and homogenous for the same reasons). – Koschi Nov 28 '23 at 14:20
  • nothing, cosmic time is preffered from cosmological point of view. But nothing stops you from re-setting it to some other time parameter either – Kosm Nov 28 '23 at 14:54
  • @Koschi, Thanks. Nothing would change for events and observers in their rest frames, relativistic effects would remain exactly the same. As far as I can see, we can always appeal to the cosmological rest frame (as a third party) in order to settle any disagreements between observers (in their rest frames.) – RedDot Nov 28 '23 at 15:06

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The cosmological time is the longest proper time possible since the big bang and the only hypersurface of constant t in which the universe is isotropic, but other than that it is not special, the equations of general relativity also hold in any other reference frame so you can do it, but you don't have to.

Also the comoving observers who experience t to be their τ are not at rest relative to each other, so that alone is already different than under Newton. Nevertheless, a fast neutrino created at the big bang will always have a lower proper time than a comoving observer when they fly past each other and compare their clocks.

Yukterez
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  • indirectly related: https://physics.stackexchange.com/a/789119/24093 – Yukterez Nov 28 '23 at 18:25
  • Cosmological time is analogous with absolute newtonian time insofar as it allows us to map events and observers independently of their rest frames. Beyond that, nothing would change for events and observers in their rest frames, relativistic effects would remain exactly the same. – RedDot Nov 28 '23 at 22:03
  • @RedDot: But we can label them independently of their rest frames in other coordinates as well, relativity is coordinate independend. The comoving coordinates have the simplest form though, but we can transform into whatever coordinates we like and label all the events in them too. – Yukterez Nov 28 '23 at 22:11
  • It is already the universal time in the sense that most observers in the universe are more or less comoving and it also is their proper time since the big bang, but since it's not it's not universal in the sense that it's everybody's proper time like it would be under Newton that term could be misleading, although it does apply for the majority of observers, and that only if you neglect their local peculiar velocities (which are small anyway, but nevertheless). – Yukterez Nov 28 '23 at 22:21
  • Absolutely. It's a preferred frame, in no way a mandatory one. – RedDot Nov 28 '23 at 22:24
  • It is indeed the preferred frame for most calculations in cosmology. The other coordinates are used rather seldom, but they do exist and they also get the job done. – Yukterez Nov 28 '23 at 22:26
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Lee Smolin argues for an alternate conception of relativity in which there exists a preferred cosmological time throughout the universe. See his book Time Reborn for an introduction to this idea.

niels nielsen
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