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OK...before everyone blasts this with references to the relativistic invariance of the physical laws, time dilation, etc let me add some context. Also, I am an amateur with an interest in physics, so I don't know the details of the physics. However, I have read enough about the cosmological principle and cosmic time to develop this "confusion". My confusion is more philosophical than technical, so it seemed appropriate to post here.

Based on evidence gathered to-date, it appears that the universe is both isotropic and homogenous to a high degree, as evidenced by observations and the usefulness of the Freedman-Lemaitre-Robertson-Walker (FLRW) metric. This principle, when combined with the FLRW metric, allows space-time to be divided into non-intersecting slices and therefore establish a "cosmic time."

Now, the existance of such a time in no way suggests that it is the "right" time or a true "now", hence violating relativity -- although some presentist philosophers (e.g., W.L. Craig) have tried to make this argument, but it is not generally accepted in philosophical circles.

However, this leaves me confused about an apparent disconnect between relativity and the cosmological principle:

Isotropy only holds if we are at rest relative to the cosmic microwave background radiation (universal rest frame), o/w anisotropy is present - yet each reference frame is supposed to allow equally valid observations. How can these two be reconciled when most reference frames would lead us to conclude that the universe is NOT isotropic?

It seems that only by appealing to the idea of being at rest relative to "universal rest frame" can we explain away any discrepancies from isotropy as due to "peculiar motion". However, doesn't this give this "universal rest frame" and its associated time an empirically privileged status, even though physical laws work just fine in every reference frame?

For example (pardon any abuse of astronomy): if look off in some part of space and see only quasars, and then in another part of space and see only brown dwarfs, but I measure both as apparently the same distance from me, then can I conclude that we are in motion, since otherwise we would have an empirical contradiction (kind of like finding dinosaur and human remains in the same strata)?

Any help on where I am going wrong would be helpful. Intuitively, I don't think there should be a way to establish absolute ordering between space-like separated events, but the above reasoning suggests otherwise.

  • Not sure this is a philosophy question rather than physics. – Rex Kerr Nov 15 '13 at 17:54
  • I was torn between the two, although cosmology tags are present on this forum. The essential philosophical part regards how you make absolute statements in a relativistic world? The cosmological principle certainty seems like such a statement, at least prima facie. – user4634 Nov 15 '13 at 18:04
  • But the answer hinges upon details of the mathematical formulation of the physical laws and models, and not upon any deep thought about absolute statements. Everything is adequately defined; you just need to know how the math works out (or fails to). – Rex Kerr Nov 15 '13 at 18:06
  • Its bascially an argument for presentism vs. eternalism with physics garb. It seems that if we insist that the universe is truly isotropic, then we cannot equate all reference frames, and hence we can single out a frame that represents "now", with the rest of us using distorted clocks. – user4634 Nov 15 '13 at 19:00

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