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Imagine a Universe spinning on the x-axis. So there is a centripetal directed away from the x-axis. According to General Relativity this is entirely equivalent to a non-spinning Universe with a gravitational force directed away from the x-axis.

Either way, objects will feel pulled away from a line running through the Universe.

My question is this: Is there any way to distinguish from a Universe spinning clockwise or anti-clockwise. Equivalently, is there any difference in the gravitational field directed outwards that is equivalent to this state?

Would an object feel some kind of twisting, torque or tidal effects that could distinguish these two possibilities? Or is a clockwise spinning Universe entirely equivalent to a counter-clockwise spinning Universe?

(I suppose another way to put this, is would an ant on a spinning drum in empty space know if the drum was spinning clockwise or anticlockwise?)

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    $\begingroup$ "Clockwise" is not an absolute notion. $\endgroup$ – fqq Dec 8 '15 at 16:08
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    $\begingroup$ @fqq: that strikes me as a rather petty criticism. The question is whether you tell the direction of the spin. Whether you choose to label the direction clockwise or anticlockwise doesn't affect the point of the question. $\endgroup$ – John Rennie Dec 8 '15 at 16:11
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    $\begingroup$ Perhaps in a spinning Universe it would be an absolute notion. That's my question. $\endgroup$ – zooby Dec 8 '15 at 16:18
  • $\begingroup$ I don't think spin applies to the universe actually. And I don't know of any measurements or observations that suggest there is any rotation. But see Galaxy sized twist in time pulls violating particles back into line: "However Dr Mark Hadley, of the Department of Physics at the University of Warwick, believes he has found a testable explanation for apparent Charge Parity violation..." $\endgroup$ – John Duffield Dec 8 '15 at 17:06
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Let's take your last paragraph first as it's easiest to answer. You ask:

would an ant on a spinning drum in empty space know if the drum was spinning clockwise or anticlockwise?

and this is more or less what the Kerr metric describes. The answer is that yes you can determine the direction of rotation by measuring the frame dragging. Whether the rotation is clockwise or anticlockwise is ambiguous because an observer rotated 180º relative to you will measure an opposite direction. If you called the rotation clockwise they would call it anticlockwise and vice versa. Which of you is correct is the sort of empty question that wars get fought over.

Anyhow, aside from that easy bit the rest of your question cannot be answered because you haven't defined what a rotating universe is. Specifying a universe is far harder than I'd guess you appreciate. You have to find a spacetime geometry and matter distribution that are linked by the Einstein equation, and that's hard. I would guess that you are assuming a rotating universe would be the same as a static universe with a rotating coordinate system, but that is most emphatically not the case.

The only rotating universe for which we have an analytic solution to the Einstein equations is the Gödel universe. This an infinite homogenous universe where the curl of the matter velocity field is everywhere constant. It is rotating, but it doesn't have a central axis. Any observer anywhere in the universe would observe the universe to be rotating about them at the same rate.

It's easy to tell the direction of motion in a Gödel universe because it's possible to determine your own rotation by measuring the motions of test particles near you. Adjust your own rotation until the universe around you looks static, then you can determine your own rotation rate using the test particles.

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