Considering that we only have the observable
universe in view, how can we possibly comment on The Universe
. Politely, it would be speculation at best.
"That is really my question - could we figure it out if it is really there?"
"We certainly know that spin can be detected in a smaller closed system
inside the universe" My emphasis.
So what? Not wishing to be impolite.
Ok, try to think of it as a limiting situation for experiments that we can actually do.
I am assuming that you agree that a person can detect that he/she is in a rotating frame of reference when in a small, closed space with no visibility to the outside, by seeing that a mass set into unperturbed motion follows a curved path instead of a straight path.
Jut think about expanding that enclosed space until you reach the edge of the universe. Unless something suddenly changes when the boundaries of your test space reach the total space in our universe, you will get the same result for the universe that you can get in the smaller space that is a subpart of the universe.
Granted, detectability in any amount of space requires the ability to see enough deflection from a straight path to exceed your measurement threshold.
The thing that would get difficult in large amounts of space would be determining what is a straight line. Typically, we assume that a laser makes a straight line in space. But, we do know that light bends around mass. So, if
light does not travel in mostly straight lines across our universe, then we might not be able to detect that matter is not traveling in a straight line, either. However, from what I keep reading, most people agree that our universe is "flat" rather than "curved". On the other hand, people used to think that was true of the Earth's surface at one time, and we laugh at them today. Maybe people will be laughing at us about space being flat some day, too.