This article tries to explain the Hubble tension by saying the entire universe rotates very slowly.
I understand that if the entire universe rotated there would be a centrifugal “force” that pushed the galaxies apart faster than otherwise. But what independent confirmation could there be that it was rotation causing it rather than just dark energy?
And what the heck would it be rotating with respect to? My only thought is with respect to the universe beyond the observable universe.
Earlier thread
Dr. Becky’s take on it (she’s an astrophysicist at the University of Oxford):
There would be a coriolis effect.
I don’t know enough about dark energy to answer your first question, but you can determine if your frame of reference is rotating.
An inertial frame of reference is one that is moving at a constant velocity. That velocity doesn’t have to be in relation to anything; it can zero, negative, or positive, depending on an arbitrary external frame of reference. The important thing is that Newton’s first law holds within the frame. If you are in an inertial frame, you can tell, but you can’t tell your velocity without an external reference. The Earth’s diameter is so large, we feel like we are sitting still but at the equator you would be moving at about 1000 miles per hour. However, you are actually going in a circle and can take measurements to demonstrate that you are actually in an accelerated frame (where the acceleration is a vector perpendicular to a tangent of your path and inwards towards the center).
A rotating body rotates with respect to its center of rotation. You can take measurements to determine this without caring about anything external. We think of the Earth rotating with respect to its orbital path, or the Sun, or any other arbitrary celestial object, but if none of those things were available we could still determine that we were rotating.
If it were not so, you could climb into a centrifuge and be immune to G-forces simply by declaring that it’s the rest of the universe spinning, while you stay still.
The problem I have with a rotating universe is that if the universe is vast enough there comes a point where the universe rotates faster than the speed of light. And if the universe was infinite the speed of rotation would become infinite too. How do you avoid that?
Are we talking about the universe rotating, or the contents of the universe rotating? Your objection stands if the latter. But not the former.
How so?
If you’re in a closed box that is moving perfectly smoothly in a straight line, you can’t do any experiment inside the box that would differentiate whether or not you are moving.
If you’re in a closed box that is rotating, you only need to roll a marble on the floor of the box and observe whether it moves in a straight line relative to the floor of the box - if you’re rotating, it won’t.
Interesting, thanks.
Another question: If the universe is rotating, doesn’t that imply there is a center?
I think it does - assuming the rotation isn’t some weird chaotic thing that isn’t what we normally think of as geometric, concentric rotation, a rotating universe has to have a centre of rotation.
I wonder how there could be a center of rotation if there is no center of expansion, or a center in any other sense.
But rotating with respect to what? Locally we could say the Earth or the Sun or the galaxy. But if you mean the universe (and not the observable universe) then what do you measure the rotation with respect to.
If the entire universe is rotating, doesn’t that mean there is no natural inertial frames of reference. You’d have to move in some fashion to counteract that.
Also the Coriolis effect varies with the distance from the center. Does this mean there is a center of the universe, or is the universe’s rotation something like quantum spin which is not the same as rotation. Rather it is an intrinsic property of the universe and the same everywhere?
It’s possible to have no centre of expansion if you have a grid of objects where the spaces between the objects is increasing at the same rate across the whole grid. You can pick a point and sit there and it will look like everything is going away from you, so you reason that you must be at the centre of expansion, but you can do that for any point and the experience will be the same there - therefore there is no definitive centre of expansion.
Rotation simply doesn’t work like that.
The center in how many dimensions?
I think you’re conflating reference frames. I don’t know if this will help, but to expand on the previous examples:
If you’re in a closed box that is rotating and is also moving perfectly smoothly in a straight line, you can determine whether it is rotating by rolling a marble on the floor, but there is nothing you can do within the box to determine whether there is also a constant linear component to your motion.
Rotation isn’t the same as motion at a constant velocity - The answer to ‘rotating relative to what?’ is ‘relative to not rotating’
As above, if it were not so, you could get in a centrifuge and spin it up, and just declare the rest of the universe to be spinning wildly relative to yourself, and you should feel no G forces. That doesn’t happen.
OK let me ask the question this way. In the context of the universe rotating, presumably (ignoring my previously stated possibility that it’s like quantum spin) it must be that observers close to one another should detect rotation in the same direction and with the same angular velocity.
On different sides of the center of rotation, the Coriolis force will move things in the opposite direction. So if we can find that, have found the center of the universe’s rotation?
Yeah, I think you can probably derive the centre of rotation from measurements on one side only, assuming they’re sufficiently accurate.
If you had a universe where every object was fixed in place, but every individual object was rotating in the same direction, that would be a universe that rotates, but without having a discernible center.