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Comment by est31 | Hacker News Reader

> All objects within our universe rotate, including planets, stars, solar systems, galaxies, and galaxy clusters. Moreover, black holes, spherically symmetric objects with horizons, display near maximal rotation as presented by Daly (2019). The idea that everything revolves [...] naturally extends to the whole universe, as hinted by recent claims of anisotropic Hubble expansion in X-ray observations by Migkas et al. (2021). Furthermore, a plausible syllogism is that the universe has near-maximal rotation, motivated by cosmologies where the universe is the interior of a black hole (Pathria 1972).

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> a plausible syllogism is that the universe has near-maximal rotation

And also from the abstract:

> Curiously, this is close to the maximal rotation, avoiding closed time-like loops with a tangential velocity less than the speed of light at the horizon.

That's weird considering that in lambda CDM the universe's accelerating expansion implies that stuff falls out of our observable universe, which implies that there is more stuff beyond the edge of the observable universe, and anyways there is no center of the universe and we are only at the center of our observable universe, which also further implies that there is stuff beyond the observable universe. Are they saying that our observable universe has a rate of rotation such that the tangential velocity at the edge is about the speed of light? What about the stuff beyond the edge? And as u/BigParm wonders, doesn't having the whole universe rotate imply a center? Surely we can't be at that center. But maybe there can be an illusion that we are at the center of rotation.

If spacetime rotates, does that imply there being a centre of the universe?

And an axis.

Or an axis.

And my ax

https://en.m.wikipedia.org/wiki/Axis_of_evil_(cosmology)

Wouldn't the center be the Big Bang and the 3D Universe at the current time (if Relativity lets me write about a current time) be the 3D surface of the 4D sphere (or spheroid) that the Big Bang is creating by keeping to expand?

the big bang happened everywhere

With the caveat that by some awkward metric 'everywhere' was much smaller when it happened.

Well, that depends whether the universe is infinite. In any case, everything was closer together and hotter.

Infinities come with a spectrum, some are smaller than others . . .

and some universes are finite.

It's more correct to just say location had no meaning until something existed in the universe.

There's really no way to know if there was something in existence before the big bang however. We just lack evidence of such a thing.

I think "the big bang happened everywhere" is more correct than your take. The big bang dlrefers to the early period of a spacetime with an initial moment. At that initial moment the distance between all points is zero. But there still are a full 3d set of locations. Starting at this initial moment, distances between points grow quickly. Thats the bang.

It is true that in GR you can't speak of "before" the big bang, but the big bang itself is a feature within time. It happens at the first moments of time and everywhere in space. And if you replace the initial singularity with a dense quantum foam and thus are able to extend time into the past in some quantum sense, the big bang doesn't go away.

If the universe is the interior of a black hole, one would assume so.

Depends what you mean by centre, I guess.

If you have two stars orbiting each other, they orbit a centre of gravity and they will probably both be rotating in the same direction as their orbits.

Is that a meaningful centre for anything else though?

There is. The center of the observable universe is earth. Every alien civilization will see themselves at the center of thier observable universe. And each will observe the same rotation in faraway objects. Relativity makes things strange.

If everything started from a singularity that had a axiomatically uniform rotation, you might not be able to assess the center "axis" based from inside spacetime itself

This seems to be a variation of Mach's principle, which is one of the closest things to philosophy in physics!

To say that the universe rotates usually implies that it rotates with respect to something external. If we limit ourselves to the visible universe, this would mean that mass outside our light cones can actually influence us, by means of building the frame of reference that allows us to say that the universe rotates!

You don't need any external reference points to know that you're rotating. You could detect it by the apparent centrifugal force: objects in your own frame of reference don't move in straight lines.

It would imply that there exist privileged reference points within the universe, and that would be a major change to physics. I can't predict all of the consequences of that, and they might include some kind of external frame of reference. But that doesn't necessarily follow.

Yes, it would. Or at least a central axis.

Which would undercut practically all of modern physics. Really fundamental conservation laws like momentum and energy rely on the universe being equal in all directions. If it has a central axis, that does not hold.

So if that holds, it's potentially a major pointer to the very origins of the universe itself. But it's also one of those extraordinary claims that require extraordinary proof. I strongly doubt that this will stand up to scrutiny -- though I'll certainly be pleased if it turns out be true, because that will be a major advance in our understanding.

If the universe is a black hole, would we be able to see anything falling into it after it crosses the event horizon?

You must be able to observe the edge to see that. And afaik the scientific community agrees no edge is observable in our cone of light.

As I got right up against the wall, I noticed the little white square sign. It said,

"Obviously you are not convinced that this is the end of the universe. if you will place a quarter in the slot below, the peep-hole will open, and you can see for yourself."

And the captain was right. I paid my quarter and looked through the peep-hole. But it was nothing.

--From "Ado about Nothing, by Robert K. Ottum

So everything that ever fell into the blackhole arrives into the big bang at the start?

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