> 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).
> 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.
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?
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.
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 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.
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 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?
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.
Correct. Though the matter in it would also move, dragged along by the space-time.
It's much like the expansion of the universe, which is separate from the ordinary momentum of the objects within the universe. But the objects within the universe do move apart along with it.
I seem to recall there was an article posted here recently which noted that galaxies have a preferred direction of rotation. Seems like the universe itself rotating could be a reason why?
That article is by a crank. People pointed out that he cherry-picked the galaxies, and no one else who looked at the data objectively saw any evidence of preference for a rotation direction.
I looked around (Smithsonian, space.com, astronomy.com, etc.), but nowhere has any mention of people disputing the findings, do you have a link? Who is "people" in this case?
(I was/am skeptical just because it's a single-author study with pretty spectacular results, and have been keeping an eye out for any followups, but must have missed them)
It's unfortunately rare for this kind of straightforward falsification to make it into publications aimed at the general reader. "A lie is halfway round the world before the truth has got its boots on," as they say.
I was hoping for something a bit more authoritative than a reddit comment being copy/pasted to HN (e.g. an actual retraction, or a paper directly disputing the findings), but I'm guessing most of the community just ignored the paper due to the history of the author.
That comment has exactly what you're asking for: Links to papers that show there is no evidence for rotation asymmetry. The very first article linked in that Reddit comment is a scientific publication from the Royal Society with the title "No evidence for anisotropy in galaxy spin directions".
I was hoping to read something that addressed this specific paper. All of those links are papers published before Lior Shamir's paper was published.
The comment I originally replied to said "people pointed out he cherry-picked results". I thought "people" might be "scientists" and they might have pointed it out in a subsequent paper or something.
Universe is called universe because it is the only one. Everything that exists should be part of the universe. When we say the universe rotates, what does it mean? Rotate relative to what? Does it mean that there's a larger "universe" that contains ours?
But even then you can only tell that you are rotating relative to space itself. If I understand correctly here we are speaking about the space itself rotating, so that would not be possible unless something else contains our universe's space, right?
Nitpick: "Atoms are called atoms because they have no subdivisible parts".
Oops, it turns out we used the name too soon.
When people say "universe" these days they mean the "visible universe" (or maybe the visible universe plus the stuff we're sure is there, but that falls outside our light cone now) - and not the original definition of the word anymore.
Atoms having no subdivisible parts implies that there are parts at all. But it turns out all matter is just somewhat stable oscillations within an energy field and no real boundaries between one part and another, and certainly nothing resembling such at the scale of something as large as an atom. So again.. too early..
I don't think the paper claims that it's the observable universe that rotates. Does it? It'd be awkward if only the observable universe rotates. Observable universe is not special. It's observable just because it is "close" to the Earth.
I consider this to be a good question, but with a reasonably familiar answer:
Rotation is absolute. Unlike linear motion, you can tell if you're sitting in a rotating frame of reference or not. Experiments such as the Coriolis force, and Foucault's pendulum, are demonstrations of this principle.
In fact, a historical oddity is that when this idea was nailed down by both theory and experiment, the Catholic Church dropped its ban on heliocentrism. (Not that it mattered, the horse had already left the barn).
Rotation implies a great many observables and properties of the universe. There would be a cosmic axis, which implies a cosmic center and a cosmic north. Both concepts that are problematic in an infinite universe. If the universe is in fact inside of a black hole of a different universe - some of these problems go away as the Hubble volume would be the finite boundary of the universe.
That’s one speculative possibility, but we don’t know. It could be infinite, or finite with a boundary just out of sight (beyond the edge of the observable universe), or some other more exotic geometry. There’s no solid evidence either way.
My money’s on non-infinite, just because I’m not sure infinity is a well-defined concept outside of mathematics. We’ve certainly never observed any other infinite phenomena.
I always like to think that finity can only exist in a context of infinity. So even if the "universe" is finite, it exists in a context of infinity.
Because they're opposites, they depend on each other's existence. Like hot cannot exist without cold, or light doesn't have any meaning if there is no darkness.
I want to address this specific comment: "or some other exotic geometry..." And ask conceiveavle or inconceivable? Of conceiveable by future beings? Far future beings? Current sci-fi authors? My uncle Ian? Only Joe Haldman?
Except plenty exist - every electron is an infinite phenomena. They're point-like particles made up of no constituents. You can get infinitely close to an electron and still get closer to it.
Infinity extends both upwards and downwards in scale.
Below the Planck distance, "closer" ceases to be a meaningful concept. Similarly, distances larger than the size of the observable universe may also be meaningless.
The co-moving radius of the universe is about 3x larger than the observable universe, but I don't think there's any practical measurements larger than that. We don't know how much is beyond that because potentially a large chunk of the universe is already forever outside our past and future light cone.
Why is that? I've heard this before but don't understand what it actually implies. Like, why can't 2 things meaningfully be half a Planck distance apart?
It's just that it's physically impossible to measure such small scales. To probe a smaller scale you have to put more energy into a volume (roughly, because you have to force particles closer together), and at some point you have to put so much energy into such a small volume that it becomes a black hole, then any more energy you put in will just make the black hole bigger. So there seems to be a fundamental limit to how short a scale we can probe, and therefore we can't give a physical meaning to smaller scales. Nima Arkani-Hamed talks about htis in a few lectures online.
This is just according to our current theories, and you don't need any speculative ideas about granularity of space. Mathematically, there's no problem in considering arbitrarily small length scales, because current theories are based on continuous space and time dimensions, but since we can't give physical meaning to small length scales, that's a clue that a more fundamental theory will somehow not be based on continuous space and time
Here's a summary of my layman's understanding, which could be wrong.
To my understanding there's nothing special about it except once you get to quantum field theory, and specifically spin-2 particles, which are the ones that hypothetically carry gravity (i.e. gravitons). (They're hypothetically gravitons because mathematically they result in conservation of the "stress-energy tensor", which is the same thing that Einstein's field equations give. And this isn't coincidence; it's because both are second-order field theories preserving Lorentz transforms aka special relativity). But notably if you only get through non-relativistic quantum mechanics, there's nothing special that happens at Planck length.
Planck length becomes special in QFT because at Planck length, the feedback (my force on you affects your force on me affects my force on you etc) of the quantum field mathematically explodes in something called "UV divergence". Notably, this also happens for other force carriers, not just gravitons, but a mathematical trick called Renormalization fixes that. Renormalization is based on shady techniques like assuming the sum of all whole numbers is -1/12. (IIUC this is a fairly straightforward pure mathematical result from complex analysis, required to ensure consistency of Fourier transforms for infinite sequences, known as analytic continuation / Riemann zeta function. Of course it makes no sense that nature should behave that way, but one day some physicists thought "let's try this weird math" and it matched experiments). However that technique only works for lower-spin fields (E&M, weak, strong) because the feedback is linear, hence sum(1+2+3+...). But in spin-2 fields the feedback is (quadratic? exponential?), and the same trick still gives an infinite result.
Ultimately one could say this is the only thing standing in the way of a complete theory of quantum gravity. If we could get the math for gravitons not to blow up at tiny scales, we'd be done (experimental verification notwithstanding). String theory is one attempt: "Let's say there are no point particles, only strings, and this problem goes away". Other approaches exist too. But ultimately right now, there's no way to say for sure whether sub-Planck exists, doesn't exist, exists in some ways but not in others, or whether all of physics is wrong and we have to start over.
A quick addendum: Planck energy is approximately the chemical energy in a tank of gas. Planck mass is about that of an amoeba (so an antimatter bomb made out of an amoeba would explode like a tank of gas (Hiroshima was 0.6 grams of mass), a photon with that energy would have a Planck-length wavelength, and an amoeba-mass black hole is Planck-length). Given there's nothing overly special about Planck energy or mass, my money is on there being nothing special about Planck length either.
Cool stuff. I don't know enough about QFT to evaluate it, but involvment in basic physics of the zeta function to evaluate diverging series and so on is something really amazing. This video discusses L-functions and QFT:
About the Planck length, while the length itself might not be physically significant, black holes are definitely physically significant, and according to the current thinking there will be some smallest length that can be probed before what you're looking at becomes a black hole. I guess I was lazily assuming that would be around the Planck length. I would have to look into it more to come up with a calculation for that
The only reason I've heard that isn't super handwavy is that measuring it would require a wavelength with enough energy to create a black hole. IDK if that's true or not, and IDK if that also means the distance doesn't exist or if it's just not measurable.
I've also heard that there's nothing special about planck length other than it being universal constant that we and any conceivable aliens would agree on as a standard of measure. So, idk.
Imagine the universe is made out of graph paper, with only the points of intersection of the lines being actually "real". That is, space is actually discrete. If that is true, then you cannot meaningfully talk about things being "half a square" apart.
People (some people) think that the universe really is that way, at the Planck distance. Actual experimental confirmation is somewhat lacking at this time...
More importantly, this idea doesn't work in the context of space dilation. Per special relativity, the distance between objects is arbitrarily different for different observers, so any grid you draw will be wrong for me if I'm moving at a different velocity than you. This is especially bad because it doesn't just depend on the magnitude of the velocity, but also the direction - space is compressed in the direction of motion. So two observers moving at an angle to each other will have very different views of the grid.
Imagine that at some small distance, space itself is quantum, so quantized. I.e. at the bottom, space is a discrete graph with incredibly small edges. Possibly very tangled and not even layed out in obvious dimensions over short distances.
We don’t know if that is the case. That is only one possibility.
But it seems very clear that whatever happens at the Planck distance or lower isn’t simple smooth space as we model it for larger scales.
Unfortunately for this idea, special relativity tells us that Planck length is an observer-dependent phenomenon. That is two objects that are at a distance that is more than the Planck length apart for one observer will be closer than the Planck length for another observer and vice versa. So Planck length can't be a fundamental property of space-time, unless special relativity breaks down at some point even for non-accelerated observers.
I would suspect since general relativity would break down that special relativity certainly would.
The “relativity” aspect will almost certainly still apply in some way, and still form an emergent basis for the special relativity effects you point out.
All of general relativity has to be emergent from the to-be-discovered laws of the underlying small scale structure of space.
(I.e. general relativity isn’t wrong, it is just not complete. Similar relationship as with Newton’s Law of Gravity, which was also correct, but breaks down beyond the conditions it covers well, because it was not complete.)
The smallest scale is also where we expect more “light” shed on the initial conditions of the universe and potentially the insides of black holes. Two other conditions where general relativity already breaks down.
“Breaks down” just means our equations don’t know what to say about a situation.
It doesn’t that any particular phenomena we understand today will break down. Just we will be able to see richer behavior, that has been there all along, than our models cover today. And so be able to understand more conditions than we do today. And perhaps new capabilities to engineer things than we have today.
Obviously quantum field theory, as it exists today, breaks down at the Planck distance too, since it can’t tell us what is happening at smaller distances either.
This isn’t the least bit controversial or surprising. It just means that even with both theories, we can’t explain everything yet.
When we do have an accurate theory of the fine structure of space and time, it will also be a theory of the fine structure of fields. Since space-time is integral to both theories.
It may even be the long sought after unification.
But for distances much larger than the Plank distance, the fine structure theory will still simplify into the GR and QFT we have today.
Just as GR under many conditions we encounter every day, further simplifies to Newton’s Law of Gravity.
I don't think this is the right idea. A new theory can invalidate core assumptions of a successful old theory, revealing it to be a coincidence that it happened to work mathematically in some regimes, it's not always a case that the old theory is simply an approximation of the new one.
For examples of the "good" kind, Newton's laws of motion are indeed just an approximation of special relativity. The Schrodinger equation is just an approximation of QFT.
In contrast, GR came in and showed that Newton's law of universal attraction is completely wrong, at the fundamental level. Sure, it predicts certain phenomena correctly, but so did the epicycles that it replaced. Similarly, QM/QFT showed that Newton's laws of motion are also completely wrong, that objects (or at least particles) don't even move according to some laws of motion, they are only described by a probability wave that moves according to some laws, and between two interactions they have no definite state and are not even localized.
And of course, GR and QFT disagree on these parts - GR generally agrees with Newton's laws of motion (with the SR corrections), and QFT generally agrees with Newtoninan gravity. But you can't use GR's gravity with QFT's laws of motion, so we know one or both are broken. A new theory is very likely to also "overrule" one or both of them and show that they are not just an approximation, but completely wrong, working only "by accident" on the scenarios where they have been tested. Especially if the new theory requires there to exist fixed space distances that all relativistic observers agree on.
Was the universe infinitely small or was it just extremely or infinitely dense?
Anyway, we don't actually know that, since we do not have a time machine. The theory that the universe used to be much smaller is something we infer based on the snapshot we see today, just like we infer rotation based on the snapshot we see today. Either one could yet be proven wrong.
yes but if x is finite, any f(x) will still be finite. So the universe has either always been finite, or always been infinite, and given that it started out being very small in a big bang, it would imply the former
> yes but if x is finite, any f(x) will still be finite.
The number line is not finite, even though every value on it is. The universe may be like the number line.
> and given that it started out being very small in a big bang, it would imply the former
The universe started out denser, which is not quite the same as "small" when talking about infinities. The currently visible universe gets both things at the same time, but only because the visible bit is finite.
Apply f(x) = x * 1e-60 to the number line, and things which were previously spaced out every integer, are now much more densely packed, but the number line as a whole is still the same size — infinite.
not if x is the set of all Real numbers. All points could have been infinitely close to each other at the big bang, and the universe could still have been infinite in size.
The observable universe measures as flat, as far as we are able to tell. The universe may be curved overall, but the patch we can see doesn’t let us deduce that, so far.
The center of a black hole lies in the future. It’s called a singularity, and the equations of general relativity break down at it. So no, we are not at the center.
“We show that a Gödel inspired slowly rotating dark-fluid variant of the concordance model resolves this tension…”
Gödel wait what? I thought of him as “only” the logician who killed Hiblert’s dreams and caused people to question the very foundations of mathematics.
Dude then took up physics as a hobby and trolled Einstein by discovering closed time-like curves?
At the level of what he did, physics is “just” math (finding solutions for Einstein’s general-relativity equations).
Also, from Wikipedia: “At age 18 [so around 1924], Gödel joined his brother at the University of Vienna. He had already mastered university-level mathematics. Although initially intending to study theoretical physics [we can assume that’d include an interest in relativity at that time], he also attended courses on mathematics and philosophy.”
based on napkin math, the rotation they land on: 4.34 x 10^-20 rad/s is one order of magnitude (not a lot on a cosmological scale) slower than necessary for a causal disconnect at the boundary of the observable universe due to tangential velocity (at ~C) - obviously my maths could be off-by-one.
> 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).
> 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?
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.
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 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.
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.
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?
If the universe is the interior of a black hole, one would assume so.
And an axis.
https://en.m.wikipedia.org/wiki/Axis_of_evil_(cosmology)
Or an axis.
And my ax
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?
If I understand what I skimmed, the idea is that space-time itself rotates, not all the collective matter in it?
Correct. Though the matter in it would also move, dragged along by the space-time.
It's much like the expansion of the universe, which is separate from the ordinary momentum of the objects within the universe. But the objects within the universe do move apart along with it.
Thanks.
These things really challenge my physics intuition!
I seem to recall there was an article posted here recently which noted that galaxies have a preferred direction of rotation. Seems like the universe itself rotating could be a reason why?
That article is by a crank. People pointed out that he cherry-picked the galaxies, and no one else who looked at the data objectively saw any evidence of preference for a rotation direction.
I looked around (Smithsonian, space.com, astronomy.com, etc.), but nowhere has any mention of people disputing the findings, do you have a link? Who is "people" in this case?
(I was/am skeptical just because it's a single-author study with pretty spectacular results, and have been keeping an eye out for any followups, but must have missed them)
This comment has a good summary of the debunking: https://news.ycombinator.com/item?id=43540635
It's unfortunately rare for this kind of straightforward falsification to make it into publications aimed at the general reader. "A lie is halfway round the world before the truth has got its boots on," as they say.
Thanks.
I was hoping for something a bit more authoritative than a reddit comment being copy/pasted to HN (e.g. an actual retraction, or a paper directly disputing the findings), but I'm guessing most of the community just ignored the paper due to the history of the author.
That comment has exactly what you're asking for: Links to papers that show there is no evidence for rotation asymmetry. The very first article linked in that Reddit comment is a scientific publication from the Royal Society with the title "No evidence for anisotropy in galaxy spin directions".
>That comment has exactly what you're asking for
No, it doesn't.
I was hoping to read something that addressed this specific paper. All of those links are papers published before Lior Shamir's paper was published.
The comment I originally replied to said "people pointed out he cherry-picked results". I thought "people" might be "scientists" and they might have pointed it out in a subsequent paper or something.
Thanks though.
The chronology or papers isn’t important here.
The cherry picking is clear from the difference in results not their timing.
Download the Pubpeer extension. There's often blips from someone.
Do you mean "common" rather than "rare"?
I think "rare" is right but the way I phrased it is probably misleading, sorry.
I meant the rebuttal usually doesn't get published with the same high profile as the original story.
The universe being a giant toilet in the process of flushing is delightfully Doug Adams inspired.
It's that constant sucking sound ...
Universe is called universe because it is the only one. Everything that exists should be part of the universe. When we say the universe rotates, what does it mean? Rotate relative to what? Does it mean that there's a larger "universe" that contains ours?
Rotation isn't relative. You can tell how quickly you're rotating without reference to any other objects.
But even then you can only tell that you are rotating relative to space itself. If I understand correctly here we are speaking about the space itself rotating, so that would not be possible unless something else contains our universe's space, right?
Nope, rotation is not relative because it's an acceleration.
There is no "space itself" because then you're assuming it is relative, like the speed of light. Basically you're trying to rediscover the aether.
That was the question: light moves at speed C, but relative to what?
Nitpick: "Atoms are called atoms because they have no subdivisible parts".
Oops, it turns out we used the name too soon.
When people say "universe" these days they mean the "visible universe" (or maybe the visible universe plus the stuff we're sure is there, but that falls outside our light cone now) - and not the original definition of the word anymore.
(Not that we have "found" anything else yet.)
Nitpick: atoms really have no subdivisible parts with the same properties as the whole.
They are aptly named.
Atoms having no subdivisible parts implies that there are parts at all. But it turns out all matter is just somewhat stable oscillations within an energy field and no real boundaries between one part and another, and certainly nothing resembling such at the scale of something as large as an atom. So again.. too early..
Good point.
“Universe” is often used in the sense of “observable universe”.
I don't think the paper claims that it's the observable universe that rotates. Does it? It'd be awkward if only the observable universe rotates. Observable universe is not special. It's observable just because it is "close" to the Earth.
Rotation relative to what?
I consider this to be a good question, but with a reasonably familiar answer:
Rotation is absolute. Unlike linear motion, you can tell if you're sitting in a rotating frame of reference or not. Experiments such as the Coriolis force, and Foucault's pendulum, are demonstrations of this principle.
In fact, a historical oddity is that when this idea was nailed down by both theory and experiment, the Catholic Church dropped its ban on heliocentrism. (Not that it mattered, the horse had already left the barn).
Thanks! That makes sense, but didn't come to mind right away for some reason.
[dead]
ICYMI: a slick link from the article with a video that demonstrates how closed timelike curves work in Godel's time traversal solution to Einstein's field equations. https://iopscience.iop.org/article/10.1088/1367-2630/15/1/01...
Wait did nobody seriously consider rotation yet? Everything is rotating. Angular momentum is conserved.
That would imply a preferred direction in the universe (the axis of rotation) and thus anisotropy.
There are stringent constraints on anisotropy from the cosmic microwave background.
In particular, one can use the Doppler effect to check whether the CMB dipole is compatible with our velocity with respect to the CMB frame.
Well, the universe has an axis, found in the CMB:
https://en.m.wikipedia.org/wiki/Axis_of_evil_(cosmology)
The article says right up front that the universe does not have an axis:
However, a 2016 study compared isotropic and anisotropic cosmological models against WMAP and Planck data and found no evidence for anisotropy.
That was only one observation, most of the observations do indicate there is an axis
Rotation implies a great many observables and properties of the universe. There would be a cosmic axis, which implies a cosmic center and a cosmic north. Both concepts that are problematic in an infinite universe. If the universe is in fact inside of a black hole of a different universe - some of these problems go away as the Hubble volume would be the finite boundary of the universe.
Are we in an infinite universe? I thought the current thinking was that it was finite but unbounded; that is, that it curved back on itself.
That’s one speculative possibility, but we don’t know. It could be infinite, or finite with a boundary just out of sight (beyond the edge of the observable universe), or some other more exotic geometry. There’s no solid evidence either way.
My money’s on non-infinite, just because I’m not sure infinity is a well-defined concept outside of mathematics. We’ve certainly never observed any other infinite phenomena.
I always like to think that finity can only exist in a context of infinity. So even if the "universe" is finite, it exists in a context of infinity.
Because they're opposites, they depend on each other's existence. Like hot cannot exist without cold, or light doesn't have any meaning if there is no darkness.
My money is on Joe Haldmans sawtooth cosmogony. Build an accelerator to simulate the big bang... Instead it recreates it.
I want to address this specific comment: "or some other exotic geometry..." And ask conceiveavle or inconceivable? Of conceiveable by future beings? Far future beings? Current sci-fi authors? My uncle Ian? Only Joe Haldman?
Except plenty exist - every electron is an infinite phenomena. They're point-like particles made up of no constituents. You can get infinitely close to an electron and still get closer to it.
Infinity extends both upwards and downwards in scale.
Below the Planck distance, "closer" ceases to be a meaningful concept. Similarly, distances larger than the size of the observable universe may also be meaningless.
The co-moving radius of the universe is about 3x larger than the observable universe, but I don't think there's any practical measurements larger than that. We don't know how much is beyond that because potentially a large chunk of the universe is already forever outside our past and future light cone.
Why is that? I've heard this before but don't understand what it actually implies. Like, why can't 2 things meaningfully be half a Planck distance apart?
It's just that it's physically impossible to measure such small scales. To probe a smaller scale you have to put more energy into a volume (roughly, because you have to force particles closer together), and at some point you have to put so much energy into such a small volume that it becomes a black hole, then any more energy you put in will just make the black hole bigger. So there seems to be a fundamental limit to how short a scale we can probe, and therefore we can't give a physical meaning to smaller scales. Nima Arkani-Hamed talks about htis in a few lectures online.
This is just according to our current theories, and you don't need any speculative ideas about granularity of space. Mathematically, there's no problem in considering arbitrarily small length scales, because current theories are based on continuous space and time dimensions, but since we can't give physical meaning to small length scales, that's a clue that a more fundamental theory will somehow not be based on continuous space and time
Here's a summary of my layman's understanding, which could be wrong.
To my understanding there's nothing special about it except once you get to quantum field theory, and specifically spin-2 particles, which are the ones that hypothetically carry gravity (i.e. gravitons). (They're hypothetically gravitons because mathematically they result in conservation of the "stress-energy tensor", which is the same thing that Einstein's field equations give. And this isn't coincidence; it's because both are second-order field theories preserving Lorentz transforms aka special relativity). But notably if you only get through non-relativistic quantum mechanics, there's nothing special that happens at Planck length.
Planck length becomes special in QFT because at Planck length, the feedback (my force on you affects your force on me affects my force on you etc) of the quantum field mathematically explodes in something called "UV divergence". Notably, this also happens for other force carriers, not just gravitons, but a mathematical trick called Renormalization fixes that. Renormalization is based on shady techniques like assuming the sum of all whole numbers is -1/12. (IIUC this is a fairly straightforward pure mathematical result from complex analysis, required to ensure consistency of Fourier transforms for infinite sequences, known as analytic continuation / Riemann zeta function. Of course it makes no sense that nature should behave that way, but one day some physicists thought "let's try this weird math" and it matched experiments). However that technique only works for lower-spin fields (E&M, weak, strong) because the feedback is linear, hence sum(1+2+3+...). But in spin-2 fields the feedback is (quadratic? exponential?), and the same trick still gives an infinite result.
Ultimately one could say this is the only thing standing in the way of a complete theory of quantum gravity. If we could get the math for gravitons not to blow up at tiny scales, we'd be done (experimental verification notwithstanding). String theory is one attempt: "Let's say there are no point particles, only strings, and this problem goes away". Other approaches exist too. But ultimately right now, there's no way to say for sure whether sub-Planck exists, doesn't exist, exists in some ways but not in others, or whether all of physics is wrong and we have to start over.
A quick addendum: Planck energy is approximately the chemical energy in a tank of gas. Planck mass is about that of an amoeba (so an antimatter bomb made out of an amoeba would explode like a tank of gas (Hiroshima was 0.6 grams of mass), a photon with that energy would have a Planck-length wavelength, and an amoeba-mass black hole is Planck-length). Given there's nothing overly special about Planck energy or mass, my money is on there being nothing special about Planck length either.
Hopefully at least some of the above is correct.
Cool stuff. I don't know enough about QFT to evaluate it, but involvment in basic physics of the zeta function to evaluate diverging series and so on is something really amazing. This video discusses L-functions and QFT:
https://www.youtube.com/watch?v=-OxVsVUesSc
About the Planck length, while the length itself might not be physically significant, black holes are definitely physically significant, and according to the current thinking there will be some smallest length that can be probed before what you're looking at becomes a black hole. I guess I was lazily assuming that would be around the Planck length. I would have to look into it more to come up with a calculation for that
The only reason I've heard that isn't super handwavy is that measuring it would require a wavelength with enough energy to create a black hole. IDK if that's true or not, and IDK if that also means the distance doesn't exist or if it's just not measurable.
I've also heard that there's nothing special about planck length other than it being universal constant that we and any conceivable aliens would agree on as a standard of measure. So, idk.
Imagine the universe is made out of graph paper, with only the points of intersection of the lines being actually "real". That is, space is actually discrete. If that is true, then you cannot meaningfully talk about things being "half a square" apart.
People (some people) think that the universe really is that way, at the Planck distance. Actual experimental confirmation is somewhat lacking at this time...
More importantly, this idea doesn't work in the context of space dilation. Per special relativity, the distance between objects is arbitrarily different for different observers, so any grid you draw will be wrong for me if I'm moving at a different velocity than you. This is especially bad because it doesn't just depend on the magnitude of the velocity, but also the direction - space is compressed in the direction of motion. So two observers moving at an angle to each other will have very different views of the grid.
Imagine that at some small distance, space itself is quantum, so quantized. I.e. at the bottom, space is a discrete graph with incredibly small edges. Possibly very tangled and not even layed out in obvious dimensions over short distances.
We don’t know if that is the case. That is only one possibility.
But it seems very clear that whatever happens at the Planck distance or lower isn’t simple smooth space as we model it for larger scales.
Unfortunately for this idea, special relativity tells us that Planck length is an observer-dependent phenomenon. That is two objects that are at a distance that is more than the Planck length apart for one observer will be closer than the Planck length for another observer and vice versa. So Planck length can't be a fundamental property of space-time, unless special relativity breaks down at some point even for non-accelerated observers.
I would suspect since general relativity would break down that special relativity certainly would.
The “relativity” aspect will almost certainly still apply in some way, and still form an emergent basis for the special relativity effects you point out.
All of general relativity has to be emergent from the to-be-discovered laws of the underlying small scale structure of space.
(I.e. general relativity isn’t wrong, it is just not complete. Similar relationship as with Newton’s Law of Gravity, which was also correct, but breaks down beyond the conditions it covers well, because it was not complete.)
The smallest scale is also where we expect more “light” shed on the initial conditions of the universe and potentially the insides of black holes. Two other conditions where general relativity already breaks down.
If special relativity breaks down, so does QFT - since QFT is based on special relativity just as much as on QM.
“Breaks down” just means our equations don’t know what to say about a situation.
It doesn’t that any particular phenomena we understand today will break down. Just we will be able to see richer behavior, that has been there all along, than our models cover today. And so be able to understand more conditions than we do today. And perhaps new capabilities to engineer things than we have today.
Obviously quantum field theory, as it exists today, breaks down at the Planck distance too, since it can’t tell us what is happening at smaller distances either.
This isn’t the least bit controversial or surprising. It just means that even with both theories, we can’t explain everything yet.
When we do have an accurate theory of the fine structure of space and time, it will also be a theory of the fine structure of fields. Since space-time is integral to both theories.
It may even be the long sought after unification.
But for distances much larger than the Plank distance, the fine structure theory will still simplify into the GR and QFT we have today.
Just as GR under many conditions we encounter every day, further simplifies to Newton’s Law of Gravity.
I don't think this is the right idea. A new theory can invalidate core assumptions of a successful old theory, revealing it to be a coincidence that it happened to work mathematically in some regimes, it's not always a case that the old theory is simply an approximation of the new one.
For examples of the "good" kind, Newton's laws of motion are indeed just an approximation of special relativity. The Schrodinger equation is just an approximation of QFT.
In contrast, GR came in and showed that Newton's law of universal attraction is completely wrong, at the fundamental level. Sure, it predicts certain phenomena correctly, but so did the epicycles that it replaced. Similarly, QM/QFT showed that Newton's laws of motion are also completely wrong, that objects (or at least particles) don't even move according to some laws of motion, they are only described by a probability wave that moves according to some laws, and between two interactions they have no definite state and are not even localized.
And of course, GR and QFT disagree on these parts - GR generally agrees with Newton's laws of motion (with the SR corrections), and QFT generally agrees with Newtoninan gravity. But you can't use GR's gravity with QFT's laws of motion, so we know one or both are broken. A new theory is very likely to also "overrule" one or both of them and show that they are not just an approximation, but completely wrong, working only "by accident" on the scenarios where they have been tested. Especially if the new theory requires there to exist fixed space distances that all relativistic observers agree on.
That's only if you view the electron classically, which is an inadequate picture.
It's only been a finite time since the universe was very small, so unless it's expanding infinitely fast, I don't see how it could be infinite
Was the universe infinitely small or was it just extremely or infinitely dense?
Anyway, we don't actually know that, since we do not have a time machine. The theory that the universe used to be much smaller is something we infer based on the snapshot we see today, just like we infer rotation based on the snapshot we see today. Either one could yet be proven wrong.
It's (mathematically) allowed for it to have always been infinite, and still grow.
If you take the number like and apply f(x) = 2x, it was infinite before and after.
yes but if x is finite, any f(x) will still be finite. So the universe has either always been finite, or always been infinite, and given that it started out being very small in a big bang, it would imply the former
> yes but if x is finite, any f(x) will still be finite.
The number line is not finite, even though every value on it is. The universe may be like the number line.
> and given that it started out being very small in a big bang, it would imply the former
The universe started out denser, which is not quite the same as "small" when talking about infinities. The currently visible universe gets both things at the same time, but only because the visible bit is finite.
Apply f(x) = x * 1e-60 to the number line, and things which were previously spaced out every integer, are now much more densely packed, but the number line as a whole is still the same size — infinite.
not if x is the set of all Real numbers. All points could have been infinitely close to each other at the big bang, and the universe could still have been infinite in size.
f(x) = tan(x)
f(x) = 1/(1-x)
So if x is finite, f(x) can still be infinite.
we don't know that the universe was ever small, just that it used to be a lot denser than it is today.
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The observable universe measures as flat, as far as we are able to tell. The universe may be curved overall, but the patch we can see doesn’t let us deduce that, so far.
It's so close to flat that we can't tell.
>If the universe is in fact inside of a black hole of a different universe
The observation could also be due to an area of relatively high density inside an area of relatively no density.
Except why would there only be one axis?
and we would be at the center?
My dog is quite sure she’s at the center, if we’re trying to be specific
Main Character Energy? My cat has that too.
Yeah, we are.
https://en.m.wikipedia.org/wiki/Axis_of_evil_(cosmology)
The center of a black hole lies in the future. It’s called a singularity, and the equations of general relativity break down at it. So no, we are not at the center.
Isotropy is a fundamental assumption that is very hard to let go of.
Where would the axis of rotation be?
https://en.m.wikipedia.org/wiki/Axis_of_evil_(cosmology)
it's like shooting bullets in the side of a barn and drawing circles around them.
“We show that a Gödel inspired slowly rotating dark-fluid variant of the concordance model resolves this tension…”
Gödel wait what? I thought of him as “only” the logician who killed Hiblert’s dreams and caused people to question the very foundations of mathematics.
Dude then took up physics as a hobby and trolled Einstein by discovering closed time-like curves?
At the level of what he did, physics is “just” math (finding solutions for Einstein’s general-relativity equations).
Also, from Wikipedia: “At age 18 [so around 1924], Gödel joined his brother at the University of Vienna. He had already mastered university-level mathematics. Although initially intending to study theoretical physics [we can assume that’d include an interest in relativity at that time], he also attended courses on mathematics and philosophy.”
Godel and Einstein were actually quite close.
There's a scene in the recent movie Oppenheimer about that.
If your paper has a question title, it should be sent back to you with the note "change the title to summarize the paper".
And Betteridge's Law applies, so based on this alone: the Universe does not rotate.
based on napkin math, the rotation they land on: 4.34 x 10^-20 rad/s is one order of magnitude (not a lot on a cosmological scale) slower than necessary for a causal disconnect at the boundary of the observable universe due to tangential velocity (at ~C) - obviously my maths could be off-by-one.
Is this a reasonable conclusion?
And the Hubble tension is around 4%, within a factor of 2 of that 10%. This sounds reasonable.
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