Northwestern University's press-release (which one can find repeated verbatim across the web e.g. at phys.org) points at an open access (CC:BY) Astrophysical Journal page <https://iopscience.iop.org/article/10.3847/1538-4357/ace051>. Unfortunately IOP (the official online home for ApJ) occasionally has fits of "I'm really not sure you're a human, let's loop around until you get bored" that may affect HN readers in particular. The corresponding preprint, Kaaz, Liska et al. 2022, "Nozzle Shocks, Disk Tearing and Streamers Drive Rapiod Accretion in 3D GRMHD Simulations of Warped Thin Disks" can be found at <https://arxiv.org/abs/2210.10053>.
Each version is about fifteen pages including a substantial references section and many graphs and graphical depictions.
As several early commenters here have focused on the GR-MHD (general relativitistic magnetohydrodynamics) simulation basis, the second author is a well-known computational astrophysicist and leader of the software collaboration used in this paper and his site may be of interest: https://www.matthewliska.com/
> Unfortunately IOP (the official online home for ApJ) occasionally has fits of "I'm really not sure you're a human, let's loop around until you get bored"
I have my privacy turned up somewhat (uBlock Origin with custom rules), and I didn't notice any issue. I see that its calling out to CloudFront and Google, can you detail what the possible issue is?
Why? Are you in a position to help everyone? (As you probably guessed while reading the comment you replied to, I don't really need help; more on that below).
"Occasional" is not universal; as you aren't getting the problem in a here-and-now sense you can probably play around with the "here" part by using Tor Browser to see if you can get to the article via the link in the fine press release from northwestern. Many times Tor (say if one wants to be sure one is not transmitting some credential to the publisher) works fine, sometimes you'll have to start a new Tor circuit (or maybe be more patient than me). Likewise, with uBlockOrigin on medium mode <https://github.com/gorhill/uBlock/wiki/Blocking-mode:-medium...> sometimes it works, sometimes it doesn't.
I waver between wondering if it's caching (seems to be more frequent on "cold" papers hosted at iop.org, during a trawl of a paper's references), sometimes location (in the IP range sense), sometimes defences triggered by "hug-of-death" on a very fresh paper.
When it happens it's usually very easy to find the preprint and start with that, so I haven't been minded to pursue anything like formal process with anyone's IT department.
To be clear, IOP is pretty good by learned society standards, and even the worst of those is much less annoying to use (and less likely to have annoying glitches) than by major for-profit academic publishers, even (sometimes especially) when the paper is open access. For that last set, for cold papers, one is more than tempted to start with sci-hub for the latter's greater reliability and better user interface. And of course, there's also https://unpaywall.org/ which is also tremendously useful.
It's really interesting that there are two.discs in different planes because of the frame dragging effect. And that this might help explain the phenomenon of quasars.
Astronomy continues to advance and discover new things all the time!
Question: is the misalignment of the accretion disc planes due to the black hole rotation axis not matching the host galaxy's rotation axis? Or, does the frame dragging cause misalignment even when the black hole and galaxy rotations are aligned?
(Not an astrophysicist, but I've always assumed that a black hole's rotation axis would tend to be aligned with that of the galaxy hosting it, for angular momentum reasons alone. Then again, galaxies and black holes do merge...)
It's not the galaxy's rotational axis, exactly; it's the rotational axis of the (currently present) infalling gas. That may tend to align with the galaxy's axis, but there's probably a lot of variation. (When a star is getting sucked in, it's going to contribute pretty heavily, and star orbits in the core are not at all correlated with the axis of rotation of the disk.)
All this is my understanding only. I'm not a working physicist.
The paper isn't particularly mass-dependent. it contemplates AGN masses down to the 10 M_sun stellar black hole in a previous paper by the second author and others.
In general stars, stellar black holes, and bigger black holes have random spin axes. Central black hole binaries exist but their spins can point in random directions compared to the plane of the binary orbit; and the plane of the binary orbit is not necessarily aligned with that of the host galaxy (if discoid). As you say, galaxies can merge and in ellipticals the dominant rotation curve may be radial (blobs of (possibly star-forming!) hydrogen gas and the like cycling in and out rather than around some axis).
Black hole-star binaries exist (with stellar multiplicities up to perhaps five) too, and rotation is faster with greater multiplicities, but the spin axes need not be aligned or anti-aligned, and the several stars need not all orbit in the same plane. This is an active area of research in galactic evolution.
Consequently, the spilling of gas from a star to a BH companion, especially in a triple+ (and especially considering intrinsic-variable stars' structured outpourings), could lead to an accretion disc that is highly non-equatorial and/or retrograde. The paper press-released at the top contemplates the forcing the innermost part of such discs onto a more equatorial and prograde orbit, and how that warps, tears or ablates the disc compared to a "gentle" picture of a uniform disc in prograde equatorial orbit around a slow-spinning BH.
> does frame dragging cause misalignment
No, Lense-Thirring torques "cure" (parts of) a non-equatorial accretion disc by pulling inner elements of it into the equatorial plane.
Going a little further than above, a non-equatorial disc could arise when an eruptive-variable companion spits out matter that sails just above or below the pole of its companion BH, even if the spin axes of both were completely parallel. Probably even more likely is that a large eruption spits out lots of matter and the near region of its companion BH only subtends a fraction of it. That might lead to a complex accretion structure that might settle into multiple discs. Lots of observatories are looking for that kind of thing. :-)
ETA: Bardeen & Petterson (1975), The Lense-Thirring Effect and Accretion Disks Around Kerr Black Holes ends with: "We remark that both Shakura (1972) and Katz (1973) have made the suggestion that tilted disks may form in binary systems if the optical star rotates around an axis which is not aligned with the system’s orbital angular momentum. Wilson (1974) has pointed out that an asymmetric supernova explosion may change the direction of the orbital angular momentum." This also gives you an idea of how old this idea of LT precession "untilting" non-equatorial accretion discs is (I believe it originated with Bardeen & Petterson). The broad question, even older, is "why are accretion structures around stars and Saturn essentially always equatorial? Does that apply to black holes too?"
Question for astrophocists. Does frame dragging originate at the event horizon or at the singularity/ringularity? If it's the latter, does it mean that there's a limit in size for black holes above which this phenomenon becomes negligible?
It feels similar to the counter intuitive notion that tidal forces are more extreme when closer to a small black hole.
You can ask it only for an in-falling observer. To extend that to someone who remains outside the event horizon, it becomes meaningless. There are many hints that there is something special going on at the event horizon itself.
With respect, I disagree. A pair of comments I made elsewhere in this thread bear that out.
> > Does frame dragging originate at the event horizon or at the singularity/ringularity?
> You can ask it only for an in-falling observer
Both the geodetic effect and the Lense-Thirring (LT) effect, forms of "frame dragging", have been demonstrated in Earth orbit, for example by Gravity Probe B <https://en.wikipedia.org/wiki/Gravity_Probe_B>.
I think we can agree that there is neither an event horizon nor a singularity associated with Earth, and that since GP-B continues to orbit Earth after almost twenty years, it was not doing any sort of in-falling during its mission.
The tl;dr answer to the question you think is a bad one is that Kerr exterior spacetimes -- even those that are not black holes -- induce LT, which is roughly an altitude-dependent mismatch between a vector pointing to something far away that is parallel-transported around the spinning central mass and around the same mass if it were not spinning. On extended objects occupying a range of altitudes above a spinning mass, an LT torque is induced.
No black hole needed.
The paper press-released in TFA the spinning object is a black hole, which lets one get much closer to the centre of mass (because a BH is compact compared to a planet) and because can spin much faster than a planet (which would tend to fly apart), which lets the LT effect be stronger. It also contemplates a large accretion disc in which LT torques can create pronounced observable effects.
The singularity is not relevant to the analysis (it is not even mentioned in the paper). The horizon is barely relevant: towards the horizon they reduce the resolution of the adaptive mesh they calculate with and justify doing so in a couple of ways -- most of the interesting physics are at least 2R and mostly above 4R, where R := r_g which is the Kerr equivalent of the Schwarzschild radius r_s.
Thinking about this more, it should be at the singularity level since the event horizon doesn't have mass to cause it. Unless my basic understanding is black holes is wrong.
> We shouldn't be able to see tides caused by changes on the mass distribution inside the horizon.
... if we hold the horizon fixed. If we don't let the horizons move at all we get all sorts of useful results like Hawking radiation via linear analysis. But if we let apparent horizons evolve dynamically taking into account anisotropic radiation reaction, the backreaction on the metric of the accretion of mass and angular momentum, and so forth, the full nonlinear theory becomes so complicated that I think it is fair to say that it is not in general well understood.
We do know (empirically) that merged black holes ring down, which involves values sloshing about in the Weyl tidal tensor (in the 3+1 formalism). I am not certain treating the apparent horizon as a substance that settles down (emitting gravitational radiation) is conceptually useful, as opposed to the dynamical apparent horizon being generated by the movement of whatever's inside the merged binary plus most of the outgoing waves falling back towards the merged binary.
The paper press-released at the top is not about merger and ringdown, though, so the internal configuration of the Kerr BH is irrelevant. Also, Lense-Thirring torques, the topic of the paper, arise even in non-BH Kerr spacetimes. We even see them in Earth orbit, and in due course probably will have good data from solar orbits too.
>>> Does frame dragging originate at the event horizon
> That "originates" word is funny
I think it would be enlightening if you were to expand on that.
Finally, whether or not it's tidal forces comes down to an interpretation of how one decomposes the Riemann curvature tensor. In some ways, the Weyl tidal tensor part is good for understanding the disc's warping and tearing. However, if one decomposes the fluid into a zillion particles of dust, this approach seems (to me) to be less intuitive (and certainly harder to compute) compared to the GRMHD approach in the paper. Additionally, for these effects the black hole's spin parameter is more relevant than the black hole's mass, and notably the paper is concerned about quasars and other active galactic nuclei, which implies supermassive black holes, so the tidal tensor will in general be quite gentle even at the ISCO.
For none of the above do we need to know anything about the microscopic states within the horizon. This is entirely about solutions to the geodesic equations and equations of motion in the spacetime outside the horizon, and matter-matter interactions of disc material (or "figure-skater-like spacecraft" material).
Let's start with the most major feature of General Relativity:
Freely falling objects (around which one will build inertial frames of reference, with the object always at the spatial origin) are in geodesic motion, with the choice of all possible geodesics determined by the whole spacetime, and the particular geodesic determined by "initial" position and velocity.
The Schwarzschild metric for a black hole generates a set of available geodesics such that there is a large set of innermost stable circular orbits (ISCOs) permitted by the spherical symmetry and the absence of any outside matter to break the time-symmetry ("eternal, never growing or shrinking"). There are many initial <position, velocity> pairs that leave a test particle on an ISCO forever, and the ISCOs will generally be found at 3R_s, where R_s is the Schwarzschild radius.
If we break the spherical symmetry and make the system cylindrically symmetrical or axisymmetric, there are fewer ISCOs available: just as there are many great circles around a sphere, there are fewer around a cylinder. Rotating black holes generally do not have spherical symmetry, even if they are fully time-symmetric ("never grows nor shrinks, never changes its angular momentum").
Very roughly, frame-dragging captures the difference in 'equatorial' orbits between a spherically symmetric black hole ("one cannot distinguish an equatorial orbit from a polar orbit") and an equatorial orbit around a black hole with reduced symmetry (and between equatorial and nonequatorial orbits).
A rotating black hole with an axis of rotation lets one distinguish between an orbit that goes above both poles and an equatorial orbit (and orbital planes in between those extremes). It also lets one distinguish between a prograde and retrograde orbit. This affects available ISCOs: for an initial <position, velocity> where the velocity is prograde, the ISCO will be closer to the rotating black hole than if the velocity were retrograde. Notably, as the black hole rotation is increased to a maximum, the prograde ISCO "just touches" the event horizon.
On every ISCO, the local physics of a test free-faller is that of Special Relativity. This is essentially the content of the weak and Einstein equivalence principles, which you can get a feel for in the two sections starting at <https://en.wikipedia.org/wiki/Equivalence_principle#The_weak...>. Free-fallers behave like astronauts in the ISS or other orbiting or Earth-Moon spacecraft. Things just float about weightlessly, but the speed of light, Planck's constant, and so forth are unchanged compared to a lab on the surface of the Earth.
Now, ignoring the further breaking of symmetry by introducing "fixed stars at great distances" to the spacetime, let's have a pair of counter-rotating equatorial ISCO-freefallers use a system of local East-North-Up coordinates (see <https://en.wikipedia.org/wiki/Local_tangent_plane_coordinate...>) where we choose an equatorial plane at random where zero black hole rotation does not pick one out. "Up" is normal to the black hole's surface. North follows the right hand rule convention. The observers can counter-rotate on the same orbit because they are magically transparent to each other ("test objects", which are mathematical tools used explore the geodesics of a spacetime).
Let's have the initial <position, velocity> of our counter-rotating observers differ only by whether they are travelling East or West in these coordinates, and with "Up" pointing directly to a bright distant star. On a high-radius circular equatorial orbit above minimally-rotating black hole, each "year" our two observers will meet and look Up to see the bright star immediately overhead. As we harden the orbit towards ISCO, there is a growing identical mismatch (the geodetic effect) between the locally maintained "Up" and the bright star that was initially "Up". The geodetic effect is not the subject of the paper press-released by northwestern, so I won't dwell on that here.
If the black hole has significant rotation, as we harden the initially-soft orbit (with a long "year") there is a further but opposite "annual" mismatch between "Up" and the direction of the bright star. This is the Lense-Thirring effect. One can think of this in many ways, but it is related to prograde/retrograde geodesic motion. The <position, velocity> and <position, -velocity> starting points can only take one to a stable circular equatorial orbit. The other, with the opposite sign, must accelerate to be on the same circle but in the opposite direction. Or conversely, it will evolve into an orbit that is a combination of less-circular, less-equatorial, and different-height. We stop having the "annual" meetings below the distant star; each time the opposite-direction observers on approximately the same orbit meet, they will notice that the distant star is offset by an increasing angle. Or conversely, if they measure the year by being directly under the distant star, the prograde/eastbound observer will decide that the retrograde observer will arrive later and later every year, and the retrograde/westbound observer will decide that the prograde observer will arrive earlier and earlier every year.
We might consider a simple accretion disc as a "dust" of many observers confined to the equatorial plane and circular orbits in the same direction. We have to make a perturbation like a dust-dust collision to accelerate a mote off its particular orbit and onto another one. This tends to cause an avalanche of interactions, so one then is inclined to treat the disc as a fluid. Additionally, one would draw some analogies between the orbital differences in the previous paragraphs and gyroscopic precessions or processions of electromagnetically-charged objects in a magnetic field. This is the root of "GRMHD", where the last three letters stand for magnetohydrodynamics, which is a technique used in the paper press-released by northwestern.
In this approach one would not want to consider the local coordinates attached to each microscopic fluid element, but would work in some other coordinates suitable for studying the bulk fluid, such as spherical polar coordinates on the black hole itself. In those coordinates, the local "East-North-Up" of individual microscopic fluid elements along a <-ct,r,\theta=towards_our_bright_star,\phi=equator> will have to unalign, e.g., t_BH!=t_particle or \theta is at an angle to local Up/Down. The unaligning is stronger for fluid elements closer to the spinning black hole, and stronger as the black hole's spin increases.
This unalignment is the 'general-relativistic "frame-dragging of the Kerr BH"' that the paper contemplates.
We have to add something else: the fluid elements couple to each other non-gravitationally.
The usual way of explaining this is to consider a figure skater who can extend and retract his arms, changing the velocity of his spin (while keeping angular momentum constant, since the latter is the former times the radius squared). If the figure skater were non-interacting dust, he would simply fly apart under rotation; it is the stickiness of the molecular forces in his body that keeps him intact, with his core preventing his fingertips from flying away by accelerating them back onto the core's preferred trajectory.
In a far orbit around a spinning black hole, a freely falling spinning object that can extend and retract arms will notice nothing special compared to our figure skater: the arms' extremeties will be pulled towards the core by molecular forces and the like. The whole extended object will follow a free-falling trajectory.
However, in a close orbit, the effects above manifest: the bits of the object closer to the black hole have to be accelerated more strongly back onto the freely-falling orbit of the centre of mass-momentum of the whole object compared to the bits further from the black hole. This relatively-higher drag on the closer-to-the-black-hole parts of the spinning generates a torque. If the orbiting object extends arms like a figure skater, but with a hand dipping closer to the black hole each rotation, the torque is increased. This is "paid for" by the whole spacetime, but for simplicity one usually points to the angular momentum of the black hole: the black hole's spin alters the spin of the (much much less massive) spinning orbiting object.
In the accretion disc fluid, there are lots of small cells which are like the spinning object in the previous paragraph, and they bump into each other transferring angular momentum. The result is that inner regions of the disc experience a greater torque. The torques create differential misalignments of the black hole spin vector (in the spherical polar coordinates) and the spin vectors transported by each fluid element (in their local East-North-Up coordinates) that one an treat as an orbital precession. But an element initially travelling Eastbound in the equatorial plane will keep doing so no matter what sort of coordinates one puts on the bulk of all elements + the black hole, unless the element is acted on by some non-gravitational force. And fluid-element-to-fluid-element coupling does exactly that. The result is that the unalignment is forcibly realigned by bulk interactions, with the result that the disc warps in some combination of all four coordinates. The warp in \phi can be dramatic if the bulk is initially moving in a non-equatorial plane: the inner regions will be forced into the equatorial plane, leading to what the authors call a "torn disk".
> While previous researchers have hypothesized that black holes eat slowly, new simulations indicate that black holes scarf food much faster than conventional understanding suggests.
Title should be changed to:
Models of Black holes eat faster than previous models expected.
Yes, although "observation" is perhaps a bit loose. More on that at the end.
For around fifty years, relativists have been interested in how general relativity drives the structure of rings and discs of matter where the inner edges are exposed to general relativity. The motivations are to better anticipate observations that may test general relativity, to understand how to distinguish relativitistic orbits from Newton-Kepler orbits in various ways, and to try to expand our weak understanding of why there are many disc-like structures around ordinary stars (protoplanetary nebulae and so on) and why there are so many discoid galaxies like typical spirals.
The very broad question, decoupled from any theory of gravitation, is: given an initial random ball of matter around a central mass, what drives the ball into a disc?
How this works for accretion structures around a spinning black hole has been the stuff of research for decades, and a 1975 paper (by Bardeen & Petterson) spawned a direction of research from which "today's" particular paper descends: "frame dragging" can evolve a fluid disc surrounding a black hole from a non-equatorial plane into an equatorial plane. ("Today's paper" adds some complications to this decades-old idea by using computers to study the fluid dynamics and magnetics within a somewhat more realistic disc than could be considered decades ago. Additionally, this remains an area of ongoing research and "today's" authors are not the only active group).
I notice I used "scare quotes" above in three ways; the first is simply shorthand for the paper discussed at the northwestern press-release linked at the top of the page. The second is quoting your use of the word observation, and I'll return to that. The third is because "drag" in "frame dragging" gives me itches because it is so hard to understand non-mathematically. Bear with me, I'll spend the next couple of paragraphs on that, scratching my own itch. (Also it turns out I'll use "" in a fourth way, namely surrounding search terms that may be useful for anyone interested in finding out more).
Although circular orbits around a spinning black hole are stationary (they are not accelerated or decelerated by the black hole spin; but for a constant radius from the black hole they may be longer or shorter) the orbits may be deformed in direction orthogonal to the direction of the orbit.
That is, an orbiting body (in free-fall on a circular geodesic) may, without feeling any accelerations or body forces, find itself on an orbit which is in a different plane, or is at a different radius, or which takes a different amount of proper (as in wristwatch) time for a full orbit, or some combination of these.
The effect is called "frame dragging", but again, nothing is really feeling a drag like in a viscous medium (like running into or along with a strong wind, for example). Einstein gets the blame for the use of "drag" since he wrote a letter to Mach in 1913 talking about the plane of an idealized Foucault's pendulum being dragged about by (an idealized) Earth's rotation. Lense and Thirring (1918-1921) were interested in the vanishing of Coriolis forces in certain frames of reference. Cohen (1965) generalized that to the finding of inertial frames of reference within and near rotating masses. These papers all discussed a freely-falling observer feeling no forces (especially not "fictitious forces") who by looking well outside the frame (e.g. at distant "fixed stars") tell that the observer's orbit was influenced by the rotation of the black hole, even if the black hole left no local imprint on experiments. The free-falling laboratory locally inertial frame of reference of the observer was said to be "dragged" with respect to those fixed stars, like Foucault's pendulum's plane.
"Drag" is unfortunate because the presence of these special frames of reference are a feature of the (curved) spacetime under study; they trace out orbits, and one can set up a body so that it follows one of these orbits. The orbits are a special set among a (larger) infinity of possible orbits, but oddly such special orbits seem more popular with matter in an accretion disc. Why? Because the matter is "dragged" there like one would drag a toy truck by its string or iron filings with a bar magnet? No, the matter feels no pushes or pulls, only its orbit changes, and spatially only in directions orthogonal to the orbital direction.
Today's paper introduces pushes and pulls within the matter of the accretion disc. The matter feels those (scatterings, collisions), even in the directions along the orbit itself. This results in observable structure when matter is deposited randomly around a black hole (rather than placed very very carefully into an equatorial orbit).
Those structures easily remove energy from matter in the disc, allowing that matter to fall onto closer orbits (and ultimately into a plunging orbit into the black hole itself).
If they exist, those structures should be visible soon enough by astronomers, so it's a decent prediction. They also differ from the structures predicted in alternative answers to the questions, "are accretion discs entrained to the equators of spinning black holes? what is the mechanism?".
Finally, "observation". Except in X-rays (thanks to Inverse Compton Scattering) we have few direct observations of accretion disc structure. We have not proven that astrophysical accretion happens in discs. There may be non-disc accretion systems, and at least in principle we are not in position to say which is the exception (are accretion discs rare? are non-disc structures rare?).
We do know (thanks to the background luminosity and larger subtended angle) that there are polars, which are very X-Ray bright white dwarfs and neutron stars which channel accretion into columns down their magnetic poles, rather than into discs. There are pulsars which have both this type of columnar accretion and disc accretion, too.
If these compact objects collapse further into black holes, what happens to these columnar accretion structures?
Hopefully astronomers will figure out how to better see known black holes and black hole candidates in various parts of the electromagnetic spectrum. We may also get some ideas about large accretion structures from central-black-hole mergers in distant galaxies, and maaaybe even in open clusters in our own galaxy (the Hyades cluster has been in recent news, and it's close). Also globular clusters in our galaxy like M15 might turn out to have a central black hole with lots of matter liable to be drawn into a close orbit around it.
(Our own central black hole Sgr-A* is not just obscured by dust and gas along our line of sight, it's also really really quiet in the wavelengths which are least obscured; it does not seem to be accreting much of anything. It only got the "*" designation because in radio it stood out strongly against the other things in Sagittarius. Even Jansky noticed it in 1933 on his apparatus <https://en.wikipedia.org/wiki/Karl_Guthe_Jansky#/media/File:...>. But most known central black holes in other galaxies are much brighter in radio than ours).
No. This could be about observations of black holes indicating rapid mass ingestion, more than the current models can explain. Instead, the story is about a new model which shows quicker mass ingestion than previous models.
Each version is about fifteen pages including a substantial references section and many graphs and graphical depictions.
As several early commenters here have focused on the GR-MHD (general relativitistic magnetohydrodynamics) simulation basis, the second author is a well-known computational astrophysicist and leader of the software collaboration used in this paper and his site may be of interest: https://www.matthewliska.com/