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What Gravitational Waves Can Say About Dark Matter (symmetrymagazine.org)
61 points by scottie_m 34 days ago | hide | past | web | favorite | 27 comments



Dark matter seems to interact with normal matter only through gravity, but, based on the way known particles interact, theorists think it’s possible that dark matter might also interact with itself.

If dark matter were to interact with itself (as regular matter does), wouldn't we expect it to clump together and form the same structures as regular matter does, especially given that there's supposed to be much more dark matter to interact together than the regular stuff? AIUI one of the defining features of dark matter is that it doesn't clump together.


Actually dark matter clumps, e.g. so called dark matter halo of a galaxy[1]. It is seen from rotational curves of galaxies and can be also checked with with strong and weak lensing effects [2].

[1] https://en.wikipedia.org/wiki/Dark_matter_halo

[2] https://en.wikipedia.org/wiki/Gravitational_lens


No, dark matter will form halos even if it's interactions are solely gravitational. "Clumping" refers to much stronger concentrations from non-gravitational interactions.


Yes, it is correct that clumping can be solely gravitational effect, though so far we have pretty vague idea about the density in those clumps and halos, which are mostly derived from N-body simulation, which are obviously model dependent, e.g. NFW profile [1]. If dark matter interacts with itself one can expect deviations from purely gravitational description due to dark matter analogues of pressure, temperature, viscosity, etc.

[1] https://en.wikipedia.org/wiki/Navarro%E2%80%93Frenk%E2%80%93...


Yes, but this just upper bounds the interaction strength rather than constrains it to be zero. Additionally, if there are multiple species of dark matter, there may be subpopulations (up to 10%, I think) that interact very strongly without conflicting with observation.


Perhaps tangential, but is there a frequency of a gravity wave high enough to "cusp" due to one of the quantization limits?


In even the classic theory, when the Swartzchild radius of the gravitational wavelet exceeds its width, wouldn’t it naturally form a black hole and end up a cusp?

Like a gravity Kugelblitz.


I think by definition it wouldn’t form a black hole, it would be a black hole. Gravitational waves are propagating disturbances through spacetime, so the kind of wave you’re describing would begin as a singularity. I don’t think (but am not sure) that the math allows for the emission of such a thing. It sounds non-physical, and I’d suspect that if you do the math you’d discover that you’d need to have giant black holes merging to generate such a wave, or FTL. In the former case I’d bet that it turns out the wave would form within the event horizon of the hole, and that’s a good as saying it would never form.


>In the former case I’d bet that it turns out the wave would form within the event horizon of the hole, and that’s a good as saying it would never form.

Sorry if this is a bit naive and tangential, but I've always stumbled at the thought of how does gravity-information about the interior of a black hole propagate out of the event horizon? ...Gravitons/gravity waves travel at, c?


That’s a bit of a tricky question, because it’s math-heavy. The best way to describe it is to think of the event horizon as the black hole, and forget that there is even an interior. The black hole can be fully described by the conditions at the event horizon after all, and everything else is cut off from the surrounding universe completely. In that sense there is no propagation from the interior at all, which is good because if information could escape then theories describing black holes would be broken.

Instead the black hole has mass, charge, and momentum (three kinds of momentum actually, but that’s not important). Whatever is going on beyond the event horizon, whatever that might be, has no effect that anyone can detect. Matter is accreted “onto” the event horizon which then expands in proportion to the mass of the volume of the hole. Maybe it’s destroyed beyond that point, or maybe it goes to another universe, but we can never know. The event horizon can also shrink if the surroundings are sufficiently cold (really really really cold) and the horizon is sufficiently hot.

Still, all of this is surface phenomena, like dropping a bowling ball into a tub of water. The water only “knows” about the surface of the ball, which which gets properties from the whole ball without exposing the center. A bowling ball in water creates waves, but the interior isn’t interacting with the water anymore than the black hole interior interacts with space (assuming an idealized perfectly rigid bowling ball). In the same way gravity waves or fractions would be a function of how the space just beyond the event horizon is warped.

Does that help?


I have some naive questions too. This is basically just me rephrasing the question I understood wallace_f to be asking:

- The event horizon is a two-dimensional sphere and, being two-dimensional, has zero mass and cannot exert any gravitational force.

- The black hole within the horizon is a three-dimensional massy object and can and does exert a lot of gravitational force.

- Assume at equilibrium our black hole is somehow exerting gravitational forces on its surroundings which are what you would predict if you accurately knew the black hole's actual mass.

- Assume the black hole moves, e.g. because of inertia.

- Now it should be exerting more force than previously on one half of the universe (the half it moved toward), and less force on the other half.

- Say it moved toward you. After a speed-of-light delay, you should actually perceive more force on yourself towards the black hole. But this can't be because a messenger particle was transmitted from the black hole to you. How can it be?

Assuming this shows that black holes cannot move seems unsatisfactory, given the recession of galaxies from one another, observations believed to show black holes colliding, etc. Where are my mistakes?

Followup: one black hole collides with another black hole of roughly ten times its size. Is it necessarily the case that the center of mass of the new, combined black hole ends up at the point that was the center of mass of the small-hole/big-hole system just as the small hole crossed the big hole's event horizon?


> - The event horizon is a two-dimensional sphere and, being two-dimensional, has zero mass and cannot exert any gravitational force.

Stop right there. A two dimensional surface can have mass if it has infinite density. And infinite density makes as much sense as any other sort of singularity...


Where's the singularity that occurs if we assume it's just a region of space with nothing in it? I didn't call the black hole a zero-dimensional point.


We don’t know, and may have no way of knowing. There are conjectures that the event horizon is it, that inside the event horizon is a quantum fuzz ball, or strings, or 1D points, or a whole universe. We don’t know, and may well never know. What we do know is that it seems a 2D horizon can encode the information required to describe a 3D volume, and that goes for event horizons, as well certain classes of cosmic horizons in some models. This weirdness is the core of the holographic principle conjecture.


Ok, I’ll do my best here. I’m going to pass the first question because there’s already a discussion about it below.

I have some naive questions too. This is basically just me rephrasing the question I understood wallace_f to be asking: ... - The black hole within the horizon is a three-dimensional massy object and can and does exert a lot of gravitational force.

The black hole includes the event horizon, which marks the point at which we stop knowing anything or have theories to predict anything. We really have no idea what’s beyond the event horizon, and almost anything you can imagine has been conjectured as being there, from firewalls to elder gods. We’re talking about a region which can’t be properly described by he theories we have, where the manifold ceases to well behaved. Everything beyond the event horizon is causally disjoint with the rest of the universe, and may as well not exist for anything that isn’t falling past the event horizon.

- Assume at equilibrium our black hole is somehow exerting gravitational forces on its surroundings which are what you would predict if you accurately knew the black hole's actual mass.

Right, mass is one of the “hairs” a black hole has along with charge and momentum.

- Assume the black hole moves, e.g. because of inertia. - Now it should be exerting more force than previously on one half of the universe (the half it moved toward), and less force on the other half.

It sort of does, this is the basis of frame dragging when the hole is spinning. The hole warps spacetime around it, dragging reference frames in the direction of its motion.

- Say it moved toward you. After a speed-of-light delay, you should actually perceive more force on yourself towards the black hole. But this can't be because a messenger particle was transmitted from the black hole to you. How can it be? Assuming this shows that black holes cannot move seems unsatisfactory, given the recession of galaxies from one another, observations believed to show black holes colliding, etc. Where are my mistakes?

The theory of gravity we actually have doesn’t involve bosons, it’s a geometric theory describing a continuous manifold. How that squares with theories containing gravitons is well above my pay grade, sorry. The classical theory says that the warping of spacetime is continuous, and so the hole moves like something being dragged through water, including a wake and bow wave. Since the hole can’t move at c, being massive, there is always an acceptable delay for the light-speed propagation of disturbsnces in spacetime to reach you first.

Followup: one black hole collides with another black hole of roughly ten times its size. Is it necessarily the case that the center of mass of the new, combined black hole ends up at the point that was the center of mass of the small-hole/big-hole system just as the small hole crossed the big hole's event horizon?

They end up merging like two legs of a pair of pants meeting at the crotch, with the new center of mass at the barycenter of the previous orbiting pair. I loved these questions by the way, I can tell you put some real thought into them.


Thanks for writing that out, very interesting.

I find this concept of a black hole's surface having the contents inscribed on it to be really difficult for me to imagine; but I can take people's word for it, and it does explain away the paradox.

This reminds me that from some vantage points the universe seems so arbitrary sometimes. This is probably just my human intuition, but personally it just appears to me that nature is not really always elegant, but rather has these work-arounds and different layers to it to keep it working. Not unlike my terrible code.


Tangential, and possibly revealing ignorance here. I definitely don't get why clouds of dark matter surrounding galaxies don't fall into the black hole at the center. Dark matter is there to explain why the outer stars of a galaxy rotate faster than expected, but why is dark matter not distributed roughly in the same density distribution as visible matter?


It is presumed that the dark matter also rotates about the galactic center. It can’t fall in, any more than the stars in the galaxy can, because it collectively can’t dump its angular momentum. However, there is controversy whether dark matter can dissipate momentum that is parallel to the rotational axis either by self interaction or by interaction with ordinary matter. If it can, then the distribution of dark matter could resemble a disk, as opposed to a spherical distribution.

Lisa Randall has written a provocative book about this which will answer your question in depth: Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe.


As others have pointed out, if they’re in a stable orbit they won’t fall in (at least not for a very long time), because unlike luminous matter it won’t experience any force other than gravity. If you imagine a dust cloud of luminous matter around a black hole, it will tend to experience frictional heating the closer it gets, there is the chance of a collision or radioactive decay, and other forces acting to draw it in or send it far away. Dark matter won’t do that, it just couples to gravity. Our usual intuition about how a halo of matter behaves has a lot to do with interactions other than gravity. Clumping for example, aggregation and accretion pretty much work because of interactions other than gravity, until a body becomes massive enough.


They do. But there's nothing special about the gravity of a black hole versus the gravity of "ordinary matter". The vast majority of the matter in a galaxy is in orbit and won't get anywhere near close enough to the central black hole to fall into it. This is true of stars, planets, gas, and dust just as it's true of dark matter.


TFA doesn't mention recent evidence that primordial black holes are rare. It also doesn't address issues around condensation of dark matter. That is, two dark-matter objects can't collide, because they'll just pass through each other.


They can still get rid of momentum in N-body interactions via gravitational waves (N>=2) or by transferring to it to another body (N>2). The question is whether those mechanisms are significant enough to lead to compact bodies.


Thanks.

Is it generally accepted that the large-scale structure of the universe (presumably including dark matter) reflects quantum-scale structure before inflation?


Dark matter objects interact with regular matter only via gravity. But the article seems to suggest that it might interact with itself by some other force, which may allow for "collision":

"Dark matter seems to interact with normal matter only through gravity, but, based on the way known particles interact, theorists think it’s possible that dark matter might also interact with itself. "


They say, in a whisper, "Don't get a degree in particle physics! It's oversaturated, and the field can't even justify building another particle accelerator because it doesn't know what to look for!"


If you did a PhD (Emphasis on PhD because an undergraduate degree isn't close to enough) on particle physics solely with career prospects in mind then you probably have bigger issues


Exactly what I was getting at!




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