
What Gravitational Waves Can Say About Dark Matter - scottie_m
https://www.symmetrymagazine.org/article/what-gravitational-waves-can-say-about-dark-matter
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andreareina
_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.

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nuccy
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](https://en.wikipedia.org/wiki/Dark_matter_halo)

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

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

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nuccy
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...](https://en.wikipedia.org/wiki/Navarro%E2%80%93Frenk%E2%80%93White_profile)

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

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FakeComments
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.

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scottie_m
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.

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wallace_f
>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?

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scottie_m
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?

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thaumasiotes
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?

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cperciva
> \- 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...

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thaumasiotes
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.

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scottie_m
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.

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wwarner
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?

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wrycoder
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.

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mirimir
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.

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the8472
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.

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mirimir
Thanks.

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

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craftinator
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!"

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mhh__
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

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craftinator
Exactly what I was getting at!

