What Gravitational Waves Can Say About Dark Matter

[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.

[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

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

[jessriedel]

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.

[casual_slacker]

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

[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.

[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?

[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.

[scottie_m]

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.

[InclinedPlane]

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.

[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.

[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.

[canjobear]

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. "

[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!"

[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