Gravitational Waves research and Dark Matter

[Gerinski]

A lot is being written about the recent aLIGO observations and in general the implications of GW research for our scientific understanding of the universe.
We read about mostly 2 grand areas where GW research can provide novel knowledge. The astronomical / astrophysical area (i.e. mapping gravitational events in the "modern" universe such as black holes and neutron star collisions etc) and the cosmological area with the research of primeval GWs imprinted in the CMBR, which would help improving our understanding of early cosmology, inflation and fundamental high energy physics such as the Grand Unification energy scales etc.

What about Dark Matter? Since it only interacts gravitationally with ordinary matter, one might think that any progress in the understanding and observation of gravitational effects might eventually help in observing and understanding DM and any events caused by it.

However I presume that DM distribution seems to be too dilute to produce any GW of observable magnitude?

Can Gravitational Waves research help in any way in the understanding of Dark Matter?

TX

 

[Dr. Courtney]

I think you need some kind of event in the dark matter where large dark matter objects are experiencing large accelerations.

I'm no expert, but my understanding is that masses need to accelerate to produce gravitational waves.

 

[Dr.AbeNikIanEdL]

Would gravity observations even in principle provide insights into dark matter? I always had the impression that from the gravity side it is pretty clear: if you want to explain the observed facts, you have to assume a certain amount of matter. Just if you want to match that amount of matter to other observations, you get a discrepancy. But as far as I am aware, gravity doesn't distinguish dark matter and usual matter, so as long a gravity experiments confirm that general relativity is the correct theory of gravity, we would just become more sure that dark matter actually exists.

 

[Gerinski]

Thanks to both, I appreciate and sort of agree with both of you.
Indeed gravitational waves are only emitted when the massive body gets accelerated.

But let's imagine, a huge black hole is swallowing its surrounding galaxy. This galaxy consists of the ordinary matter we can infer by its EM radiation and of its Dark Matter contents, probably forming some halo in its outer edges.

Observing the gravitational behaviour between the central black hole and the infalling matter (ordinary and dark), could we not learn about the Dark Matter halo which we can not observe through EM observations, but might cause that the gravitational behaviour of the whole system black hole - ordinary matter galaxy - dark matter galactic halo shows some signs containing information about the dark matter component?

 

[PAllen]

I think you need some kind of event in the dark matter where large dark matter objects are experiencing large accelerations.

I'm no expert, but my understanding is that masses need to accelerate to produce gravitational waves.

They actually need changing acceleration. A uniformly accelerating body produces no GW. Unlike the EM case, there is no controversy about this - it falls right out of the quadrupole nature of GW. As I'm sure you know, for the EM case of uniform acceleration (including no direction change), Nobel Laureates have disagreed. Remarkably, Feynman believed the answer was no (for EM) while the modern consensus is yes.

 

[PAllen]

Would gravity observations even in principle provide insights into dark matter? I always had the impression that from the gravity side it is pretty clear: if you want to explain the observed facts, you have to assume a certain amount of matter. Just if you want to match that amount of matter to other observations, you get a discrepancy. But as far as I am aware, gravity doesn't distinguish dark matter and usual matter, so as long a gravity experiments confirm that general relativity is the correct theory of gravity, we would just become more sure that dark matter actually exists.

GW undergo gravitational lensing, and thus could detect dark matter that way. However, that is pointless, as it is so much easier to observe lensing for light, and gravitational lensing has already been used to probe distributions of dark matter. Such lensing results are extremely direct and conclusive (including finding dark matter COM shifted from stellar COM). The only unknown about dark matter is not its existence or distribution (at least in cases where lensing can be used) but what it is made of.