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Oh sorry I missed the "switch one reservoir to dark matter". In that case, if the DM and matter can only interact gravitationally (and DM-DM interactions are a

Oh sorry I missed the "switch one reservoir to dark matter". In that case, if the DM and matter can only interact gravitationally (and DM-DM interactions are also gravitational), then you have a dance wherein the collisional normal matter sheds momentum via radiation, letting the normal matter fall into the DM eventually. The momentum exchanged between the DM and the normal matter will be pretty small. (This is similar to what people pursuing the cosmic web hypothesis try to model, essentially, and is what's conjectured to be happening during the dark ages). ETA: the key thing here is that the collisionless cold dark matter will stay cold, while the normal matter will get much hotter (and light up normal matter further away still). Isn't this the nub of the EDGES result?

raattgift 9y ago
I suppose you could attack your question by considering a DM galaxy and a baryonic galaxy binary. On any given spacelike slice you can probably represent the two as a barbell with the barycentre on the infinitesimally thin bar, although there are lots of 3+1 formalism gotchas that would have to be dealt with. If you arrange the matter galaxy so that its internal structure is sufficiently stable then the whole binary system would lose momentum-energy to gravitational radiation. It won't be much, and that's a big "if" (and the "if" also works against trying to see the matter galaxy cool, especially as the energy loss would tend to contract the "bar").
- raattgift 9y ago
  You might enjoy this crazy 1998 paper, which I just found, which adds a bit of heft to the the "not much" guess above. It seeks to examine gravitatonal waves shed by a 2-galaxy cluster. "pessimistic point of view justified by the combination of great numerical difficulties (three-dimensional calculations) together with very disappointing small values expected for the gravitational-wave luminosity, the amplitude, etc" "In brief, our simulated cluster produces changes in the relative distance of the order of 10-22—detectable with current technology—in a short period of 4 days." (LIGO 2015 was about the same relative length change (h), but with a frequency of 35 - 250 Hz. I'm not sure how catching part of a gravitational wave of ~ 10^-17 Hz could be caught by a LIGO-like detector, since only higher derivatives of h are useful for identifying gravitational radiation). http://iopscience.iop.org/article/10.1086/311446/fulltext/985163.text.html http://iopscience.iop.org/article/10.1086/311446/fulltext/98... (but of course for cooling we're more interested in the luminosity, which is small compared to that of an ordinary galaxy, but a bit more interesting compared to the blackbody radiation of a galactic-mass diffuse cloud of atomic hydrogen, although that's only considering the normal-matter member of the binary, rather than the total energy leaving the region of spacetime the binary is in.)

raattgift 9y ago

I suppose you could attack your question by considering a DM galaxy and a baryonic galaxy binary. On any given spacelike slice you can probably represent the two as a barbell with the barycentre on the infinitesimally thin bar, although there are lots of 3+1 formalism gotchas that would have to be dealt with. If you arrange the matter galaxy so that its internal structure is sufficiently stable then the whole binary system would lose momentum-energy to gravitational radiation. It won't be much, and that's a big "if" (and the "if" also works against trying to see the matter galaxy cool, especially as the energy loss would tend to contract the "bar").

raattgift 9y ago
You might enjoy this crazy 1998 paper, which I just found, which adds a bit of heft to the the "not much" guess above. It seeks to examine gravitatonal waves shed by a 2-galaxy cluster. "pessimistic point of view justified by the combination of great numerical difficulties (three-dimensional calculations) together with very disappointing small values expected for the gravitational-wave luminosity, the amplitude, etc" "In brief, our simulated cluster produces changes in the relative distance of the order of 10-22—detectable with current technology—in a short period of 4 days." (LIGO 2015 was about the same relative length change (h), but with a frequency of 35 - 250 Hz. I'm not sure how catching part of a gravitational wave of ~ 10^-17 Hz could be caught by a LIGO-like detector, since only higher derivatives of h are useful for identifying gravitational radiation). http://iopscience.iop.org/article/10.1086/311446/fulltext/985163.text.html http://iopscience.iop.org/article/10.1086/311446/fulltext/98... (but of course for cooling we're more interested in the luminosity, which is small compared to that of an ordinary galaxy, but a bit more interesting compared to the blackbody radiation of a galactic-mass diffuse cloud of atomic hydrogen, although that's only considering the normal-matter member of the binary, rather than the total energy leaving the region of spacetime the binary is in.)