4 ms·

I don't understand your idea. Gravitation squashes collisional matter together, heating it. The hot matter radiates, allowing it to fall deeper into dense st

I don't understand your idea. Gravitation squashes collisional matter together, heating it. The hot matter radiates, allowing it to fall deeper into dense structures. I don't see how this cools non-infallen hydrogen, rather than lighting it up.

chopin 9y ago
I'll try to elaborate: Let's say we have to reservoirs of baryonic matter, one cool and one hot. If these interact through radiation (electromagnetic forces) both reservoirs eventually get the same Boltzmann distribution and thus temperature. Now we have the same situation but we switch one reservoir to dark matter (the cooler one) and the force between the reservoirs is purely gravitational. Beside the time scale, would that be any different? In a large collection of masses, particles can exchange momentum and energy through gravitation, afair. Therefore one could expect that the two reservoirs equilibrate. I think what differs may be that gravitation is only attractive and contraction may be faster than equilibration. But is this the case if the initial temperature of the constituents is high enough, especially of the baryonic stuff?
- raattgift 9y ago
  What would source the gravitational radiation that would thermalize the two reservoirs? The metric sourced by a uniform cloud of hot gas doesn't really lend itself to periodic change (under a slicing) unless it's collapsing far from spherically symmetrically, and even then the momentum-energy carried out by electromagnetic radiation would dwarf the gravitational radiation. Also, if you let the hot and cold clouds fall into each other, then electromagnetic scattering will do the bulk of the work of thermalization (or more likely the heating of both clouds of inelastically colliding matter; cf. collisonless dark matter clouds sliding through one another).
- raattgift 9y ago
  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.)

chopin 9y ago

I'll try to elaborate: Let's say we have to reservoirs of baryonic matter, one cool and one hot. If these interact through radiation (electromagnetic forces) both reservoirs eventually get the same Boltzmann distribution and thus temperature. Now we have the same situation but we switch one reservoir to dark matter (the cooler one) and the force between the reservoirs is purely gravitational. Beside the time scale, would that be any different? In a large collection of masses, particles can exchange momentum and energy through gravitation, afair. Therefore one could expect that the two reservoirs equilibrate. I think what differs may be that gravitation is only attractive and contraction may be faster than equilibration. But is this the case if the initial temperature of the constituents is high enough, especially of the baryonic stuff?

raattgift 9y ago
What would source the gravitational radiation that would thermalize the two reservoirs? The metric sourced by a uniform cloud of hot gas doesn't really lend itself to periodic change (under a slicing) unless it's collapsing far from spherically symmetrically, and even then the momentum-energy carried out by electromagnetic radiation would dwarf the gravitational radiation. Also, if you let the hot and cold clouds fall into each other, then electromagnetic scattering will do the bulk of the work of thermalization (or more likely the heating of both clouds of inelastically colliding matter; cf. collisonless dark matter clouds sliding through one another).
raattgift 9y ago
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.)