Good current paper on mass / luminosity and rotation curves
in [Physics]
|
From: John Park on 10 Jul 2010 23:03 eric gisse (jowr.pi.nospam(a)gmail.com) writes: > John Park wrote: > >> eric gisse (jowr.pi.nospam(a)gmail.com) writes: >>> John Park wrote: >>> [...] >>> >>>> I was using electrons as my "particles" and assuming a velocity based on >>>> the temperature. >>> >>> The problem is the 'temperature' can be whatever he wants it to be. All >>> that can really be done is produce bounds, and point out there is no >>> physical basis for what he's assuming. >>> >>> Well I guess you could argue that the upper bound for temperature is kT < >>> 1/2 m v_esc^2. Probably make further refinements on that based upon >>> backscatter, as the energy lost in a collision will be ferocious. >> Incidentally there's a rule of thumb (due to Jeans?) that for the component of an atmosphere to persist indefinitely its mean thermal velocity must be less than 1/6 the escape velocity--this may be the tighest a priori bound we can find. >> I realise I don't understand enough about bremsstrahlung--the resulting >> energy spectrum for instance. > > It is proportional to the acceleration felt by the protons/electrons upon > collision, but I have no idea how to determine from my current knowledge of > particle physics. > > I do know the acceleration is found to be something absurd like 10^10 gee, > though. I tried the basic formula from the undergrad EM texts--not sure I did it right--and got a rather feeble total. It was probably enough to pull out the thermal energy of the plasma in less than a million years, but I was wondering about observable effects. The total rate of emission was unimpressive (equivalent to an effective temperature of about 3.5 K)--but it would make a difference whether that was a pseudo black-body spectrum or occasional flashes of much higher energy. > >> >> But I just found that with the assumed plasma temperature of 25 >> megakelvin, the protons (not even the electrons) have a a thermal speed >> almost three times that of the sun's rotation about the galactic centre or >> about twice the local escape velocity... >> >> I think an implicit part of the original argument was that at very high >> energies the particle velocities are so great that most collisions won't >> change them appreciably--hence no "friction" in the plasma. > > But you exchange one plead with two more by doing that. > > 1) What made the hydrogen that way, and why didn't the rest of the visible > matter suffer that fate? > 2) What's keeping it that way? > >>> >>> All the change between protons and electrons will do is change energies >>> by a factor of 1800. Given the billions of years involved, that's not >>> going to change much. >>> >> Not sure what you're saying here. Doesn't equipartition apply? > > kT = 1/2 mv^2 > > Two ensembles with the same thermodynamic temperature will have kinetic > energies of the constituent parts that's proportional to the mass. 1.5 kT to be pedantic--and yes that is the kinetic energy of each ensemble. It's the velocities or momenta that are influenced by the relative masses. Surely. --John Park>
From: John Park on 10 Jul 2010 23:29 John Park (af250(a)FreeNet.Carleton.CA) writes: > eric gisse (jowr.pi.nospam(a)gmail.com) writes: >> John Park wrote: >> >>> eric gisse (jowr.pi.nospam(a)gmail.com) writes: >>>> John Park wrote: >>>> [...] >>>> >>>>> I was using electrons as my "particles" and assuming a velocity based on >>>>> the temperature. >>>> >>>> The problem is the 'temperature' can be whatever he wants it to be. All >>>> that can really be done is produce bounds, and point out there is no >>>> physical basis for what he's assuming. >>>> >>>> Well I guess you could argue that the upper bound for temperature is kT < >>>> 1/2 m v_esc^2. Probably make further refinements on that based upon >>>> backscatter, as the energy lost in a collision will be ferocious. >>> > Incidentally there's a rule of thumb (due to Jeans?) that for the component > of an atmosphere to persist indefinitely its mean thermal velocity must > be less than 1/6 the escape velocity--this may be the tighest a priori > bound we can find. > This of course screws up my collision-frequency estimate. --John Park
From: eric gisse on 11 Jul 2010 00:47 John Park wrote: > eric gisse (jowr.pi.nospam(a)gmail.com) writes: >> John Park wrote: >> >>> eric gisse (jowr.pi.nospam(a)gmail.com) writes: >>>> John Park wrote: >>>> [...] >>>> >>>>> I was using electrons as my "particles" and assuming a velocity based >>>>> on the temperature. >>>> >>>> The problem is the 'temperature' can be whatever he wants it to be. All >>>> that can really be done is produce bounds, and point out there is no >>>> physical basis for what he's assuming. >>>> >>>> Well I guess you could argue that the upper bound for temperature is kT >>>> < 1/2 m v_esc^2. Probably make further refinements on that based upon >>>> backscatter, as the energy lost in a collision will be ferocious. >>> > > Incidentally there's a rule of thumb (due to Jeans?) that for the > component of an atmosphere to persist indefinitely its mean thermal > velocity must be less than 1/6 the escape velocity--this may be the > tighest a priori bound we can find. Yeah, what I wrote was the absolute upper bound. If you want the halo of dark matter to stay, you have to either have the rms speed be significantly below escape velocity, demand non-Maxwellian distributions, or accept the fact the halo will evaporate. > >>> I realise I don't understand enough about bremsstrahlung--the resulting >>> energy spectrum for instance. >> >> It is proportional to the acceleration felt by the protons/electrons upon >> collision, but I have no idea how to determine from my current knowledge >> of particle physics. >> >> I do know the acceleration is found to be something absurd like 10^10 >> gee, though. > > I tried the basic formula from the undergrad EM texts--not sure I did it > right--and got a rather feeble total. It was probably enough to pull out > the thermal energy of the plasma in less than a million years, but I was > wondering about observable effects. The problem is the plasma has to persist for _billions_ of years. Galaxies weren't created yesterday. Feeble integrated over 'a long time' causes problems for such models. > The total rate of emission was > unimpressive (equivalent to an effective temperature of about 3.5 K) An interesting order of magnitude. Consider what the _primary_ irritant for astronomy in the short wavelengths is. Look at the all-sky maps from WMAP and now Planck. Hydrogen is quite visible there, and anything above the CMB temperature is going to be noticed. Furthermore, anything below the CMB temperature will be brought _up_ to the CMB temperature. This is how we have observations of the CMB temperature at high redshifts. > --but > it would make a difference whether that was a pseudo black-body spectrum > or occasional flashes of much higher energy. You only get a blackbody spectrum for thermal emission, and occasional flashes from collisions doesn't count. I'm not sure what the spectrum would be - that would require serious thinking, which this problem does not merit. The debate is not, contrary to what David thinks, whether the model could be correct but rather _wrong_ it is. >> >>> >> But I just found that with the assumed plasma temperature of 25 >>> megakelvin, the protons (not even the electrons) have a a thermal speed >>> almost three times that of the sun's rotation about the galactic centre >>> or about twice the local escape velocity... >>> >>> I think an implicit part of the original argument was that at very high >>> energies the particle velocities are so great that most collisions won't >>> change them appreciably--hence no "friction" in the plasma. >> >> But you exchange one plead with two more by doing that. >> >> 1) What made the hydrogen that way, and why didn't the rest of the >> visible matter suffer that fate? >> 2) What's keeping it that way? >> >>>> >>>> All the change between protons and electrons will do is change energies >>>> by a factor of 1800. Given the billions of years involved, that's not >>>> going to change much. >>>> >>> Not sure what you're saying here. Doesn't equipartition apply? >> >> kT = 1/2 mv^2 >> >> Two ensembles with the same thermodynamic temperature will have kinetic >> energies of the constituent parts that's proportional to the mass. > > 1.5 kT to be pedantic--and yes that is the kinetic energy of each > ensemble. It's the velocities or momenta that are influenced by the > relative masses. Surely. Forgot the 3/2 - one 1/2 for each translational degree of freedom. Thermodynamics has never been a serious interest and it is starting to have been awhile since I've worked with it seriously. > > --John Park>
From: nuny on 11 Jul 2010 07:48 On Jul 2, 10:41 pm, Yousuf Khan <bbb...(a)spammenot.yahoo.com> wrote: > On 7/3/2010 6:49 AM, eric gisse wrote: > > > Turbulence typically refers to self-interactions within a fluid. I'm > > pointing out the specific exception now rather than having to point out > > later and then deal with 'but you said there's no turbulence!!!!' response. > > > Its' like people aren't listening when I say dark matter is modeled as a > > perfect fluid. What the hell do folks think that implies? > > Then it's not a perfect fluid if there are self-interactions. You can > say it's *close* enough to a perfect fluid at the galactic scale. At > bigger scales, it is not. Second of all, you do realize that all of > these "if...then...but if...else..." type behaviours of Dark Matter is > exactly what Dark Fluid is supposed to address. > > Yousuf Khan Is Dark Fluid a superfluid, with quantized vortex lines and rings around which galaxies and clusters form? Could such vortex lines map to the observed really large scale foamy structure of the universe? Mark L. Fergerson
From: John Park on 11 Jul 2010 10:37
eric gisse (jowr.pi.nospam(a)gmail.com) writes: > John Park wrote: > >> tighest a priori bound we can find. > > Yeah, what I wrote was the absolute upper bound. If you want the halo of > dark matter to stay, you have to either have the rms speed be significantly > below escape velocity, demand non-Maxwellian distributions, or accept the > fact the halo will evaporate. [see reply at sci.astro] --John Park |