Scopes: Refractors: Orion ST80 (mods for white light solar), SV ED80 f7 Newtonians: Z12 f5 Catadioptrics: VMC110L, Intes MK66 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binocular Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker III, Vixen Porta II, Celestron CG5, Orion Sirius EQG
hahahahahahahaha
nFA,
I read about the radiation, matter and then (and current) dark helmet oops sorry, I meant dark energy,
dominated eras in the scale factor wiki:
https://en.wikipedia.org/wiki/Scale_factor_(cosmology)
and the wheels start turning up stairs, and presto, a thought occurs to me.
why is there no dark matter dominated era?
any ideas ?
Charlie
It is easy. In the periods when different forms of mass and energy, what is the meaning of dominant? Itâ€™s that one form dominates the contribution to the equation of state, i.e. the pressure as a function of density. Because DM doesnâ€™t interact with other forms, its pressure is zero or nearly so .
Scopes: Refractors: Orion ST80 (mods for white light solar), SV ED80 f7 Newtonians: Z12 f5 Catadioptrics: VMC110L, Intes MK66 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binocular Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker III, Vixen Porta II, Celestron CG5, Orion Sirius EQG
Here's a little more on how cold dark matter has zero pressure (and viscosity) and so never dominates the pressure of the universe even though gravitationally it dominates normal matter. The example is the Bullet Cluster:
https://www.forbes.com/sites/startsw.../#47fd31611738
Two galaxy clusters collide. Because cold dark matter exerts no fluid pressure force on the gas, the DM and galaxies pass through while fluid pressure and viscosity cause the gas to stick and stay behind.
The equation of state of dark matter (pressure as a function of density) is, as far as we know, p = 0. This equation of state has a long history in cosmological models. In simple FLRW models the equations of state are listed here:
https://en.wikipedia.org/wiki/Equati...te_(cosmology)
Scopes: Refractors: Orion ST80 (mods for white light solar), SV ED80 f7 Newtonians: Z12 f5 Catadioptrics: VMC110L, Intes MK66 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binocular Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker III, Vixen Porta II, Celestron CG5, Orion Sirius EQG
thank you very much nFA.
all very interesting.
hey nFA,
if you feel like it, would you please say more about this?
this is what in particular, I am thinking about. the 46 / 13 = ~3.5 value is
very simple and easy to understand.
the 1000 scale factor, I wonder a couple things.
if the current value of the scale factor is 1,
does that mean that the scale factor at the time the CMB was emitted,
was .001?
also, would you say more about the radiation temperature?
and the direct relationship between it decreasing from 3000K to 3K,
and the 1000 fold scale factor?
Yes, exactly.
First a simple hand waving argument to show that for relativistic matter photons T ~ 1/a while for cold matter T ~ 1/(a^2).also, would you say more about the radiation temperature?
and the direct relationship between it decreasing from 3000K to 3K,
and the 1000 fold scale factor?
For a relativistic particle E = pc where p is the momentum so the dependence of the energy (and therefore temperature) of a photon is the same as the dependence of p on a.
For a non relativistic particle (normal matter) E = p^2/(2m) so the dependence of the energy (and therefore temperature) of an atom or other material particle is the same as the dependence of p^2 on a.
From the form of the FLRW metric we can guess that p ~ 1/a by dimensional analysis. (Momentum ~ Planck's constant / wave length.)
So for a photon T ~ 1/a while for non relativistic particles T ~ (1/a)^2.
So.... that's pretty hand waving. But if you want rigor and detail it gets ugly awfully fast!
Here is a rigorous treatment for photons that deals with the thermodynamics of radiation as a prerequisite. It's the way I was taught in grad school.
https://www.cv.nrao.edu/course/astr534/CMB.html
Here is an even more beautiful and rigorous treatment that looks at solving the geodesic equations of motion in GR:
https://arxiv.org/pdf/physics/0603087.pdf
But you need to have comfort in the use of geodesic equations of motion and the Christoffel symbols of the FLRW metric.
Cheers.
Scopes: Refractors: Orion ST80 (mods for white light solar), SV ED80 f7 Newtonians: Z12 f5 Catadioptrics: VMC110L, Intes MK66 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binocular Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker III, Vixen Porta II, Celestron CG5, Orion Sirius EQG
a is wavelength, right?
edit: yes I see it in your post now.
thank you very much nFA!!
Last edited by chas53; 04-17-2019 at 04:12 AM.
Scopes: Refractors: Orion ST80 (mods for white light solar), SV ED80 f7 Newtonians: Z12 f5 Catadioptrics: VMC110L, Intes MK66 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binocular Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker III, Vixen Porta II, Celestron CG5, Orion Sirius EQG
helicon64 (04-17-2019)
ah, ok.
excellent.
thank you very much nFA!!