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Dark matter is:



 
 
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  #31  
Old November 8th 17, 08:21 PM posted to sci.astro.research
Steve Willner
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Posts: 1,102
Default Dark matter is:

In article ,
jacobnavia writes:
If the sea of galaxies extends to infinity (or to huge distances) the
farther you look, the more galaxies you will observe for a given solid
angle. At great distances you will see a wall of galaxies that fills
completely the view. The (very red-shifted) light from those galaxies is
the CMB.


As JT pointed out, this is closely related to Olbers' Paradox; the
resolution of which is well known. The point is that the "light from
these galaxies" cannot fit the spectral energy distribution (SED) of
the CMB. If you don't know what an SED is, have a look at
http://coolwiki.ipac.caltech.edu/index.php/SED_plots
which defines it. (The article is over-simplified in places, and I
don't agree with the "Units Matter" section, but the definition of
SED and basic explanation of it are accurate.)

"Sea of galaxies" was a possible model for the CMB until COBE flew,
but that model is inconsistent with COBE and later data.

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  #32  
Old November 11th 17, 09:48 PM posted to sci.astro.research
Nicolaas Vroom
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Posts: 210
Default Dark matter is:

On Tuesday, 7 November 2017 00:19:18 UTC+1, Phillip Helbig wrote:

You don't need a simulation. There is acceleration if

Omega/2 - lambda 0.


Which are the present values for Omega and Lambda?

When you visit
https://lambda.gsfc.nasa.gov/toolbox/tb_camb_form.cfm
they use Omega(b), Omega(c), Omega(nu) and Omega(k)
Lambda is not mentioned.
This makes it tricky to decide if the above formule
is a good yardstick.

Nicolaas Vroom

[[Mod. note -- Ned Wright's cosmology tutorial is excellent:
http://www.astro.ucla.edu/~wright/cosmolog.htm
-- jt]]
  #33  
Old November 13th 17, 01:50 AM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
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Posts: 204
Default Dark matter is:

In article ,
Nicolaas Vroom writes:
On Tuesday, 7 November 2017 00:19:18 UTC+1, Phillip Helbig wrote:

You don't need a simulation. There is acceleration if

Omega/2 - lambda 0.


Which are the present values for Omega and Lambda?


Omega = 0.3, lambda = 0.7.

When you visit
https://lambda.gsfc.nasa.gov/toolbox/tb_camb_form.cfm
they use Omega(b), Omega(c), Omega(nu) and Omega(k)
Lambda is not mentioned.


The dynamics of the universe depend on the total value of Omega, dark
matter, baryons, etc. If you're interested in the contents, then it
makes sense to have different symbols, otherwise you have to add them
all up.

In one notation (unfortunately, there are several)

Omega_M + Omega_Lambda + Omega_K = 1

or

Omega + lambda + Omega_L = 1

Hence Omega + lambda = 1 if the universe is flat.

Yes, different notation schemes can be confusing, on the other hand
understanding them can help one learn other things.
  #34  
Old November 13th 17, 07:00 AM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
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Posts: 204
Default Dark matter is:

In article , "Phillip Helbig (undress to
reply)" writes:

In one notation (unfortunately, there are several)

Omega_M + Omega_Lambda + Omega_K = 1

or

Omega + lambda + Omega_L = 1


Above line should be

Omega + lambda + Omega_K = 1

Hence Omega + lambda = 1 if the universe is flat.

Yes, different notation schemes can be confusing, on the other hand
understanding them can help one learn other things.


In the scheme above,

Omega_K := 1 - Omega - lambda

or

Omega_K := 1 - Omega_M - Omega_lambda

One can also find

Omega_K := Omega_M + Omega_lambda - 1

or

Omega_K := Omega + lambda - 1

i.e. the sign of Omega_K is opposite.

I prefer the latter, since then Omega_K (or K, in some notation) has the
same sign as k, the curvature constant in the Friedmann equation.
 




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