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Old November 30th 18, 11:16 PM posted to sci.astro.research
Nicolaas Vroom
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Posts: 216
Default dark matter hypothesis

On Wednesday, 28 November 2018 21:49:18 UTC+1, Richard D. Saam wrote:
On 11/27/18 1:03 PM, Phillip Helbig (undress to reply) wrote:

Also, big-bang nucleosynthesis tells us what fraction of the universe is
in baryons; there is no way that stars, being baryonic, could make up a
significant fraction of dark matter.


The Big-bang nucleosynthesis hypothesis does not warrant
such an absolute telling baryon fraction statement
in terms of on going BBN mechanistic derivation efforts
https://arxiv.org/abs/1810.05976v2
RDS


This interesting (up to date) article mentions the word baryonic,
however nothing about darkmatter and baryon fraction.

[Moderator's note: Since we have a pretty good idea of the total
density, the difference between that and the baryonic density is the
dark-matter density, more or less by definition. -P.H.]

At the beginning of the article we reed: "Nevertheless, it is physics
that needs to be considered in any calculation of BBN."
I agree if you want to understand the early evolution of the universe
it is physics.
At the end we read: "The revised abundances exacerbate the deviation
of BBN etc perhaps suggesting a crucial greater need for new physics
and/or astrophysical explanations."
My interpretation is that the birth of non-baryonic matter is not part
BBN and started later.

[Moderator's note: We don't know what dark matter is, but as far as I
know there is no plausible scenario where it forms after BBN. -P.H.]

Secondly any explanation requires a definition what darkmatter
(i.e. non-baryonic matter) physical is.

[Moderator's note: He whatever is not baryonic. -P.H.]

The title of the article https://arxiv.org/abs/astro-ph/0501171 is:
"Detection of the Baryon Acoustic Peak in the Large-Scale
Correlation Function of SDSS Luminous Red Galaxies"
Baryon fraction = Omegab/Omagam is dicussed at page 2.
Here we read:
"A simple way to understand this is to consider that from an initial
point perturbation common to the dark matter and the baryons,
the dark matter perturbation grows in place while the baryonic
perturbation is carried outward in an expanding spherical wave"
IMO this is not simple.

[Moderator's note: While the process is relatively simple compared to
some other things, what is meant is that this is an easy-to-understand
rough sketch, not that the entire process is extremely simple. -P.H.]

Nicolaas Vroom