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Old July 13th 18, 12:46 AM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
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Default Missing matter found in the cosmic web

In article ,
Nicolaas Vroom writes:

The article "Missing matter found in the cosmic web" in Nature of 21
June 2018 (See https://www.nature.com/articles/d41586-018-05432-2)
Starts with the following sentence: "We live in a dark Universe: just 5%
of it consists of ordinary matter such as that found in atoms, whereas
the rest is `dark' matter and energy that cannot currently be detected
directly" The word dark is written within '' indicating doubt.


Not really doubt, but probably to indicate that it is not to be taken
too literally. First, "transparent" might be a better term. Yes, it's
dark, since it doesn't emit electromagnetic radiation, but neither does
it interact with electromagnetic radiation at all.

"Dark energy" is really a stupid term, modelled on "dark matter" (which
does make at least some kind of sense). Substitute "cosmological
constant" as there is no evidence at all against the idea, and much for
it, that "dark energy" is just the good old cosmological constant.

Accordingly to https://en.wikipedia.org/wiki/Dark_matter: "In the
standard Lambda-CDM model of cosmology, the total mass-energy of the
universe contains 4.9% ordinary matter and energy, 26.8% dark matter and
68.3% of an unknown form of energy known as dark energy. Thus, dark
matter constitutes 84.5% of total mass, while dark energy plus dark
matter constitute 95.1% of total mass-energy content."


Right.

Next we read in the nature article: "However, observations of the nearby
Universe suggest that up to 40% of this ordinary matter---which is made
up primarily of particles known as baryons---is missing" This is a
strange twist. What we observe/measure are 1) galaxy rotation curves and
2) an expanding universe. What we also observe is 3) stars and baryonic
matter throughout the universe. However the amount found as #3 is not
enough to explain #1 and #2. To solve this issue we introduced the
concepts of dark (missing) matter and dark energy.


Right so far.

And this missing
matter is supposed to be nonbaryonic.


Not all of it. Big-bang nucleosynthesis tells us rather precisely how
many baryons there are. The difference between this and what is
observed in baryons are the missing baryons. The rest of the missing
matter is the dark matter, thus most of it is non-baryonic. Often "dark
matter" is used as a synonym for "unknown non-baryonice matter".
(Neutrinos, and electrons, for that matter, are known baryonic matter,
but their contribution to the mass budget is much smaller than that of
baryons.) Until recently, the uncertainty in the total mass density was
greater than that of the mass of baryons, so, at least as far as the
numbers go, saying that all dark matter is unknown non-baryonic matter,
or vice versa, was an acceptable approximation.

However accoringly to Wikipedia there is also a Missing baryon problem.


See above.

See: https://en.wikipedia.org/wiki/Missing_baryon_problem. That means
there are two problems: 1) A dark matter problem and 2) a Missing baryon
problem.


Right.

(In reality there are more issues: CMBR and BB
nucleosynthesis)


They aren't issues, but observations, and both tell us how many baryons
there are.

What this article indicates is that there is much more baryonic matter
in the cosmic web (Universe) as original thought. To me this seems
logical because more and more ordinary matter becomes visible because
technology improves.


Right; no surprise here. (While I am sympathetic to MOND, and think
that most critics don't really understand it, one of my main objections
to "MOND philosophy" is the assumption, explicitly stated or otherwise,
that there is something strange about matter that we cannot see. This
doesn't challenge standard physics any more than the discovery of
gorillas challenged Linnaeus's binomial classification system.)

My question is why is newly found matter 'clasified' as a solution for
problem #2 (and not #1)


Because there is much too little to solve problem #1.

Different question: Why are there two problems
in the first place?


Problem 2: We don't see all the baryons (but might be seeing more now).
No surprise there. Problem 1: the difference between what is deduced
from BBN and CMB (which is more than directly observed---the difference
is the missing baryons) and what we need to explain rotation curves of
galaxies as well as cosmological observations. It is conceivable (in my
view, even likely) that something like conventional dark matter is
needed for the latter and something MOND-like for the former. Perhaps,
as Khoury (who has done some of the most interesting work in
astrophysics in the last few years) suggests, these are two sides of the
same coin.