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Old July 13th 18, 12:49 AM posted to sci.astro.research
Steve Willner
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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)


The above is a useful summary of the article. The actual paper is at
http://www.nature.com/articles/s41586-018-0204-1
but behind a paywall. There is a free preprint at
https://arxiv.org/abs/1806.08395

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.


That looks like a fair summary. The evidence for this census is
diverse and has been discussed here and elsewhere. I haven't read
the Wikipedia article and can't vouch for it, but it describes the
evidence.

Next we read in the nature article: "However, observations of the nearby
Universe suggest that up to 40% of this ordinary matter


"Ordinary matter" refers to the 4.9% (which I'll round off to 5%).

---which is made up primarily of particles known as baryons---is
missing"


See Table 1 of the paper. There are large uncertainties, especially
in the hot gas components. The "primarily" is because electrons
count in this portion even though they aren't baryons, but they
contribute a trivial amount of mass.

What we observe/measure are 1) galaxy rotation curves and


and many more things than that, all of which add up to about 3% of
the total density, not 5% as they should.

However accoringly to Wikipedia there is also a Missing baryon problem.
See: https://en.wikipedia.org/wiki/Missing_baryon_problem.


Which is what is described above: 3% 5%.

there are two problems: 1) A dark matter problem and 2) a Missing baryon
problem.


I'm not sure what you mean by "problems," but missing baryons have
nothing to do with non-baryonic matter.

more and more ordinary matter becomes visible because technology
improves.


Indeed. The observations reported were from a heroic effort using a
premier space observatory.

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


What they have found is oxygen, which they extrapolate to give a mass
of hydrogen associated with the oxygen. These elements are, of
course, baryonic, and they add something like 2% to the 3% already
known, potentially making up the 5% that baryons constitute.

There are large uncertainties and possible systematic errors in the
observations, and there have been other papers along these same
lines. Many have been discussed in this newsgroup. The upshot is
that the missing baryons are almost certainly hot gas, but the
distribution of this hot gas is far from clear.

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


I am not sure I understand the question. There are two forms of
matter in the universe. Baryonic matter makes up 5% of the total
density, but only 60% of this (3% of the total) has been accounted
for. It would be nice to know what the rest is, and this paper
provides evidence towards an answer.

Non-baryonic matter makes up 27% of the total energy density, and we
have little evidence of what it is. Some hypotheses are ruled out by
existing observations, but others are still possible. Non-baryonic
matter may be a mix of different things, and some or all may be
something we haven't thought of yet. This has nothing to do with
accounting for the baryons.

Dark energy, the remaining 68%, is something different still. There
is little evidence for what it is, but all the evidence I'm aware of
is consistent with its being a cosmological constant. I personally
have no problem with that. The cosmological constant has to have
_some_ value, and there's no reason that value must be zero.

Maybe Fig 4 at page 408 shows the answer.


You mean Fig 4 of the article? That shows the new baryon census
based on the results of the paper. It is far from the final word but
is plausible.

They mention the word Local Universe which makes everything much
more complex.


Why more complex? Measurements such as the one reported can only
address the local universe. Presumably the census changes over time,
for example as gas is converted to stars, but the baryon fraction
should not change.

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