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.

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."

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. And this missing
matter is supposed to be nonbaryonic.

However accoringly to Wikipedia there is also a Missing baryon problem.
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. (In reality there are more issues: CMBR and BB
nucleosynthesis)

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.

My question is why is newly found matter 'clasified' as a solution for
problem #2 (and not #1) Different question: Why are there two problems
in the first place?

Maybe Fig 4 at page 408 shows the answer. They mention the word Local
Universe which makes everything much more complex.

Nicolaas Vroom
http://users.pandora.be/nicvroom/

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In article <d8807611-944f-43e7-ab60-555c5df4b15a@googlegroups.com>,
Nicolaas Vroom <nicolaas.vroom@pandora.be> 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.

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In article <d8807611-944f-43e7-ab60-555c5df4b15a@googlegroups.com>,
Nicolaas Vroom <nicolaas.vroom@pandora.be> 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.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123 swillner@cfa.harvard.edu Cambridge, MA 02138 USA

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"Phillip Helbig (undress to reply)" <helbig@asclothestro.multivax.de> wrote:

"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.

[...]

Yes, but can't they simply call it "vacuum energy" as in "vacuum"
displacement or polarisation when talking about eps_0 in EM?

(I confess to be a little motivated by Carlo Ovelli "Reality Is Not
What It Seems: The Journey to Quantum Gravity")

--
ciao,
Bruce

drift wave turbulence: http://www.rzg.mpg.de/~bds/

[[Mod. note --
Calling it "vacuum energy" would be making an implicit statement
that it has something to do with vacuum energy/polarization in the
sense you're using it. I don't think we know that.

(On the other hand... calling it "cosmological constant" is also
making an implicit statement that it's trully *constant*, i.e., that
it enters into the Einstein equations in a certain way, with NO terms
involving the spacetime derivatives of the "cosmological constant".
We don't know that, either. About all we know today is its average
value over the past 10^10-or-so years. We probably won't know much
about its time variation or lack thereof for another decade.)
-- jt]]

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In article <pia9mt$1aik$3@gwdu112.gwdg.de>, "Bruce D. Scott" <bds@gateafs.rzg.mpg.de> writes:

"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.

[...]

Yes, but can't they simply call it "vacuum energy" as in "vacuum" displacement or polarisation when talking about eps_0 in EM?

As Jonathan noted, this makes an assumption about its origin.

(I confess to be a little motivated by Carlo Ovelli "Reality Is Not
What It Seems: The Journey to Quantum Gravity")

I'm reading that myself at the moment. :-)

[[Mod. note -- To clarify, the following 7 quoted lines were written
by me (Jonathan Thornburg), not Bruce D Scott. -- jt]]

(On the other hand... calling it "cosmological constant" is also
making an implicit statement that it's trully *constant*, i.e., that
it enters into the Einstein equations in a certain way, with NO terms involving the spacetime derivatives of the "cosmological constant".
We don't know that, either. About all we know today is its average
value over the past 10^10-or-so years. We probably won't know much
about its time variation or lack thereof for another decade.)

True. On the other hand, there is no evidence that it is not constant,
and people have looked for such a deviation. Obviously, one can put
only upper limits on such deviations. The traditional cosmological
constant was there long before observations made it clear that it or
something like it actually exists. As long as a constant value fits the
data, there is no reason to assume otherwise, unless someone has a
really convincing theory (which should predict variation at some level
which could, at least in principle, be confirmed). However, one should
be open to a more complicated form, not repeating the mistakes of
assuming it is zero until forced otherwise by the data, as happened 30
or so year ago.

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On 2018-07-12, Steve Willner <willner@cfa.harvard.edu> wrote:

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.

Nice summary in general, just want to comment on this... you can take
the position it should be zero unless you have a reason for it. Fitting
the data is well enough (but I've seen that go wrong many times in
plasma physics where the underlying asumption of the thing and its cause
both being totally wrong and the community taking >20 years to wake up
to it). I take the position that zero is a reasonable a priori assumption,
but that if it has a value there should be a reason for it (ie, why is
it not very large). It may be like the photon mass, so small as not to
rock the boat with a theory in which it is zero and which is successful
for anything else which is known (at least below whatever it is... 5 MeV
or so for the nonlinearity in the electron scattering cross section).

Do we have solid evidence that it is _different from zero_ and if so
what does the curvature of the universe have to be? I guess if we say
68 percent of the curvature is due to the quoted dark energy fitting
then this should be something. I think if we know enough it may be a
property of space-time rather than a species of field/particle... but
I guess this is the same thing as "cosmological constant".

(does this follow from universe accelaration as per the supernovae
observations from 20 years ago? but that's negative curvature isn't it)

--
ciao, Bruce

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In article <slrnpo07t9.vfi.bds@macbook-air-3.local>, Bruce Scott <bds@ipp.mpg.de> writes:

On 2018-07-12, Steve Willner <willner@cfa.harvard.edu> wrote:

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.

Nice summary in general, just want to comment on this... you can take
the position it should be zero unless you have a reason for it.

Actually, it is usually the reverse in science. If nature has a degree
of freedom, she uses it. If it is NOT observed, then there is a reason
for it---a conservation law, for example. In particle physics, "if it
can happen, it will" is the standard approach. If something doesn't
happen, then it has to have a reason. The burden of proof is then on
the person claiming that it is zero if this is just a theoretical, as
opposed to observational, claim.

Fitting
the data is well enough (but I've seen that go wrong many times in
plasma physics where the underlying asumption of the thing and its cause
both being totally wrong and the community taking >20 years to wake up
to it).

Not really an issue here, as we are talking 1920s cosmology. If
anything, the surprise is that despite the high quality and quantity of
current data, all of it can be fit with ideas which have been around for decades.

I take the position that zero is a reasonable a priori assumption,

No; see above.

but that if it has a value there should be a reason for it (ie, why is
it not very large).

Large compared to what? Some people complain it is way to small,
compared to the expectation from quantum field theory. Others are
surprised that the energy density in the cosmological constant is
comparable to that in ordinary matter (with, for some, the additional
puzzle that this is not always the case, but is now).

It may be like the photon mass, so small as not to
rock the boat with a theory in which it is zero and which is successful
for anything else which is known (at least below whatever it is... 5 MeV
or so for the nonlinearity in the electron scattering cross section).

I'm sure that upper limits on the photon mass are much smaller than 5
MeV.

Do we have solid evidence that it is _different from zero_

Yes. This is essentially what the Nobel Prize in physics for 2011 was
awarded for.

and if so
what does the curvature of the universe have to be?

The curvature of the universe depends on the sum of the cosmological
constant and the density parameter. Observations indicate that the
universe is close to being flat and perfect flatness is not yet ruled
out.

I guess if we say
68 percent of the curvature is due to the quoted dark energy fitting
then this should be something. I think if we know enough it may be a property of space-time rather than a species of field/particle... but
I guess this is the same thing as "cosmological constant".

Right. This is essentially the question whether the cosmological
constant is "geometric" and belongs on the left side of the Einstein
equation, or is a source term with a certain equation of state and
belongs on the right side. This goes back to a discussion between
Einstein and Schr=F6dinger:

E. Schr=F6dinger, _Physikalische Zeitschrift_, 19, 20, 1918.
A. Einstein, _Physikalische Zeitschrift_, 19, 165, 1918.

(does this follow from universe accelaration as per the supernovae observations from 20 years ago? but that's negative curvature isn't it)

Yes, the supernova observations are an important reason for believing in
a positive cosmological constant. But even without them, the data point
to it. Not negative curvature, though; both matter and a (positive) cosmological constant make the curvature more positive. You might be
thinking of negative pressure. Contrary to what one might think,
positive pressure acts like normal matter: causes deceleration.
Negative pressure thus causes acceleration. Since matter thins out as
the universe expands and the cosmological constant doesn't (which is why
it is called the cosmological CONSTANT), early on matter dominates, then
with time (already in our past) the cosmological constant dominates.

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In article <slrnpo07t9.vfi.bds@macbook-air-3.local>,
Bruce Scott <bds@ipp.mpg.de> writes:

Do we have solid evidence that it is _different from zero_

If by "it" you mean dark energy, yes. There are three independent
lines of evidence: the CMB fluctuations, the SN distances, and baryon
acoustic oscillations. All three agree on the values within their
respective uncertainties.

and if so what does the curvature of the universe have to be?

The universe is flat to within 0.5% or so.

You can find a good summary in the _Planck_ 2015 paper at https://www.aanda.org/articles/aa/abs/2016/10/aa25830-15/aa25830-15.html

The paper is open access, and there are links to both html and pdf
versions. Look in the various tables, but you will have to read the
text for the symbol definitions. In particular, watch out for h =
H_0/100 =~ 0.67, which is not very close to 1.

I guess [dark energy] is the same thing as "cosmological constant".

The term "dark energy" means something like a cosmological constant
but allows for a more general case where the (negative) pressure
varies with time. So far there is no evidence it does.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123 swillner@cfa.harvard.edu Cambridge, MA 02138 USA

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In article <pmc4fv$2am$1@dont-email.me>, willner@cfa.harvard.edu (Steve Willner) writes:

In article <slrnpo07t9.vfi.bds@macbook-air-3.local>,
Bruce Scott <bds@ipp.mpg.de> writes:

Do we have solid evidence that it is _different from zero_

If by "it" you mean dark energy, yes. There are three independent
lines of evidence: the CMB fluctuations, the SN distances, and baryon acoustic oscillations. All three agree on the values within their
respective uncertainties.

In the lambda-Omega diagram, each indicates a region where the
combination of lambda and Omega are compatible with the data. Each
region is approximately an elongated ellipse. So, each individual test
allows a range of values (though much smaller than the total parameter
space, i.e. that which was not ruled out before). Since these ellipses
have different orientations, they cross. The interesting thing is that
the intersection of any two also intersects with the third. This
indicates that if the data are wrong, a) several things have to be wrong
which b) just happen to conspire to make it look like a small region of
the parameter space is likely, one which is compatible with other tests
(age of the universe, gravitational lensing, etc). (One can get a
similar ellipse from gravitational-lensing statistics, but those for the
other tests have become so small recently that gravitational lensing is
not competitive at the moment, so while this doesn't appreciably narrow
down the parameters, it does provide an independent consistency check).

and if so what does the curvature of the universe have to be?

The universe is flat to within 0.5% or so.

Meaning |lambda + Omega - 1| < 0.5% or so.

I guess [dark energy] is the same thing as "cosmological constant".

The term "dark energy" means something like a cosmological constant
but allows for a more general case where the (negative) pressure
varies with time. So far there is no evidence it does.

Right. That doesn't mean that one shouldn't look for it, but as
observations get better and better and the cosmological constant still
fits the data best, then our confidence in it increases.

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