Detecting dark matter

As Stuart Levy has indicated, a recent issue of Science has a good
summary of the current state of affairs with respect to both dark
matter and dark energy.

"g" == greywolf42 writes:

g Joseph Lazio wrote in message
g ...

greywolf42 The only reason we "need" dark matter "to keep galaxies
greywolf42 bound in the cluster, given their motions relative to the
greywolf42 cluster" is to "save" the Big Bang.
Levy was correct. Zwicky was postulating "unseen" matter in the
Coma cluster of galaxies in the 1930s, about 30 years before the
Big Bang had achieved its current prominence.

g LOL! Zwicky was producing a steady-state theory expressly to
g counter the early big-bang. [...]. However, Zwicky's "unseen"
g matter is not the "dark matter" that we discuss today. Unseen
g simply means we haven't yet observed it. "Dark" matter -- by
g definition -- cannot interact by EM, and cannot be directly
g observed by us.

Huh? I'm looking at Zwicky's 1937 paper, "On the Masses of Nebulae
and of Clusters of Nebulae." Nowhere does he make any statements
about constructing a steady-state cosmology. He's concerned simply
with the masses of galaxies and clusters of galaxies.

Some fraction of the "unseen" matter that Zwicky inferred is
baryonic. We see it today in the X-ray emitting gas in clusters.
Even if you take that into account, though, there still needs to be
more matter in the cluster to keep it bound.

Ignore the Big Bang if you'd like, but if you think that Newton was
even close to correct in his description of gravity, the motions of
galaxies in clusters require more matter than is seen.

g Your two separate assertions in the statement above are both
g unsupported and incorrect.

g 1) Newton has nothing to do with it. As gravity is not the only
g force in the universe.

I notice that you provide no evidence to suggest that other forces are
important on cluster scales.

g 2) Even if we required gravity as the be-all and end-all of
g cosmology, we don't have any problems at all with cluster motions
g outside of the big bang. It is the fact that the observations of
g apparent motion (filtered through the theory of the BB) explicitly
g contradict the big bang that gives the problem.

Nope. Ignore the Big Bang. Just look at the motions of the galaxies
in the Coma cluster (to take one example). Treat it as an isolated
system. What's the mass?


It's also worth pointing out that we need dark matter to exist
because we've detected some. Both neutrinos and black holes are
dark matter. Neither exist in a sufficient quantity to explain all
of the dark matter required, but both exist. If we have two
examples of dark matter, it is not unreasonable to suggest that
there might be a third example of dark matter.

g Neither neutrinos nor black holes are dark matter. Dark matter is
g not merely matter that cannot be seen. Dark matter is -- by
g definition and theoretical requirement -- unable to interact with
g matter EXCEPT by gravity. [...]

No, your earlier definition was correct. Dark matter is matter that
does not interact via the EM force. Neutrinos are dark matter because
they interact only via the weak force and (presumably) gravity. As
for your assertion that black holes don't exist, what's Sgr A*?



greywolf42 Does your background allow you to detect any difference
greywolf42 between a molecule of free gas and a star? Do you think
greywolf42 that magnetic fields might affect one more than the other?
I notice that you provide no evidence to suggest that gas and
stellar motions differ.
[...]
(I've posted pointers of rotation curves derived from gas motions,
check Google.)

g And your point would be what?

You assert that stellar and gaseous rotation curves are different. I
haven't seen you post any references.


It's not obvious to me that magnetic fields should be important.
First, the gas motions are measured for *neutral* gas. How do the
magnetic fields and neutral atoms couple?

g Neutral molecules have charges, though the total charge is zero.
g They are accelerated by their paramagnetic and diamagnetic
g properties.

Second, the kinetic energy in the rotation is of orders of
magnitude larger than the magnetic energy density. How can the
magnetic field be comparable in influence to the rotation?

g Non sequiteur. The EM FORCE is stronger on gas molecules than the
g gravitational FORCE. Rotation is the result, not a competing
g effect.

'Fraid I still don't understand. If the total energy in the magnetic
field is orders of magnitude smaller than the kinetic energy of
rotation, how does the former produce the latter?

--
Lt. Lazio, HTML police | e-mail:
No means no, stop rape. | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html

In article , greywolf42 wrote:

Joseph Lazio wrote in message
...
[...]
Some fraction of the "unseen" matter that Zwicky inferred is
baryonic. We see it today in the X-ray emitting gas in clusters.
Even if you take that into account, though, there still needs to be
more matter in the cluster to keep it bound.

Why would it need to be bound? Answer: Big bang needs it.

Well no -- if it's not bound somehow, then the cluster will
fall apart, since gas and galaxies are moving at faster than
escape velocity. I.e. we are assuming that we're not observing
at a an especially special time -- that the cluster is a long-lived
entity.

[...]
Nope. Ignore the Big Bang. Just look at the motions of the galaxies
in the Coma cluster (to take one example). Treat it as an isolated
system. What's the mass?

Unknown without first assuming the big bang. If no big-bang, then there is
no assumption that redshift is pure speed. If redshift is not pure speed,
then there is no way that redshift implies distance. No distance estimate,
no speed estimate.

Whoa -- go and read a little history of astronomy! Of course there are
non-redshift distance estimates -- if not for them, we'd have no
idea what value to adopt for the Hubble constant.

And if those distance estimates didn't more or less agree
(galaxy brightnesses and sizes, Cepheid variables, supernova expansion,
SN Ia standard candles, brightest planetary nebulae, etc.)
I don't think there'd be much confidence that redshift really did
correlate well with distance.

There surely are holes in modern astronomy and astrophysics,
but you need to know a little more about what those fields
do and how they got where they are, before you'll find them.

If you attack a straw man, you'll probably win, but
nobody other than you will care much.

Stuart Levy

[I really should be reading this paper on pulsar parallaxes, so let me
be brief. Again, I'll recommend highly a recent issue of Science
dealing with the dark side.]
"g" == greywolf42 writes:

g Joseph Lazio wrote in message
g ...

g 2) Even if we required gravity as the be-all and end-all of
g cosmology, we don't have any problems at all with cluster motions
g outside of the big bang. It is the fact that the observations of
g apparent motion (...) explicitly contradict the big bang that gives
g the problem.

Nope. Ignore the Big Bang. Just look at the motions of the
galaxies in the Coma cluster (...). Treat it as an
isolated system. What's the mass?

g Unknown without first assuming the big bang. If no big-bang, then
g there is no assumption that redshift is pure speed. If redshift is
g not pure speed, then there is no way that redshift implies
g distance. No distance estimate, no speed estimate. Hence no mass
g estimate available.

Ignore the systematic redshift of the Coma cluster. Take the average
redshift of all of the galaxies in the Coma cluster and subtract that
from the individual redshifts. You'll be left with a residual
redshift. For some galaxies, it's positive, some it's negative.
Equate this residual redshift with a velocity. Assume that the
cluster is a long-lived object (otherwise why are we seeing it?) and
apply the virial theorem.

As Zwicky found in the 1930s (at least some of his papers describing
this on are ADS, freely available), you'll find that this analysis
requires a lot more mass than the individual galaxies would
contribute. Now take into account the mass of the gas that's emitting
in X-rays. You still don't have enough to explain the mass derived
above.


It's also worth pointing out that we need dark matter to exist
because we've detected some. Both neutrinos and black holes are
dark matter. Neither exist in a sufficient quantity to explain
all of the dark matter required, but both exist. If we have two
examples of dark matter, it is not unreasonable to suggest that
there might be a third example of dark matter.

g Neither neutrinos nor black holes are dark matter. Dark matter is
g not merely matter that cannot be seen. Dark matter is -- by
g definition and theoretical requirement -- unable to interact with
g matter EXCEPT by gravity. [...]
No, your earlier definition was correct. Dark matter is matter
that does not interact via the EM force. Neutrinos are dark matter
because they interact only via the weak force and (presumably)
gravity. As for your assertion that black holes don't exist,
what's Sgr A*?

g Not a black hole. "I don't know" is a valid answer. My guess
g would be a magnetic pinch effect.

Why would a magnetic pinch effect affect stars? Haven't you been
trying to convince us that stellar motions aren't affected by magnetic
fields? While "I don't know" can be a valid answer, within the
context of general relativity, there is an explanation for Sgr A*.
Thus, you have to show why "I don't know" is a better explanation than
the explanation suggested by GR.


Second, the kinetic energy in the rotation is of orders of
magnitude larger than the magnetic energy density. How can the
magnetic field be comparable in influence to the rotation?

g Non sequiteur. The EM FORCE is stronger on gas molecules than the
g gravitational FORCE. Rotation is the result, not a competing
g effect.
'Fraid I still don't understand. If the total energy in the
magnetic field is orders of magnitude smaller than the kinetic
energy of rotation, how does the former produce the latter?

g Again, non sequiteur. "Energy" doesn't move matter. FORCE moves
g matter. There are two forces: EM and gravity here. Each can
g "drive" the rotation. EM force is stronger on gas molecules than
g gravitation in free space.

I'm assuming that the magnetic energy acts like a potential energy.
In your scheme, conversion of the magnetic energy to kinetic energy
would produce rotation. This is similar to how the potential energy
of a ball at the top of a hill can be converted to kinetic energy of
the ball at the bottom of the hill. Indeed, conversion of magnetic
energy into kinetic energy of *charged* particles is seen all the time
in the Earth's magnetosphere. You still haven't shown how magnetic
energy can be converted into kinetic energy of *neutral* gas.

I'm off to read some real astronomy ....

--
Lt. Lazio, HTML police | e-mail:
No means no, stop rape. | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html

Steve Willner replied to "greywolf42":

No, they don't "count" as dark matter, because their presence has
already been "counted" in the big bang cosmology as normal matter.

I'm curious to know whether you think these objects don't "create
gravity" (curve spacetime, if you prefer) or whether their effect
is somehow shielded from the rotation curves and cluster velocity
dispersions that we measure.

Steve,

From other evidence, "greywolf42" is apparently yet another
mental case, but you misinterpreted his above comment. He was
asserting that some forms of non-luminous matter have already
had their gravitating masses taken into account when totalling
up the amount of matter in galaxies, so they aren't counted as
"dark" even though they are non-luminous. I don't know to what
extent that assertion is correct in general, but many specific
articles about the problem do estimate the amount of mass in
planets, asteroids, dust, and whatever, before going on to show
that it still isn't enough (by far!) to account for observed
motion of stars in galaxies and galaxies in clusters.

-- Jeff, in Minneapolis

..

Hi:

There is no need for dark matter to explain the flat rotation velocity
curves of spiral galazies (Vera Rubin) or the motion of remote groups of
galaxies (Fred Zwicky) if the gravitational constant is slightly modified.

This also explains the real use of Hubbles constant.

A detailed explaination is provided at my web site:

http://inventing-solutions.com/new-universe.htm

You can email me with comments and questions. Would like comments.

Sol Aisenberg



"Ed Keane III" wrote in message
m...

Nicolaas Vroom wrote in message
...

"Ed Keane III" schreef in bericht
m...


If results from the Planetary Nebula Spectrograph show
that elliptical galaxies do not contain internal dark matter

Correct

while their motions with respect to other galaxies still
require it around them

Where did you read that ?


Ninety percent of proposed dark matter surrounds
galaxies and does not effect internal dynamics.
It is needed to explain the motion of galaxies with
respect to each other. Observations of motion in
galaxies have been done by measuring the motion
of gas halos with spectrometers as individual stars
cannot be resolved. A new scope, the Planetary
Nebula Spectrograph, can measure the motion
of individual novas in elliptical galaxies that do
not contain gas. The unexpectedly close mass to
light ratio needed to explain the motion within these
galaxies shows that they do not contain a lot of
invisible matter. I am not aware of it having been
shown that there is not a need for dark matter
outside of the visible halo to explain their motion
within their galactic clusters.

How do we know that in order to explain the behaviour
of galaxies you need DM?


It was first noted in the 1930's that there had to be
a lot more mass than was visible through telescopes
at that time to explain the motions of galaxies within
the Coma and Virgo clusters. The uneven mass to
light ratio was not considered to be a problem for
theoretical physics at that time.

This is completely different than to introduce DM,
in order to explain the rotation curve of a galaxy.


It is done to explain the rotation curve of a group
of galaxies and is exactly the same.

I think that it will be difficult to
argue that it was stripped away. I think that would indicate
that there is some problem with our understanding of the
gravitational dynamics of concentrated mass cores, assuming
that such galaxies do not have them.

-Ed