Dark matter now seen
Jacques Marcille
cannot yet be ascertained?
(If my understanding is correct, galaxies form preferentially in regions
where dark matter aggregates. This dark matter distorts the image of the
galaxy we receive ever so slightly. And by monitoring large numbers of
galaxies and their distorted images, in a given field, the amount and
position of the dark matter can be inferred. Thus revealing where it
aggregates.)
How come dark matter, which makes up 90% of the universe, won't block out
the light of ordinary matter, whereas dust clouds which represent a tiny
percentage, do so very effectively?
(I'm just enjoying myself in asking these questions. Since they crop up of
themselves now that I've read a sufficient number of books and articles in
the field of cosmology, about which I certainly do not know anything from a
mathematical perspective. This is not an attack on a given theory or
position.)
Jacques
Paul Lutus
> cannot yet be ascertained?
Define "seen." Dark matter is not seen, it is inferred.
> If my understanding is correct, galaxies form preferentially in regions
> where dark matter aggregates.
No, not necessarily. One can equally argue the opposite -- that dark matter
congregates where galaxies happen to be.
> This dark matter distorts the image of the
> galaxy we receive ever so slightly.
Not at all. Galaxies rotate in such a way that the presence of dark matter
can be inferred, even though it is not seen directly.
> How come dark matter, which makes up 90% of the universe, won't block out
> the light of ordinary matter, whereas dust clouds which represent a tiny
> percentage, do so very effectively?
How come one person with a bomb can undo in a second the combined work of
hundreds of laborers over years? How come silly questions outnumber serious
ones by at least a 100 to 1 ratio? How come people think they can make up
statistics like this on the spot?
--
Paul Lutus
www.arachnoid.com
Jacques Marcille <jmar...@videotron.ca> wrote in message
news:kZSZ4.1366$C4.4...@wagner.videotron.net...
Jacques Marcille
"Paul Lutus" <nos...@nosite.com> wrote in message
news:ftTZ4.147943$MB.27...@news6.giganews.com...
I refer you to an article in the latest Science&Vie, no 992, mai 2000 pp
79-84, where dark matter is represented for the first time in a 3D map of a
portion of the sky measuring 2x2 degrees. We now know where dark matter lies
in that portion, but not what it is made up of. "This work is being
confirmed by Anthony Tyson, of Bell Labs, New Jersey." The next generation
of high resolution cameras will allow us to know the distribution of dark
matter all over the universe within the next few years. A revolution in the
works.
> > Can anyone explain why dark matter can now be seen although its nature
> > cannot yet be ascertained?
>
> Define "seen." Dark matter is not seen, it is inferred.
>
> > If my understanding is correct, galaxies form preferentially in regions
> > where dark matter aggregates.
>
> No, not necessarily. One can equally argue the opposite -- that dark
matter
> congregates where galaxies happen to be.
>
> > This dark matter distorts the image of the
> > galaxy we receive ever so slightly.
>
> Not at all. Galaxies rotate in such a way that the presence of dark matter
> can be inferred, even though it is not seen directly.
This second question is far more general, far 'looser', much weaker. I have
read many times that 90% or more of the matter goes unseen. And nobody knows
what it consists of. I put it in just to see how to see it would be treated
by the muscle-bound inflated egos I see tramping over this part of the
beach. Your ranting won't hide the fact that you're as ignorant as I on this
one.
E. Robert Tisdale
> How come silly questions outnumber serious ones
> by at least a 100 to 1 ratio?
> How come people think
> they can make up statistics like this on the spot?
Funny.
Paul Lutus
matter is not visible. It is inferred by its effect on visible matter, and
on its influence on spacetime curvature. In regions without visible matter,
dark matter will remain undetectable.
> > > This dark matter distorts the image of the
> > > galaxy we receive ever so slightly.
> >
> > Not at all. Galaxies rotate in such a way that the presence of dark
matter
> > can be inferred, even though it is not seen directly.
>
> This second question is far more general, far 'looser', much weaker. I
have
> read many times that 90% or more of the matter goes unseen. And nobody
knows
> what it consists of. I put it in just to see how to see it would be
treated
> by the muscle-bound inflated egos I see tramping over this part of the
> beach. Your ranting won't hide the fact that you're as ignorant as I on
this
> one.
No, it is a great deal simpler than this -- you are wrong. You don't
understand how dark matter is detected. The primary current evidence for
dark matter is the velocity of various parts of galaxies, which, if they
only consisted of their visible components, would show a different velocity
profile from edge to edge.
This information is collected by comparing Doppler shifts of the various
parts of galaxy images, and comparing the inferred velocities with those
modeled in computers, assuming visible matter is the only source for the
observed orbital behavior. The difference between the model and the
observation is the result of dark matter.
--
Paul Lutus
www.arachnoid.com
Jacques Marcille <jmar...@videotron.ca> wrote in message
news:74UZ4.1386$C4.4...@wagner.videotron.net...
> > hundreds of laborers over years? How come silly questions outnumber
> serious
> > ones by at least a 100 to 1 ratio? How come people think they can make
up
> > statistics like this on the spot?
> >
> > --
> >
> > Paul Lutus
> > www.arachnoid.com
> >
>
>
>
J. Scott Miller
>
> Can anyone explain why dark matter can now be seen although its nature
> cannot yet be ascertained?
>
> the light of ordinary matter, whereas dust clouds which represent a tiny
> percentage, do so very effectively?
How dark matter interacts with light would reveal a way to trace it using
light of some wavelength. Size matters in this case, as does the character of
that matter.
For example, you ask about dark matter versus dust clouds. Dust clouds are
effective at blocking visible light because the grains of dust are comparable,
if not larger than wavelengths of visible light (though one can see through
dust at other wavelengths). So, one can infer size of the individual
particles of dark matter by the effect it has on different wavelengths of
light (I am not aware of across the spectrum searches, so I am not sure if
there have been conclusions on size based on this criteria).
Ordinary matter, neutral matter, for example, consists of a nucleus surrounded
by electron(s). EM radiation passing through a cloud of this stuff will be
absorbed depending on specific energies different atoms can absorb. In this
way, we can find clouds of gas along the line of site to a light source (a
distant star, for example) because the presence of the gas, though not
visible, is revealed by its effect on the light we receive.
In all I have read about dark matter to date, it does not seem to have either
of these effects that dust or ordinary collections of gas have. This sets
strict limits on the size of the matter, the density it might have, and
whether it is normal or exotic matter, matter that apparently doesn't interact
with light the way ordinary matter does, by absorbing certain wavelengths of
light.
So, the short summary of the long-winded explanation above is that it cannot
do what dust can do because it isn't like dust. It may be neutrinos, it may
be something more exotic, but if it isn't made of ordinary matter, as gas and
dust are, it will not effect the light passing through it the same way as gas
and dust do.
--
J. Scott Miller, Program Coordinator Scott....@louisville.edu
Gheens Science Center and Rauch Planetarium
http://www.louisville.edu/planetarium
University of Louisville
Jacques Marcille
does not contradict in any way the traditionnal methods of measuring the
velocities of galaxies within clusters, and of stars within galaxies, using
Doppler shifts. This new method goes further and allows the visualization of
where the dark matter aggregates. Dark matter is not spread uniformaly, but
is clumped. And these clumps are roughly where visible matter can be found.
The galaxies we see are slightly distorted by the gravitational effects of
these vast accumulations of dark matter. And the measurements of these
slight distortions upon 200,000 galaxies is what allowed the visualization
of the dark matter in the initial sample. The process is similar to, but not
the same as, gravitational lensing.
But don't allow yourself to be convinced by a non-authority. Wait for the
articles to come up in your regular scientific reviews. And perhaps then
will you consider the above a possibility.
I read four scientific magazines per month calibrated for the general
public, and have been doing so for years. I am well aware there is a big
problem concerning dark matter. What is it? Where is it?
The article in Science&Vie is the first mention I have seen of WHERE resides
the dark matter, and of HOW scientists did find it. It does not solve the
issue of WHAT is dark matter. But should constrain the possibilities
considerably. I thought that was big news. And wanted to be the first to
bring it up in this newsgroup.
So much for open-minded forward thinking in this group. The messenger gets
shot and his message gets burned.
T. Joseph W. Lazio
PL> Your reply, unfortunately, does not contradict anything in my
PL> post. Dark matter is not visible. It is inferred by its effect on
PL> visible matter, and on its influence on spacetime curvature. In
PL> regions without visible matter, dark matter will remain
PL> undetectable.
You didn't read the Science article to which he referred, did you?
Suppose one has a clump of dark matter between us and a light source.
The gravitational effects of the dark matter can cause gravitational
lensing as the light from the background source pass by the dark
matter. In particular, these researchers looked at the shapes of
distant galaxies. The shapes of galaxies were slightly distorted and
correlated, that is, the shape of a galaxy could be predicted if one
knew what the shape of another nearby galaxy was. The amount of the
distortion was slight (few percent?), but if one has enough galaxies
(they had over 10,000) it can still be discerned.
The actual amount of dark matter required to distort the shapes of the
galaxies was consistent with some of the more popular cosmological
models these days (those that invoke a cosmological constant).
The actual paper appears in Nature (my copy of which I can seem to
find at the moment...)
--
Lt. Lazio, HTML police | e-mail: jla...@patriot.net
No means no, stop rape. | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html
Paul Lutus
> shot and his message gets burned.
That outcome depends on the content of the message. The original didn't
bother to say "gravitational lensing" which would have put his message on a
credible footing. This possibility was raised in my post also.
--
Paul Lutus
www.arachnoid.com
Jacques Marcille <jmar...@videotron.ca> wrote in message
news:JJVZ4.2657$rr.5...@weber.videotron.net...
Paul Lutus
generic
> term for matter that did not glow, or did not glow brightly enough to be
> detected directly.
This is a confusion of two different things. Dark matter also does not
absorb light. There are examples where dark matter fails to block light from
behind its presumed position.
You are thinking of a list of normal-matter candidates compiled to explain
the universal mass shortfall. Dark matter is something else, and the term is
reserved for this meaning.
Cosmologists can easily distinguish dust clouds, which do block light, from
dark matter, which does not. The term is reserved for the second case.
--
Paul Lutus
www.arachnoid.com
Philip Plait <phil...@home.com> wrote in message
news:39385D05...@home.com...
>
>
> Paul Lutus wrote:
>
> > Your reply, unfortunately, does not contradict anything in my post. Dark
> > matter is not visible. It is inferred by its effect on visible matter,
and
> > on its influence on spacetime curvature. In regions without visible
matter,
> > dark matter will remain undetectable.
>
> This does not turn out to be the case. The term 'dark matter' was just a
generic
> term for matter that did not glow, or did not glow brightly enough to be
> detected directly. There are now several candidates for dark matter; two
of them
> are faint white dwarfs and neutral hydrogen, both outside what we normally
> consider to be the Galaxy. Both have now been directly detected.
>
> No one knows if there is enough of either of these materials to constitute
the
> amount of dark matter needed to explain things galaxy rotation curves and
the
> velocities of gaalxies in clusters, but they are definitely out there, and
there
> are lots of them. They used to be invisible, but our instruments keep
getting
> better. Dark matter won't stay dark forever.
>
> --
> * * * * * The Bad Astronomer * * * *
>
> Phil Plait bada...@badastronomy.com
> The Bad Astronomy Web Page: http://www.badastronomy.com
>
>
Philip Plait
T. Joseph W. Lazio
>> If my understanding is correct, galaxies form preferentially in
>> regions where dark matter aggregates.
PL> No, not necessarily. One can equally argue the opposite -- that
PL> dark matter congregates where galaxies happen to be.
True, one could argue that. However, various theoretical
models---backed up with limited observational data---suggest that the
former is the case. This is known as bias or galaxy biasing. That
is, galaxies form preferentially in regions that already contain an
aggregation of dark matter.
Paul Lutus
We don't know:
1. Whether dark matter is more concentrated where there are galaxies, and
2. Whether the dark matter came first, or the galaxies.
The first might be argued purely on theoretical grounds, the second cannot
be resolved.
--
Paul Lutus
www.arachnoid.com
T. Joseph W. Lazio <jla...@patriot.net> wrote in message
news:m2itvqw...@patriot.net...
Jacques Marcille
touch upon the specific topic mentioned in Joseph Lazio's last post. From:
Science & Vie, no 992, mai 2000, pp 87-94
Valérie Greffoz states that, as reported by Ludovic van Waerbeke (CITA) of
the Canadian Institute for Theoretical Astrophysics, dark matter has
coalesced into filaments one hundred million light years or more in length
which cross over themselves forming knots of denser dark matter at the
intersections.
No direct obsevations of galaxies residing preferentially along these
filaments or knots were reported in the article.
On the other hand, it does mention that computer simulations of the
evolution of matter in the Universe over time show that galaxies form where
dark matter is densest. Matter, whether visible or not, is affected by
gravity in the same way. Gravity assembles both types which collapse in onto
themselves. No sources or links given.
On a personal note, I might add that since there is far more dark matter
than visible matter, and both interact gravitationally, the former should
have a greater gravitational influence on the latter than the reverse.
The article states that the results have a direct bearing on the 'hot dark
matter' vs 'cold dark matter', and baryonic vs 'exotic' matter, models.
These new findings also shed light on the proportion of matter relative to
the vacuum energy, and thus relate to the question of the expansion of the
universe as derived from the Big Bang theory.
Jacques Marcille
"Paul Lutus" <nos...@nosite.com> wrote in message
news:nef_4.151266$MB.27...@news6.giganews.com...
T. Joseph W. Lazio
PL> T. Joseph W. Lazio <jla...@patriot.net> wrote in message news:m2itvqw...@patriot.net...
>> >>>>> "PL" == Paul Lutus <nos...@nosite.com> writes:
>>
PL> No, not necessarily. One can equally argue the opposite -- that
PL> dark matter congregates where galaxies happen to be.
>> True, one could argue that. However, various theoretical
>> models---backed up with limited observational data---suggest that
>> the former is the case. This is known as bias or galaxy biasing.
>> That is, galaxies form preferentially in regions that already
>> contain an aggregation of dark matter.
PL> But there is no observational evidence for this.
It is certainly not compelling, but there is *some* observational
evidence. I just did a quick literature search at
<URL:http://adsabs.harvard.edu/abstract_service.html>. Various
clusters, including the Local Group (i.e., our own cluster), have been
analyzed and found to be consistent with at least some galaxy biasing
models.
PL> It is a supposition. We don't know:
PL> 1. Whether dark matter is more concentrated where there are
PL> galaxies, and
Again, there is limited observational evidence to suggest that
galaxies are concentrated in regions of higher dark matter density.
PL> 2. Whether the dark matter came first, or the galaxies.
PL> The first might be argued purely on theoretical grounds, the
PL> second cannot be resolved.
IIRC, the COBE satellite results of the cosmic microwave background
suggest that dark matter concentrated first. COBE found fluctuations
in the CMBR, presumably resulting from regions of higher and lower
density in the early Universe. The size scale of those regions is far
larger than that seen among galaxies today. Ergo, dark matter was
already beginning to condense when the CMBR formed. (Again, all IIRC.)
Paul Lutus
> >> contain an aggregation of dark matter.
>
> PL> But there is no observational evidence for this.
>
> It is certainly not compelling, but there is *some* observational
> evidence.
You misunderstood. You go on to argue that there is some evidence that
galaxies are where the dark matter is. My point is there is no evidence for
which came first -- the galaxies or the dark matter.
Another point is that there may be concentrations of dark matter away from
all galaxies, where it cannot be seen. This would undercut the original
assumption that galaxies and dark matter tend to appear in the same places.
--
Paul Lutus
www.arachnoid.com
T. Joseph W. Lazio <jla...@patriot.net> wrote in message
news:m28zwif...@patriot.net...
> >>>>> "PL" == Paul Lutus <nos...@nosite.com> writes:
>
<snip>
T. Joseph W. Lazio
PL> You misunderstood. You go on to argue that there is some evidence
PL> that galaxies are where the dark matter is. My point is there is
PL> no evidence for which came first -- the galaxies or the dark
PL> matter.
You snipped the rest of my post. In that I pointed out that there is
some evidence (IIRC) that dark matter began to condense before galaxies.
PL> Another point is that there may be concentrations of dark matter
PL> away from all galaxies, where it cannot be seen. This would
PL> undercut the original assumption that galaxies and dark matter
PL> tend to appear in the same places.
Indeed, some evidence of this already exists. That was what started
this entire thread. Recent observations of weak gravitational lensing
do suggest that there exist regions of dark matter that are devoid of
galaxies.
That still does not contradict my earlier statement. I stated,
"galaxies form preferentially in regions that already contain an
aggregation of dark matter." That does mean that regions of dark
matter may exist in which there are no galaxies.