Space Forum / Astronomy / May 2007

The observation is discussed more definitively at:

http://arxiv.org/abs/0705.2171

Richard Saam

Richard Saam <rds...@att.net> wrote:

Thanks for the link, Richard. It's a bit slow going without
the appropriate education, but the pictures are pretty,
and the sense is getting through.

I should introduce them to Chris Gold's relaxation methods
technique of contouring from the original data points, though.

Imposing a regular grid on  data that doesn't start that way
(lensed galaxy locations) blends away information that could
be retained.

Now where did I put that? Ah!

"Automated contour mapping using triangular element data
structures and an interpolant over each irregular triangular
domain"
http://portal.acm.org/citation.cfm?id=563858.563887

There's lots of newer information, and perhaps some source
code, available from the search I used:

http://scholar.google.com/scholar?q=gold+triangle+contour+siggraph

since pretty much everything afterward cites Gold's paper,
but I sat through the 1977 SIGGRAPH session when he
presented it, so its the one I know to seek.

Someone hooked into the cosmology community please
pass that along.

xanthian.

I think "dark matter" is an unfortunate term.  "Gravitational scalar"
sums it up without the assumption of "matter" as the cause.  Brane
theories hold out the possibility of a geometric mechanism.

As for "dark energy", that's not much more than shorthand for "the gap
between what we observe and what we understand" -- sort of like
entropy in thermodynamics, I gather.

Eric

Can brane theory explain the DM distributions seen
in the Bullet Cluster and this new ring structure?

George

Considering these observations are just happening,
and nobody knows underlying scientific truth,
I do not know what the appropriate education would be.

In this context,
the 'dark matter' observation for Galaxy Cluster Cl0024+17
in http://arxiv.org/abs/0705.2171
is referenced (figures 10 & 11) in part to critical density

(3/(8*pi))*H^2/H = 9.56E-30 g/cm^3

Deviations from this baseline represent very small amounts of mass.

For example:
considering that this deviation from critical density
were in the form of 1 gram objects
with density 1 gram/cm^3
then the distance between these 1 gram objects
would be ~5E7 meters
and the mean free path would be ~2E10 light years
which means that this tenuous matter would gravitationally lens
but would not noticably attenuate or obscure light
from more distant light sources.
Is there an argument against such reasoning?

Richard Saam

Yes, insofar as the "DM" is seen as the original structure,
subsequently gravitating the matter to it.  Of course, brane theory
has almost as many variations as there are authors, but the general
idea is that gravity operates via an additional large dimension, and
gravitational highlands and lowlands on the brane cause matter to
coalesce accordingly.  So if this interpretation is correct, then
there is no "matter" in "dark matter".

Eric

The question though is "Can brane theory explain
the DM distributions seen in the Bullet Cluster
and this new ring structure?". This being a science
group, I see "explain" meaning that you can apply
the equations of brane theory to the distribution
of normal mass, and perhaps a contribution from the
brane, and show that it predicts effects equivalent
to the observed "DM" distributions. My question is
based on my understanding that brane theory is not
yet sufficiently developed to do that so I am
sceptical of your claim.

All the better, that gives us a way of rejecting
those that fail to make the above prediction.

Maybe, maybe not, but equallya brane theory might
require DM to explain other observations, who knows.

George

Richard Saam <rds...@att.net> wrote:

Mostly that it's incomplete.

What you need to worry about when contemplating the
ability of some density of matter to obscure objects
behind it isn't some vague "tenuous", but the
specific obscuring cross section that such a
consolidation level of the matter implies, and that
needs that you know how _thick_ the cloud of such
matter is along the line of sight.

Notice that the cross section varies dramatically
depending on what size chunks you think comprise the
obscuring matter, probably by a square-cube law
before refraction effects are considered.

The correct final obscuration effect is probably
much more complex to calculate than your simple
beginnings would want to imply.

xanthian.

[Apologies to the moderator if this arrives in
multiple copies, Google Groups has gone tharn on me.]

In article <mt2.1-25699-1180364608@xeon44.aei.mpg.de>, Kent Paul Dolan
<xanthian@well.com> writes:

Didn't the original poster say what size they should be, namely objects
of mass 1 gram with the density of water (which results in size of 1
cubic centimeter)?  OK, he didn't specify the SHAPE of the objects.

I remember a calculation someone did pointing out that, as far as
direct astronomical observations (i.e. influence on the images or
spectra of observed objects, as opposed to interpretation of other
observations) go, the dark matter could consist of bricks.

In article <mt2.1-27618-1180374871@xeon44.aei.mpg.de>,
Phillip Helbig---remove CLOTHES to reply  <helbig@astro.multiCLOTHESvax.de> wrote:

That's right.  If the dark matter is bricks, the optical depth rises
to about 1 by a redshift of about 5, so for observations at moderate
redshift, you don't get a lot of obscuration.  The original poster did
essentially that calculation, and it looked right to me.  It's been
done by others over the decades as well.

There are other reasons we can be confident the dark matter isn't
bricks, of course, but as far as optical observations of low- to
moderate-redshift objects are concerned, they work fine.

-Ted

Phillip Helbig <hel...@astro.multiCLOTHESvax.de> wrote:

Well, yes, but he merely chose that as an *example*,
he didn't claim that the size had any authority
except as one way among many that the mass
equivalent of the DM could be divided up if it were
somehow baryonic matter rather than some "new
stuff", IIRC.

The issue is, that if the cloud in the originally
URLed article is thick enough, for very large values
of "thick enough", even one centimeter cubes at 10
megameter spacing could on average entirely obscure
the background.

Since his 1 cm^3 size _was_ "just an example", it
seemed worth raising the issue that variations in
that guess could lead to dramatically different
results for a fixed cloud thickness. Subdivide it
into microgram bits rather than gram bits, and it
is 100-fold more obscuring, IIUC, and ignoring
refraction effects.

Yes, one of the real quandries of DM seems to be
that there are ways it could be baryonic matter and
be invisible in some of the possible ways to
subdivide it into condensed matter chunks, at any
sufficiently distant locale.

What seems in part to quash that notion is that the
same subdivided extra matter would be _very_
noticible if it were present in our solar system,
sleeting through our atmosphere as it would be, and
all, and whatever DM is, it seems to permeate our
galaxy, leaving no reason for it not also to
permeate our solar system, and noticiably so if it
were condensed baryonic matter.

xanthian.

In article <mt2.1-2996-1180423624@xeon44.aei.mpg.de>, Kent Paul Dolan
<xanthian@well.com> writes:

Except that there are good reasons to believe it is not baryonic.

Whatever the dark matter is, if it is smoothly distributed, then the
density would be so low that we certainly wouldn't notice it swooshing
through our atmosphere, even if it DID interact in such a way that we
could easily detect it.  The average density of the universe is
something like the mass of a proton per cubic meter.  (A consequence of
this is that all the matter in the visible universe, assuming it could
be compressed to the density of an atomic nucleus and remain stable in
that state, would fit comfortably within the radius of the solar
system.)

ebunn@lfa221051.richmond.edu wrote:

Assuming that both following statements 1 & 2 are true:

1. "There are other reasons we can be confident
the dark matter isn't (baryonic) bricks"
(nuclear product limitations resulting from Big Bang)

and

2. When Mass attributed to dark matter
is composed of (baryonic) bricks,
no obscuration will be observed because of cross sectional
mean free path considerations.

In the  context of
the 'dark matter' observation for Galaxy Cluster Cl0024+17 in

http://arxiv.org/abs/0705.2171

could the a base line critical density
conventionally expressed as:

(3/(8*pi))*H^2/G= 9.56E-30 g/cm^3

be regarded in the context of statement 1
and
deviations from this baseline representing very small amounts of mass.
as referenced (figures 10 & 11)
be regarded in the context of statement 2.

In other words, can statement 1 dark matter
be considered universally constant
with local mass deviations
observed as gravitational lensing
due to statement 2.

An image comes to mind of Star Trek Enterprise
moving through the vastness of space
and coming upon a brick or two.

Richard Saam

Phillip Helbig <hel...@astro.multiCLOTHESvax.de> wrote:

Earth makes for a _big_ flyswatter.

I just _do not_ want to sit down and "do the math
right", but front-of-the skull calculations put
intersection of the whole earth's atmosphere with
objects of size 1 cm^3 at 10^7 meter spacings on the
order of small whole numbers of hours intervals
between hits, not at all something that should be
completely indetectible, since it would be biased
toward the side of Earth facing our direction of
motion through the galaxy, compared to Oort Cloud
escapees that shouldn't show that particular bias.

Of course, I may have dropped any number of zeros
there somewhere.

Quantum valeat.

xanthian.

[[Mod. note -- It might be interesting to compare this to the meteor
flux density (for the same size range) in near-Earth space.  -- jt]]

In article <mt2.1-3605-1180526381@silentbox68.aei.mpg.de>,

It's also worth pointing out that, if the dark matter were made of
bricks, or centimeter-sized chunks of rock, or something like that,
then their density in the solar neighborhood would not be the same as
their average density throughout the Universe.  Over the past few
gigayears, the dark matter rocks that had been captured by our Galaxy
would have had complicated dynamics: collisions, friction due to
interactions with the interstellar medium, dynamical friction, etc.
I'd guess that, as a result of some combination of sublimation and
settling in towards the Galactic center, there wouldn't be very many
of them around here.  But I haven't done any calculations to verify
that.

-Ted

e...@lfa221051.richmond.edu wrote:

Yeah, I seem to recall that the lack of "frictional
settling toward the galactic center" is one of the
arguments used to disqualify dark matter from being
baryonic, since, in the case, as I understand, it
_is_ "out here where we orbit" and seems even to be
larger in total ellipsoidal extent than the visible
galaxy. It seems to interact with baryonic matter
and even with itself only by gravity, and not by,
say, collisions, friction, "gas" pressure (which is
== "collisions", I suppose), or any other putative
remote "dark matter" forces similar to magnetism or
electrical attraction in that they are much stronger
than gravity.

xanthian.

[Note to moderator: my postings seem to be threading
strangely recently, ending up linked as followups to
articles other than the ones to which I replied. Is
this you, Google Groups, or some other NNTP/Usenet
weirdness?]

[[Mod. note -- Threading should be driven by the  References:  headers
in articles.  These are supposed to be created by your news client (in
this case maybe Google Groups) when you submit an article.  -- jt]]

e...@lfa221051.richmond.edu wrote:

But that's a phony comparison.

Dark matter, Hubble surveying showed, is very clumpy
itself, and IIUC, most of the universe is as devoid
of DM as of other kinds, so that "average" _across
the universe_ is a flawed metric.

What matters to "direct detectability of Earth-local
dark matter" is the average density of dark matter
at our orbital radius from the center of the Milky
Way, not some universe-wide average.

However, per the moderator's suggestion, I went
looking for "meteors per time frame" numbers, and
this URL

http://www.sandia.gov/LabNews/LN01-30-98/plume_story.html

makes it look like seeing putative baryonic objects
at a density sufficient for accounting for "dark
matter gravitation" would be nearly impossible. If
my "one per several hours" guesstimate is correct,
the evidence would be at the noise level of existing
impacts from solar-origin meteoroids.

Oh, well.

Next candidate: how massive is the Oort cloud, and,
if every star has one, how does that fairly
invisible matter compare in mass to the requirements
for accounting for dark matter gravitation?

xanthian.

Notice that I don't for a moment believe that dark
matter IS baryonic, I'm just trying to eliminate
baryonic candidates, a task I'm sure reputable
astronomers have already accomplished in much
greater detail before me.