Dark matter avoidance of galatic centres?

We hear a lot on the "Dark Matter" subject.
But I must confess that I have yet to see any good arguments explaining
just WHY "Dark Matter" avoids the visible regions of galaxies (including
our own).
This is surely very important as if dark matter is gravitationally reactive
to "Normal" matter, we should see a lot more infall into the central "black
hole(s)" of galaxies. The dark matter would presumably greatly increase the
mass and energy output of a singularity especially a rotating one.
It strikes me that we might then find almost every galaxy would be an
active one and life would indeed be very rare in the universe.

clifford wright wrote in
:

We hear a lot on the "Dark Matter" subject.
But I must confess that I have yet to see any good arguments
explaining just WHY "Dark Matter" avoids the visible regions of
galaxies (including our own).
This is surely very important as if dark matter is gravitationally
reactive to "Normal" matter, we should see a lot more infall into the
central "black hole(s)" of galaxies. The dark matter would presumably
greatly increase the mass and energy output of a singularity
especially a rotating one. It strikes me that we might then find
almost every galaxy would be an active one and life would indeed be
very rare in the universe.

Just a little addition to my posting. Something else I had apparently
forgotten. Since "Dark Matter" will NOT experience frictional
effects with the normal kind and also Electromagnetism can play no part in
its interactions. Surely there are VERY few potential arguments available
to explain the apparent lack of observed infall of "Dark matter"
into galactic nucleii.

For years now I have been more impressed by the OBSERVED fact that WHATEVER
is causing the apparent anomallies in the large scale of the Universe is
in some way proportional to the scale involved.
In other words the bigger the scale the greater the anomally.

As a mere retired Electronics engineer and Amateur astronomer, that looks
to me like a "Field effect" of some kind rather than a mysterious
substance!

In article ,
"Robert L. Oldershaw" writes:
... the CDM speculations. They definitively predicted "cusped" or
centrally peaked CDM distributions in galactic interiors

Could you provide a reference to this prediction? It's not what the
Millennium Simulation, for example, shows.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA

In article ,
clifford wright writes:
But I must confess that I have yet to see any good arguments explaining
just WHY "Dark Matter" avoids the visible regions of galaxies (including
our own).

Why do you think dark matter "avoids the visible regions of galaxies?"

if dark matter is gravitationally reactive
to "Normal" matter, we should see a lot more infall into the central "black
hole(s)" of galaxies.

More infall than what? And why would you think so? Why should dark
matter be any more susceptible to falling into a black hole than
visible matter?

A paper recently discussed here (in a different context) shows that
for significant dark matter to fall into a black hole -- even an
enormous one -- the dark matter density has to exceed 250 solar
masses per cubic parsec. The expected dark matter density is nowhere
greater than 1 solar mass per cubic parsec.

The paper is at http://lanl.arxiv.org/abs/1002.0553

The dark matter would presumably greatly increase the
mass and energy output of a singularity especially a rotating one.

I don't see how this would follow. If dark matter interacts only
gravitationally, it can't form an accretion disk. (And by the way, a
black hole has no singularity visible from the outside.)

I think you have some misconceptions about dark matter, but I'm not
sure just which ones. For dark matter in galaxies, you might enjoy
looking at http://burro.cwru.edu/JavaLab/RotcurveWeb/main.html
It's a java tool that allows you to test various distributions of
dark matter.

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA

(Steve Willner) wrote in
:

[Mod. note: entire quoted article trimmed -- mjh]

I think you have some misconceptions about dark matter, but I'm not
sure just which ones. For dark matter in galaxies, you might enjoy
looking at http://burro.cwru.edu/JavaLab/RotcurveWeb/main.html
It's a java tool that allows you to test various distributions of
dark matter.


Firstly I am VERY well aware that singularities, if they exist, cannot be
directly observed. Except by their gravitational effects.
Just an odd thought here. How is gravity propagated across the event
horizon? That sounds like good support for the Multiverse to me!

I must confess I find it hard to see why CDM cannot form a accretion
disk. After all if it has any relative velocity to the singularity then
it must tend to spiral around it before being "swallowed up". That like
any vortex is a purely gravitational effect.
NB I am NOT saying that baryonic matter does not have electromagnetic
forces on it under such extreme theoretical conditions. This may well
modify its behaviour realtive to CDM. But "spiralling in" is what I would
expect it to do.

However the CDM enthusiasts have claimed that by far the greater
proportion of the mass of the universe is CDM. So if "normal" matter can
feed these singularities, then one would surely expect CDM to have an
effect in proportion to its abundance.
Yet the figures I have seen appear to show that the energy output of
active galactic nucleii is about what one would expect from "normal"
matter infall alone.
BTW where did the idea of my thinking that CDM was more affected by
gravity than the ordinary kind come from?

However in the event I think I will withhold my acceptance of CDM right
along there with "gravitational waves".
I'm old fashioned and much prefer experimental proof to theory piled on
theory.

clifford wright askeds:
Just an odd thought here. How is gravity propagated across the event
horizon? That sounds like good support for the Multiverse to me!

Actually, classic 4-dimensional-spacetime general relativity (GR)
has no problem explaining "how is gravity propagated across the event
horizon", so this point provides no particular evidence for (or against)
multiverse models.

The key to the the GR explanation is that gravity doesn't have to
propagate *across* the event horizon -- GR is a *nonlinear* field
theory, so when a black hole forms by collapsing matter, the field
outside can "remember" the field configuration from before the black
hole formed.

There's a brief non-technical discussion of this question in the
Usenet Physics FAQ, in the entry "How does the gravity get out of
the black hole?", which can be found at
http://www.edu-observatory.org/physi...k_gravity.html

If you want to see the analysis worked out in detail, read
Misner, Thorne, & Wheeler chapter 23 ("Spherical Stars") and
chapter 32 ("Gravitational Collapse"), & references therein,
particularly
* Oppenheimer & Snyder 1939 (Physical Review 56, 455)
* Misner & Sharp 1964 (Physical Review B 136, 571)


It's also interesting to consider a variant of Clifford Wright's
question,
How does the electromagnetic field of a charged black hole
propagate across the event horizon?

The answer turns out to be basically the same: Charge is conserved,
so to form a charged black hole where there wasn't one before requires
collapsing charged matter, and the nonlinearity of GR (the Einstein-
-Maxwell equations) allows the coupled gravitational-electromagnetic
field to "remember" the field configuration from before the black hole
formed. I don't know of any reference where this is worked out in
detail. (N.b. I strongly suspect that this question has been analyzed
in detail; my previous sentence states only that *I* do not know of
a reference to such an analysis.)

ciao,

--
-- "Jonathan Thornburg [remove -animal to reply]"
Dept of Astronomy, Indiana University, Bloomington, Indiana, USA
"Washing one's hands of the conflict between the powerful and the
powerless means to side with the powerful, not to be neutral."
-- quote by Freire / poster by Oxfam

"Jonathan Thornburg [remove -animal to reply]"
wrote in
:

http://www.edu-observatory.org/physi...ckHoles/black_
gravity.html

Thanks for that very useful pointer Johnathan!
However it does bring up yet another point-
It can explain how a single "collapse" event can produce a specific
gravitaional field quite easily. But what about the case of later
mass infall into the now extant "black hole".
Is this material "remembered" together or separately? To be slightly
facietious!

In article ,
"Robert L. Oldershaw" writes:
The 14 January 2010 issue of Nature has the "Gone With The Wind" paper
by Governato et al in which the well-known "CDM central cusp problem"
is explained "naturally" by their hypothesis.

So there's at least one easy resolution to the problem, if it exists
at all and isn't just an artifact of low resolution in the CDM
simulations.

Notice, by the way, that the same problem exists (or not) whether the
CDM particle masses are micro-eV or up to many solar masses. The
only requirement as far as I can tell is that the particle masses
must be small compared to 10^5 solar masses, the resolution of the
model. Also, of course, that the only significant interactions of
CDM particles be gravitational.

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

In article ,
clifford wright writes:
I must confess I find it hard to see why CDM cannot form a accretion
disk. After all if it has any relative velocity to the singularity then
it must tend to spiral around it before being "swallowed up".

What dissipative force causes the "swallowing up?" Consider a
particle initially at a large distance from a black hole but with low
but non-zero angular momentum. It falls toward the black hole,
swings around, and then leaves on a near-parabolic orbit. No "spiral
around," no disk. (Of course if the particle comes very close to the
event horizon, it doesn't come out, but that only occurs if the
initial angular momentum was tiny indeed. Or to put it another way,
AGN accretion disks extend far outside the event horizon.)

any vortex is a purely gravitational effect.

What do you mean by that? Accretion disks are formed by viscous
forces. Otherwise how could they collapse to disks?

However the CDM enthusiasts have claimed that by far the greater
proportion of the mass of the universe is CDM.

Yes, for good reason.

Another interesting calculator is at
http://map.gsfc.nasa.gov/resources/camb_tool/index.html

If you can match the WMAP data without including dark matter, please
tell us how.

You might also want to do some reading on the "Bullet Cluster." What
exactly do you think the lensing mass is made of?

And that's not even to mention galaxy cluster velocity dispersions or
galaxy rotation curves. Or the light nuclide abundances as they
relate to baryonic dark matter.

along there with "gravitational waves".

And your explanation for the binary pulsar orbits is...?

In fact, it's hard for me to imagine any theory of gravity that
doesn't include gravitational waves. Think about how charge
acceleration makes electromagnetic waves. Why doesn't something very
similar happen gravitationally when any mass is accelerated?

--
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA

clifford wright asked
[[how does gravity get out of a black hole]]

I replied:
| http://www.edu-observatory.org/physi...ckHoles/black_
| gravity.html

Thanks for that very useful pointer Johnathan!
However it does bring up yet another point-
It can explain how a single "collapse" event can produce a specific
gravitaional field quite easily. But what about the case of later
mass infall into the now extant "black hole".
Is this material "remembered" together or separately? To be slightly
facietious!

I'm not quite sure what you're asking, but my best guess is that you're
asking "if I have a black hole of mass M1, and I then let an additional
mass M2 fall into it, does the gravitational field now 'remember' M1+M2
together, or does it 'remember' M1 and M2 separately?".

Basically, the answer is "M1+M2 together".

In more detail, let's again idealise the whole system as spherically
symmetric (SS), so M2 is a SS shell of mass which falls in through the
event horizon. Let's say that we start with an already-existing black
hole (BH) of mass M1, and that M2's (time-dependent) radius is R2(t).
(I'm idealising M2 as being very thin here, basically having a delta-fn
cross-section in radius.)

Then (given classical general relativity) we can easily work out that
anywhere outside R2(t), the gravitational field is just that of a single
non-rotating BH of mass M1+M2.

So, the field just has to 'remember' this as M2 falls into the BH.

--
-- "Jonathan Thornburg [remove -animal to reply]"
Dept of Astronomy, Indiana University, Bloomington, Indiana, USA
"Washing one's hands of the conflict between the powerful and the
powerless means to side with the powerful, not to be neutral."
-- quote by Freire / poster by Oxfam