Dark matter avoidance of galatic centres?

Thus spake Steve Willner
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.

The problem is that their solution requires interaction between ordinary
matter and dark matter which is postulated not to exist. hmmm.

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.

contradicting the "easy resolution to the problem"

Regards

--
Charles Francis
moderator sci.physics.foundations.
charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and
braces)

http://www.rqgravity.net

Thus spake Steve Willner
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?"

obviously because it is "observed", or rather calculated from lensing
and rotation curves to have a different distribution from visible
matter.

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?

perhaps because of the absence of dark matter in central regions of
galaxies?


I don't see how this would follow. If dark matter interacts only
gravitationally, it can't form an accretion disk.

Odd that you say that. Accretion discs are down to gravity and
centrifugal force.

I think you have some misconceptions about dark matter, but I'm not
sure just which ones.

That's easy at least. Everything we know about dark matter is
misconception.


Regards

--
Charles Francis
moderator sci.physics.foundations.
charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and
braces)

http://www.rqgravity.net

Oh No wrote:
Thus spake Steve Willner
In article ,
clifford wright writes:

I don't see how this would follow. If dark matter interacts only
gravitationally, it can't form an accretion disk.

Odd that you say that. Accretion discs are down to gravity and
centrifugal force.

Accretion requires the shedding of kinetic energy.
With normal matter this happens via the electromagnetic
force (friction leading to heat radiation). This is
why accretion disks are observed to be hot.

[[I think I've unwrapped all the nested quoting correctly, but if I've
erred and attributed anyone's remarks to someone else, my apologies!]]

clifford wright wrote:
If dark matter interacts only
gravitationally, it can't form an accretion disk.

Oh No wrote:
Odd that you say that. Accretion discs are down to gravity and
centrifugal force.

Greg Neill wrote:
Accretion requires the shedding of kinetic energy.
With normal matter this happens via the electromagnetic
force (friction leading to heat radiation). This is
why accretion disks are observed to be hot.

To expand on what Greg Neill wrote:

Imagine following a chunk of matter which starts out at the outside
of an accretion disk, and eventually gets to the inside and falls into
the central object. During this process it needs to shed a lot of
gravitational binding energy (as Greg noted, this is why accretion
sisks are hot).

In addition, our chunk of matter also needs to shed a lot of angular
momentum. (That is, it had much more angular-momentum-per-unit-mass
at the start of our observation than it had at the end.) Since it's
hard to "radiate" large amounts of angular momentum, what actually
must happen is that the angular momentum gets transported *outwards*,
i.e., transferred to matter which is farther out in the accretion
disk. This "transfer" presumably happens via viscosity and/or
magnetic fields, but the details are alas messy and hard to model.


In the present context, the key point is that the phrase "dark matter"
means stuff which basically only interacts gravitationally, so it won't
have any way to quickly shed large amounts of gravitational binding
energy or angular momentum. In other words, dark matter isn't going
to form an accretion disk.

--
-- "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

Thus spake Greg Neill
Oh No wrote:
Thus spake Steve Willner
In article ,
clifford wright writes:

I don't see how this would follow. If dark matter interacts only
gravitationally, it can't form an accretion disk.

Odd that you say that. Accretion discs are down to gravity and
centrifugal force.

Accretion requires the shedding of kinetic energy.
With normal matter this happens via the electromagnetic
force (friction leading to heat radiation). This is
why accretion disks are observed to be hot.

Ok. I was thinking after I posted that this might be the case (it wasn't
specifically mentioned in the reference I looked at, but that reference
isn't 100% authoritative). But then I still had two problems. One, that
this contradicts the notion that supernovae can be responsible for
driving CDM out of the centre of gravity, as suggested in the Nature
paper refed in this thread, and two, that without doing a simulation I
can't actually be sure of the result. Accepted that the existing
simulations of accretion discs should include frictional forces, but are
we sure that a simulation without frictional forces would not still
result in a disc? Intuitively it seems to me that gravity plus
centrifugal force could still create a disc, even if kinetic energy is
not lost due to friction.

Regards

--
Charles Francis
moderator sci.physics.foundations.
charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and
braces)

http://www.rqgravity.net

Thus spake Jonathan Thornburg
[[I think I've unwrapped all the nested quoting correctly, but if I've
erred and attributed anyone's remarks to someone else, my apologies!]]

clifford wright wrote:
If dark matter interacts only
gravitationally, it can't form an accretion disk.

Oh No wrote:
Odd that you say that. Accretion discs are down to gravity and
centrifugal force.

Greg Neill wrote:
Accretion requires the shedding of kinetic energy.
With normal matter this happens via the electromagnetic
force (friction leading to heat radiation). This is
why accretion disks are observed to be hot.

To expand on what Greg Neill wrote:

Imagine following a chunk of matter which starts out at the outside
of an accretion disk, and eventually gets to the inside and falls into
the central object. During this process it needs to shed a lot of
gravitational binding energy (as Greg noted, this is why accretion
sisks are hot).

In addition, our chunk of matter also needs to shed a lot of angular
momentum. (That is, it had much more angular-momentum-per-unit-mass
at the start of our observation than it had at the end.) Since it's
hard to "radiate" large amounts of angular momentum, what actually
must happen is that the angular momentum gets transported *outwards*,
i.e., transferred to matter which is farther out in the accretion
disk. This "transfer" presumably happens via viscosity and/or
magnetic fields, but the details are alas messy and hard to model.


In the present context, the key point is that the phrase "dark matter"
means stuff which basically only interacts gravitationally, so it won't
have any way to quickly shed large amounts of gravitational binding
energy or angular momentum. In other words, dark matter isn't going
to form an accretion disk.

This describes baryonic matter falling into the central object, and the
mechanisms you describe are necessary to model observed discs, but I
thought we were describing matter which remains in an orbit in the disc,
and dark matter we cannot observe directly. For matter to remain in the
disc, it does not need to lose kinetic energy, or angular momentum,
though the angular momentum vector does have to change to align with the
disc axis. It is not obvious to me that gravitational forces cannot
cause this to happen. Certainly they can perturb the angular momentum
vector, but I think a pretty good numerical model would be needed to say
what should happen.

Clearly the absence of friction will result in a different mass
distributions for dark matter and baryonic matter, but we do know that
no model results in the matter distributions required for either flat
rotation curves or observed lensing, and that these two observations
require inconsistent mass distributions.

Regards

--
Charles Francis
moderator sci.physics.foundations.
charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and
braces)

http://www.rqgravity.net

In article ,
Oh No writes:
this contradicts the notion that supernovae can be responsible for
driving CDM out of the centre of gravity, as suggested in the Nature
paper refed in this thread,

[I'm not sure what the antecedent to your "this" was, but perhaps the
following is a partial answer.]

You don't think the normal matter has a gravitational effect on the
dark matter? Of course that effect is usually trivial because the
normal matter is a trivial fraction of the total mass, but the Nature
paper showed that's not always the case.

Intuitively it seems to me that gravity plus
centrifugal force could still create a disc, even if kinetic energy is
not lost due to friction.

You don't need a simulation. This is essentially the same problem as
solar system formation, and the basics were understood 200 years ago.
Initially the particles (interstellar gas or dark matter) have a more
or less isotropic velocity distribution. In order to get rid of the
z velocity, you need friction at each disk-crossing. Precession
won't do it.

For the case of dark matter particles around a black hole, the black
hole has trouble even capturing them into an orbit. Basically the
particles either zip by unaffected (except for a change in the
direction of their velocity vectors), or if their angular momentum is
low enough, they fall into the black hole.

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

Thus spake Steve Willner
In article ,
Oh No writes:
this contradicts the notion that supernovae can be responsible for
driving CDM out of the centre of gravity, as suggested in the Nature
paper refed in this thread,

[I'm not sure what the antecedent to your "this" was, but perhaps the
following is a partial answer.]

You don't think the normal matter has a gravitational effect on the
dark matter?

I don't know where you got that idea from.

Of course that effect is usually trivial because the
normal matter is a trivial fraction of the total mass, but the Nature
paper showed that's not always the case.

Intuitively it seems to me that gravity plus
centrifugal force could still create a disc, even if kinetic energy is
not lost due to friction.

You don't need a simulation. This is essentially the same problem as
solar system formation, and the basics were understood 200 years ago.

The process is not properly understood even now.

Initially the particles (interstellar gas or dark matter) have a more
or less isotropic velocity distribution.

and you know this how? It rather contradicts observations of structure
in the early universe.

In order to get rid of the
z velocity, you need friction at each disk-crossing. Precession
won't do it.

For the case of dark matter particles around a black hole, the black
hole has trouble even capturing them into an orbit. Basically the
particles either zip by unaffected (except for a change in the
direction of their velocity vectors), or if their angular momentum is
low enough, they fall into the black hole.

So, according to you cold dark matter is so hot that it will not be
captured by a strong gravitational field?

Regards

--
Charles Francis
moderator sci.physics.foundations.
charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and
braces)

http://www.rqgravity.net