Dark Matter Paradox / Black Hole Runaway

On 3/25/2013 8:27 PM, David Spain wrote:
However, my main issue is how does DM respond in the vicinity of a BH?
If it's really there, Swartzchild's calculations would have or should
have been way off.


I'd be a lot happier with the CDM hypothesis if it were re-framed
somewhat. Rather than think of it terms of discrete "matter", I'd prefer
to think of it in terms of a space-time "contour". Seen this way CDM
only presents at very, very large scales. As you continually reduce the
scale (volume) of the Universe under observation, essentially CDM fades
away. This would eliminate the issue I have with CDM "falling" into a
BH. The contours are essentially "straight". But at the scale of a
galaxy or esp. at a universe space-time exhibits the popular bands and
strands. I'd call this a "course-structure" model, but that'd probably
confuse the issue.

What's still missing is a QM explanation for why a CDM contour would
exist. If we can ever incorporate Gravitation into QM maybe we can start
to understand why.

Dave

In article ,
David Spain writes:
If a BH can form an accretion disk of ordinary matter why is
dark matter so selectively able to 'stay away'?

Because dark matter isn't affected by anything except gravity. A DM
particle comes in, and unless its orbit intersects the event horizon,
the particle goes right back out again, regardless of what other
particles may be nearby. (I'm over-simplifying a bit: the relevant
distance is a little larger than the event horizon but not much.) A
baryon, on the other hand, may hit another baryon, and orbits of both
get changed. In particular, motion parallel to the angular momentum
vector is damped out, forming a disk. Once a disk forms, viscosity
(baryon-baryon interaction) causes matter to flow inward, eventually
into the black hole. If baryons were collisionless, they wouldn't
form a disk or be accreted either.

An analogy for DM particles around a black hole is globular cluster
stars. Many GCs probably have black holes in the middle, but the
stars don't magically get sucked in. The stars just continue
orbiting unless (very rarely) one of the orbits is perturbed enough
to bring the star very close to the black hole.

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

Thanks for the reply Steve, your explanation helps a great deal.
A few comments tho...

On 3/26/2013 4:57 PM, Steve Willner wrote:
In article ,
David Spain writes:
If a BH can form an accretion disk of ordinary matter why is
dark matter so selectively able to 'stay away'?

Because dark matter isn't affected by anything except gravity. A DM
particle comes in, and unless its orbit intersects the event horizon,
the particle goes right back out again, regardless of what other
particles may be nearby. (I'm over-simplifying a bit: the relevant
distance is a little larger than the event horizon but not much.) A
baryon, on the other hand, may hit another baryon, and orbits of both
get changed. In particular, motion parallel to the angular momentum
vector is damped out, forming a disk.

Parallel? If you are referring to angular momentum relative to the BH,
don't you mean perpendicular?

Once a disk forms, viscosity
(baryon-baryon interaction) causes matter to flow inward, eventually
into the black hole. If baryons were collisionless, they wouldn't
form a disk or be accreted either.

An analogy for DM particles around a black hole is globular cluster
stars. Many GCs probably have black holes in the middle, but the
stars don't magically get sucked in. The stars just continue
orbiting unless (very rarely) one of the orbits is perturbed enough
to bring the star very close to the black hole.


I'm still stuck on the visualization of DM as 'particles'.
I think the problem is that DM is so 'extremely massive' it's hard to
think of it in terms of ordinary matter. I get your GC analogy (and
thanks for that as well it really helps!) however, there are several
real problems that remain. Sure DM can whiz past a BH in a
non-intersecting orbit, but there are many possible orbits. For example,
stellar pairings of stars and BH's trapped in their own mutual
gravitation are not uncommon. So why wouldn't BH's attract halo's of DM?
Especially If DM is far more common than matter? Would not this cause
measurable gravitational anomalies in the vicinity of a BH that we
should be able to observe?

I'm less unhappy if I think of DM not as 'particles' but as contours.
Contours that play out only along very very very large distances. Since
BH geometry is quite compact, it would help explain (to me anyway) why
we don't see strange BH/DM interactions.

To get a feel for what I'm talking about let's consider the problem from
a purely spacial perspective and "re-normalize". Think of the
circumference of a 'typical' BH event horizon being about the size of a
hand-held shot-put. Think of the circumference of a DM 'particle' being
the circumference of the Earth. From the BH perspective the DM
'particle' presents as a flat surface. From the Moon I can see the DM
'particle' as a discrete object and it would take mighty powerful optics
to see the shot-put at all.

Ignoring all the other differences, am I getting closer to a better analogy?

Dave

In article ,
David Spain writes:
Parallel? If you are referring to angular momentum relative to the BH,
don't you mean perpendicular?

The angular momentum _vector_ points toward the pole of rotation.
For a disk to form, the relevant angular momentum is that of the
baryons that make up the disk. Depending on the formation mechanism,
that might be close in direction to the black hole angular momentum,
but it doesn't always have to be.

I'm still stuck on the visualization of DM as 'particles'.

Can't help you with that; it works fine for me.

It might help to think about comets approaching the Sun. They come
in from all directions and at varying speeds, but very few hit the
Sun. Mostly they just go back out where they came from. If the Sun
were replaced by a black hole of the same mass, nothing much would
change except that the cross section for "hitting" would be a lot
smaller. Is that clear?

As you say later, having multiple bodies complicates things. Some
comets are perturbed by Jupiter or other planets into Sun-
intersecting orbits. However, some that would have intersected the
Sun are perturbed into orbits that miss. I doubt there's a
substantial net effect one way or the other, but it's beyond me to do
a detailed calculation. Dark matter particles should behave the same
way (except that radiation pressure and other non-gravitational
forces are zero).

The basic point is that a black hole isn't some kind of "cosmic
vacuum cleaner" that goes around sucking in everything in the
vicinity. It's just a gravitating body like any other except very
near the event horizon. Depending on how sensitively you can measure
and how close the orbit comes to the event horizon, there may be
measureable differences, but the overall appearance of orbits is the
same whether the central mass is a black hole or any other body.

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

On 3/13/2013 6:25 AM, Steve Willner wrote:
There's a nice movie at
http://www.mpa-garching.mpg.de/galform/millennium/

Thanks for posting this, I've been looking over this entire website
whenever I get a chance.

Some scholia:

1) Units of Gpc/h, Mpc/h and kpc/h not explained.

For those of us not steeped in this for a living I did a little
research. Gpc == Giga-parsecs, Mpc == Mega-parsecs kpc == kilo-parsecs.
The /h is a scalar correction in the range [0.5, 0.75] reflecting the
uncertainty in the value of the Hubble constant H for the rate of
expansion of the Universe. See Wikipedia:
http://en.wikipedia.org/wiki/Parsec

To give you a feel for the 'size' of a Mega-parsec, according to the
cited Wikipedia page, the Andromeda Galaxy is about 0.78 Mpc away from
the Earth.

2) Units of Gyr assumed to be Giga-years, i.e. 10**9 or as we say in the
US one billion years (thank your Dr. Sagan). As opposed to Phoenix
Goodyear Airports... See Wikipedia: http://en.wikipedia.org/wiki/Gyr

3) For the series of images which are taken as zoomed slices through the
density field, I'm presuming the redshift taken for each series can also
be presumed as how it would appear for various ages of the universe. For
example at a z=0 t=13.6 Gyr we are essentially looking through density
fields for the universe at its current age. At z=18.3 t=0.21Gyr we are
looking at density fields for a universe that is 'only' 210 million
years old.

Steve correct where needed.

Dave