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noncryptic
2010-Mar-31, 12:17 PM
Astronomers Find Black Holes Do Not Absorb Dark Matter
March 22nd, 2010
http://www.universetoday.com/category/black-holes/

...

"The researchers modeled the way in which the dark matter is absorbed by black holes and found that the rate at which this happens is very sensitive to the amount of dark matter found in the black holes’ vicinity. If this concentration were larger than a critical density of 7 Suns of matter spread over each cubic light year of space, the black hole mass would increase so rapidly, hence engulfing such large amounts of dark matter, that soon the entire galaxy would be altered beyond recognition.

“Over the billions of years since galaxies formed, such runaway absorption of dark matter in black holes would have altered the population of galaxies away from what we actually observe,” said Hernandez.

Their work therefore suggests that the density of dark matter in the centers of galaxies tends to be a constant value. By comparing their observations to what current models of the evolution of the Universe predict, Hernandez and Lee conclude that it is probably necessary to change some of the assumptions that underpin these models – dark matter may not behave in the way scientists thought it did."

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So now it's "dark matter does not interact with normal matter except gravitationally - except in the case of black holes."

This is an exiting time for cosmology.


The finding seems to contradict earlier findings that say dark matter played a crucial role in the formation of black holes in the early universe.

Did 'Dark Gulping' Generate Black Holes in Early Universe?
april 23 2009
http://www.physorg.com/news159695546.html

grant hutchison
2010-Mar-31, 02:59 PM
Astronomers Find Black Holes Do Not Absorb Dark MatterWhat a misleading title from UT!
That's not actually what the paper (http://arxiv.org/abs/1002.0553) reports. The assumption is that dark matter does fall into black holes, same as everything else. But the observed mass of galactic black holes constrains how much dark matter they must have absorbed, which sets constraints in turn on how dark matter is distributed within galaxies.

Grant Hutchison

noncryptic
2010-Mar-31, 03:56 PM
What a misleading title from UT!


And layman's interpretation (mine) does the rest.

Ok, so it's not as bad as it seemed at first.

Still pretty weird though, how dm hangs around concentrations of 'normal' matter (galaxies), but seems to avoid areas with the largest concentration (galactic plane and -core) - but only slightly more weird than what was known previously.

loglo
2010-Apr-02, 03:20 PM
And layman's interpretation (mine) does the rest.

Ok, so it's not as bad as it seemed at first.

Still pretty weird though, how dm hangs around concentrations of 'normal' matter (galaxies), but seems to avoid areas with the largest concentration (galactic plane and -core) - but only slightly more weird than what was known previously.


Since the Dark Matter particles do not interact electromagnetically there is no significant way for them to lose their momentum, therefore they don't clump together at the centre as that is where they have the greatest momentum.

WaxRubiks
2010-Apr-02, 04:03 PM
could it be one interpretation that when dark matter falls into a black hole, that it just flies out the other side?

just a shot in the dark.

grant hutchison
2010-Apr-02, 04:10 PM
could it be one interpretation that when dark matter falls into a black hole, that it just flies out the other side?It's matter. It interacts gravitationally with the rest of the Universe, just like baryonic matter. A black hole works the same for dark matter as it does for the rest of us.

Grant Hutchison

WaxRubiks
2010-Apr-02, 04:37 PM
It's matter. It interacts gravitationally with the rest of the Universe, just like baryonic matter. A black hole works the same for dark matter as it does for the rest of us.

Grant Hutchison

yes, but would DM flying right back out again, have the effect, of what is quoted in the OP(which I'm still not sure about after what people have said), hypothetically?

noncryptic
2010-Apr-03, 11:17 AM
yes, but would DM flying right back out again, have the effect, of what is quoted in the OP(which I'm still not sure about after what people have said), hypothetically?

That would require dm to be able to go faster than the speed of light, and there'd have to be good reason (observations) to think that it can. Iow: that hypothesis violates existing theories, it creates more problems than it solves.

WaxRubiks
2010-Apr-03, 11:34 AM
That would require dm to be able to go faster than the speed of light, and there'd have to be good reason (observations) to think that it can. Iow: that hypothesis violates existing theories, it creates more problems than it solves.

yes, well maybe the existing theories about black holes are not quite correct....I just want to know whether DM passing right through a black hole and out the other side would explain the observations in the article quoted in the OP.

If dark matter could swing through a BH in the same way as it might be able to swing through the Earth's gravity well.

Tensor
2010-Apr-03, 02:20 PM
yes, well maybe the existing theories about black holes are not quite correct....I just want to know whether DM passing right through a black hole and out the other side would explain the observations in the article quoted in the OP.

If dark matter could swing through a BH in the same way as it might be able to swing through the Earth's gravity well.

The dark matter swings through the BH's gravity well, not the BH. The article talks about the orbits of the dark matter particles. In the introduction it mentions this:

"...but only in the accretion of particles on capture orbits. While readily acreted (sic), they con- stitute only a minor fraction of those available in the dis- tribution function of halo particles."

Note the bold. It specifically mentions that the dark matter is readily accreted by the black hole when the particles are in capture orbits. What this paper is studying is this:

"Here we consider only the accretion of unbound particles, through the absorption cross section presented by the black hole through its event horizon,..."

Unbound particles are particles that have hyperbolic orbits and wouldn't normally be captured by the BH. However, as the mass of the BH grows, the size of the event horizon grows. This causes an increase in cross section of the BH. Allowing it to capture more of the particles, as orbits that were once far enough away to swing by the BH are now close enough to the BH to enter the BH. It has nothing to do with the properties of the dark matter.

They only have to look at the orbits and paths of the dark matter as dark matter wouldn't interact with the accretion disk, as normal matter would. The accretion disk can slow normal matter, by interaction with it, and thus allow the BH to capture normal matter, even if it would normally miss the BH.

Please note that the accretion of the dark matter is when the dark matter enters the BH and is separate from the accretion disk.

transreality
2010-Apr-06, 03:41 AM
Is it possible to quantify how much greater is the cross section of a blackhole and its accretion disc to normal matter, than the blackholes cross section alone to dark matter is? Presumeably only the event horizon is contributing to the capture area of a blackhole to DM.

Tensor
2010-Apr-06, 02:58 PM
Is it possible to quantify how much greater is the cross section of a blackhole and its accretion disc to normal matter, than the blackholes cross section alone to dark matter is? Presumeably only the event horizon is contributing to the capture area of a blackhole to DM.

It would depend on the size of the accretion disk. The larger the disk, the larger the cross section. Of course, it also depends on orbit the particle is traveling on. The accretion disk tends to be flattened, much like the the rings of Saturn (I'm talking general shape here). If the orbit doesn't intersect the disk, then the disk doesn't affect the cross section of the BH for that particular particle.