The Lemon Fool

Ah well.
They have replaced a Black Hole - a beastie we know at least something about - with Dark Matter that we know almost nothing about!

stewamax wrote:Ah well.
They have replaced a Black Hole - a beastie we know at least something about - with Dark Matter that we know almost nothing about!

Since dark matter is supposed to react to gravity in the same way as ordinary matter shouldn't the mass of dark matter collapse into a black hole anyway ? To have the same effect on the closest orbiting stars the dark matter would need to have the same mass and radius and hence the same density as the black hole which should surely make it a black hole.

ursaminortaur wrote:

stewamax wrote:Ah well.
They have replaced a Black Hole - a beastie we know at least something about - with Dark Matter that we know almost nothing about!

Since dark matter is supposed to react to gravity in the same way as ordinary matter shouldn't the mass of dark matter collapse into a black hole anyway ? To have the same effect on the closest orbiting stars the dark matter would need to have the same mass and radius and hence the same density as the black hole which should surely make it a black hole.

I know zip about this sort of stuff, but find it very interesting.

But as you say it's called Dark Matter for a reason. It's matter.

AiY

ursaminortaur wrote:

stewamax wrote:Ah well.
They have replaced a Black Hole - a beastie we know at least something about - with Dark Matter that we know almost nothing about!

Since dark matter is supposed to react to gravity in the same way as ordinary matter shouldn't the mass of dark matter collapse into a black hole anyway ? To have the same effect on the closest orbiting stars the dark matter would need to have the same mass and radius and hence the same density as the black hole which should surely make it a black hole.

As ordinary gas collapses it heats up. Eventually a high enough temperature will generate high pressure which would stop the collapse. The gas has to lose energy through radiation before the collapse can continue. There is a limit on how fast an accretion disk can lose energy and so how fast a black hole can grow.

Dark matter does not radiate. It may or may not have other ways to lose the energy that would prevents a black hole from forming.

At least that's one theory, but no one really knows what dark matter is or does.

9873210 wrote:

ursaminortaur wrote:

stewamax wrote:Ah well.
They have replaced a Black Hole - a beastie we know at least something about - with Dark Matter that we know almost nothing about!

Since dark matter is supposed to react to gravity in the same way as ordinary matter shouldn't the mass of dark matter collapse into a black hole anyway ? To have the same effect on the closest orbiting stars the dark matter would need to have the same mass and radius and hence the same density as the black hole which should surely make it a black hole.

As ordinary gas collapses it heats up. Eventually a high enough temperature will generate high pressure which would stop the collapse. The gas has to lose energy through radiation before the collapse can continue. There is a limit on how fast an accretion disk can lose energy and so how fast a black hole can grow.

Dark matter does not radiate. It may or may not have other ways to lose the energy that would prevents a black hole from forming.

At least that's one theory, but no one really knows what dark matter is or does.

The defining characteristic of a black hole is that its escape velocity Ve = (2GM/r)^0.5 is greater than the speed of light. That depends upon the density (M/r) and, as far as I am aware*, the limits on radius and mass of Sagittarius A* provided by studying closely orbiting stars show that it satifies the criteria for its escape velocity to be greater than the speed of light. Hence even if Sagittarius A* were made of dark matter its density would still be such that it had an escape velocity greater than the speed of light and hence would be a black hole.

Note. It doesn't matter whether the matter inside actually collapses to a singularity or not - we assume with ordinary matter that it does because we don't know of any force which could oppose that collapse but we also don't know that such a force doesn't exist. Hence whether or not dark matter would collapse to a singularity similarly doesn't matter - so long as the mass and radius are enough to make the escape velocity greater than the speed of light it would still be a black hole.

* of course these researchers may have better information about the mass and radius which may indicate that there is some leeway for the escape velocity to be less than the speed of light. However there have been other explanations for why G2 wasn't destroyed in its last pass such as it containing a star rather than being just a gas cloud.

https://en.wikipedia.org/wiki/Sagittarius_A*#Discovery_of_G2_gas_cloud_on_an_accretion_course

Other astronomers suggested the gas cloud could be hiding a dim star, or a binary star merger product, which would hold it together against the tidal forces of Sgr A*, allowing the ensemble to pass by without any effect.

ursaminortaur wrote:
The defining characteristic of a black hole is that its escape velocity Ve = (2GM/r)^0.5 is greater than the speed of light. That depends upon the density (M/r) and, as far as I am aware*, the limits on radius and mass of Sagittarius A* provided by studying closely orbiting stars show that it satifies the criteria for its escape velocity to be greater than the speed of light. Hence even if Sagittarius A* were made of dark matter its density would still be such that it had an escape velocity greater than the speed of light and hence would be a black hole.

I am pretty sure the observational limits are not that tight. To verify the density is high enough for it to be a black hole requires observations very near the event horizon. This would require observing the accretion disk, not nearby stars. To the best of my knowledge we have not resolved this for Sagittarius A*.

AIUI the closest observed stars where many Schwarzschild radii out. S2 is about 1400 Schwarzschild radii at closest approach. The conclusion that it is a black hole is drawn because there is nothing else we know of that could match the observed limits. But Dark Matter is not something we know much about so there is room for the unexpected.

ursaminortaur wrote:Since dark matter is supposed to react to gravity in the same way as ordinary matter shouldn't the mass of dark matter collapse into a black hole anyway ? To have the same effect on the closest orbiting stars the dark matter would need to have the same mass and radius and hence the same density as the black hole which should surely make it a black hole.

That's what I was thinking...

But do we know if this is true? "dark matter is supposed to react to gravity in the same way as ordinary matter"

We know ordinary matter is affected by gravity from dark matter - that's how it was discovered - but do we know it works the same way the other way around? Or that dark matter is affected by gravity in the same way as ordinary matter? I have not the slightest idea...

Reading the comments to that article is somewhat depressing, IMO.

cantdrive85
Jun 03, 2021
Black hole matter? Black holes matter! Dark holes matter! Dark hole matter?

It's all faerie tales and lies just as with the Marxist BLM.

But then, going back to the 2014 article where the unexpected result was first reported, it is no less depressing to see in the comments the usual list of self-appointed 'experts' lining up to take pot-shots. Whatever for? :

https://phys.org/news/2014-07-explanation-lack-gas-cloud-interaction.html

'Culture Wars'? - "Ain't nobody getting out of here alive..."

XFool wrote:

ursaminortaur wrote:Since dark matter is supposed to react to gravity in the same way as ordinary matter shouldn't the mass of dark matter collapse into a black hole anyway ? To have the same effect on the closest orbiting stars the dark matter would need to have the same mass and radius and hence the same density as the black hole which should surely make it a black hole.

That's what I was thinking...

But do we know if this is true? "dark matter is supposed to react to gravity in the same way as ordinary matter"

We know ordinary matter is affected by gravity from dark matter - that's how it was discovered - but do we know it works the same way the other way around? Or that dark matter is affected by gravity in the same way as ordinary matter? I have not the slightest idea...

Our best theory of gravity is General Relativity which just makes gravity into a pseudo force caused by the curving of space-time by mass.
Hence, as long as dark matter, is just made up of some sort of particles which have mass then they should both curve space in the same way as ordinary matter particles with mass and also react to that curvature in the same way as ordinary matter particles ie both normal and dark matter should create gravitional attraction and should react to that attraction in the same way.

(Of course, we know that General Relativity is not a complete theory of everything so that picture of mass curving space-time may be just approximately right in a similar way to Newton's theory of gravity being approximately correct. Thus we can't be 100% sure that dark matter reacts to gravity in the same way as normal matter but that is what our best theory says.)

Just to throw another bit of information into the mix for those that might understand more than me about the matter;

https://www.theguardian.com/science/202 ... r-einstein

“Scientists are interested in these structures because they suspect that gravity may behave very differently inside them.”

Sorry, bit late back to this one, but since there seems to be some interest in the topic

New study suggests supermassive black holes could form from dark matter

https://phys.org/news/2021-02-supermass ... -dark.html

A new theoretical study has proposed a novel mechanism for the creation of supermassive black holes from dark matter. The international team find that rather than the conventional formation scenarios involving 'normal' matter, supermassive black holes could instead form directly from dark matter in high density regions in the centers of galaxies.

SteMiS wrote:Sorry, bit late back to this one, but since there seems to be some interest in the topic

New study suggests supermassive black holes could form from dark matter

https://phys.org/news/2021-02-supermass ... -dark.html

A new theoretical study has proposed a novel mechanism for the creation of supermassive black holes from dark matter. The international team find that rather than the conventional formation scenarios involving 'normal' matter, supermassive black holes could instead form directly from dark matter in high density regions in the centers of galaxies.

I seem to remember seeing a TV program a few years ago about the problems with how quickly super massive black holes formed in the early universe which included direct collapse of dark matter into black holes as one suggested solution. The other solution suggested was dark stars - stars made from a mixture of normal hydrogen and dark matter which were much larger than normal since they were powered by dark matter annihilation.

https://astronomy.com/magazine/2018/09/dark-stars-come-into-the-light

Someone mentioned Dark Matter being 'discovered' surely inferred?

As far as I'm aware despite a huge amount of effort expended no one has proven it's existence. It along with Dark Energy remain little more that number put into calculations that then allow those calculation to model what is observed.

Great low budget film, Dark Star.

John Carpenter, 1974

https://youtu.be/ocse-0bBfo8

GrahamPlatt wrote:Great low budget film, Dark Star.

John Carpenter, 1974

https://youtu.be/ocse-0bBfo8

And some great music with the same name: https://www.youtube.com/watch?v=-Xic-CHInek

--kiloran

More interesting material on the subject

https://www.scientificamerican.com/arti ... black-hole

In particular this embedded link: “Various astrophysical constraints rule out PBHs as the dark matter if they have either low or high masses” (https://arxiv.org/pdf/2006.02838.pdf).

So what’s at the centre of the Milky Way isn’t a PBH, but may have been seeded by one, dark matter or not.

ursaminortaur wrote:I seem to remember seeing a TV program a few years ago about the problems with how quickly super massive black holes formed in the early universe which included direct collapse of dark matter into black holes as one suggested solution. The other solution suggested was dark stars - stars made from a mixture of normal hydrogen and dark matter which were much larger than normal since they were powered by dark matter annihilation.

What would cause the "dark matter annihilation"? Or do we have to envisage anti-dark matter as well?

XFool wrote:

ursaminortaur wrote:I seem to remember seeing a TV program a few years ago about the problems with how quickly super massive black holes formed in the early universe which included direct collapse of dark matter into black holes as one suggested solution. The other solution suggested was dark stars - stars made from a mixture of normal hydrogen and dark matter which were much larger than normal since they were powered by dark matter annihilation.

What would cause the "dark matter annihilation"? Or do we have to envisage anti-dark matter as well?

That’d just be matter as we know it, Jim.

GrahamPlatt wrote:

XFool wrote:

ursaminortaur wrote:I seem to remember seeing a TV program a few years ago about the problems with how quickly super massive black holes formed in the early universe which included direct collapse of dark matter into black holes as one suggested solution. The other solution suggested was dark stars - stars made from a mixture of normal hydrogen and dark matter which were much larger than normal since they were powered by dark matter annihilation.

What would cause the "dark matter annihilation"? Or do we have to envisage anti-dark matter as well?

That’d just be matter as we know it, Jim.

(Assuming you are not jesting)
Would it, why? AFAIK 'normal' anti-matter is not dark matter, so why should anti-dark matter (if it exists) be ordinary matter? Also, normal anti-matter is - just like ordinary matter - subject to electromagnetic forces. Unlike dark matter - which is why it is dark.