Dark matter and black hole.

Is it true that dark matter can't form black hole ?

http://www.forbes.com/sites/startsw...nt-dark-matter-form-black-holes/#7d134bae60d0

 

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And if a black hole takes same amount dark matters then what would be ?

 

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Forming a black hole is a question of density. As the article points out, it is very hard for dark matter to get as dense as the matter we normally run into.

In the current cosmological era, here may be situations where dark matter enters a black hole or is present when a black hole is formed, but it isn't providing the density required.

In the distant past, when the universe was far denser, there may have been black holes created by dark matter. These black holes may have since evaporated or they may be out there somewhere.

 

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I think there is some issue here, forming a black hole is not a matter of density in general. Our know-how limits ourselves till EH, once the object is beneath its schwarzschild radius, a BH is formed. We speak of BH density in reference to a sphere of schwarzschild radius, in this case density is inversely proportional to mass squared, that means higher the mass, lower the density. For a billion solar mass BH, the density would be less than water density.

 

As far as established physics is concerned, density is about all that matters.

The radius is a function of mass, which means that it is a measure of density.

 

Forming a BH is most certainly a matter of density and overcoming such barriers as EDP and NDP.
Once that is achieved and the Schwarzchild radius is reached, then compulsory collapse ensures and a BH is formed with an EH, corresponding with the Schwarzchild radius.
From that instant speaking of the BH density itself, does not make a whole lot of sense, since in effect, all a BH is, is critically curved spacetime, with a mass at its core.

 

Please be clear.
Considering at event horizon, a BH can have any density, say from less than water to as high as nucleus type. I am not able to figure out how density is about all that matters for BH.

 

May I ask if matter follows a compulsory collapse at/after the event horizon and compacts to something very small....would not the density anywhere below the horizon and down to the surface of the small central part be zero.
There is no mass in this volume.
What do I miss?
Alex

 

Once a BH is formed, speaking of density does not make much sense as previously outlined.

 

As PhysBang clearly said,

"Forming " a BH is certainly a question of density.
But once a BH is formed, speaking of density does not make much sense as previously outlined.

 

As the article points out... Dark matter doesn't interact
with electromagnetic radiation so how can it(dark matter) evaporate itself from the black hole ?( I mean how can it transform itself into energy ?)

May be...
Does strong gravitational force of black hole or other reasons help to decompose this dark matter ?

 

Look up Hawking Radiation.

 

When you say that forming a BH is certainly a question of density, that implies that an object must have certain density when it is just at EH to become a BH. But as I said a prospective BH object, can have any density when just at EH (as low or even lower than water to as high as nucleus), so there is no question of any density requirement or condition.

 

I am not sure but I have not seen any work on evaporation of dark matter through Hawking radiation. Loosely speaking anti particle neutralises the baryonic mass of a BH under Hawking radiation (The exact concept is different, but this is sufficient for understanding). Nowhere I have seen that DM mass of a BH is neutralized, unless you are creating a DM antiparticle under HR.

 

You need to stop pretending to read physics and take the time to read some physics.

 

Can you clarify how density matters for formation of BH?

And also how DM BH evaporates?

You do not have to attack me or behave rudely, just answer these two questions please. I will read your answers.

 

As you should have read in many sources, a black hole forms after a certain amount of mass is contained in a certain volume (with some additional peculiar factors absent). This means that a certain density is reached within a certain volume.

I can't tell you that one. But if DM interacts with anything, then it might be able to evaporate.

 

I am sorry Physbang, you are just pushing this density issue. BH formation has nothing to do with density, there is no question of any "certain density" requirement for a BH formation. A 10^17 kg cubic meter or 20 kg cubic meter, both objects can be just at EH, just before forming a BH.

You claimed in your earlier post that DM BH could have evaporated. Now you are saying you cannot tell and if it interacts then it might evaporate. DM is not yet known to interact other than gravitationally. You just freaked out.

Without being rude to you, I must say you need to spend sometime in reading Physics.

 

Of course it does. Lets look at how a black hole is formed. It forms where the mass of a star is compressed into a small enough volume that the gravitational field at the surface of the mass exceeds the escape velocity of light. That clearly mean that the density has increased. A star that does not have enough mass to make a black hole will form a neutron star. The density of a neutron star is about 10^17 kg/m^3. For a black hole to form the density must be higher than that.

Hopefully you will see how obvious this is and move on.

 

I believe you should stop indulging in fabricated nonsense.
To form a BH is certainly a matter of density.

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Quite ironic to say the least. You must be talking to yourself, or perhaps watching your reflection in a mirror.