Really big black holes

[David Holland]

I have a question about really big black holes. As I understand it as the mass of a black hole increases the density decreases. (Thats the density an outside observer would get if measured the mass and divided by the volume enclosed by the event horizon.) For a large enough black hole the density would be the same as a galaxy. Larger ones would have the same density as galactic cluster. Even larger ones would have the same density as the universe itself. Could these really large black holes exist? Would we be able to detect them, perhapse by their gravity? What would it look like from the inside? Would the matter in black hole with the density of a galactic cluster necessarily have to collapse into a singularity? Could you see out of it? If we were in one of these things would we even know it?

[neilzero]

I'm not sure there are mainstream answers to most of those questions as the largest super massive black holes known are (100?) times more massive than the one at the center of our galaxy, thus hypothesis about still larger ones are not testable. To the outside observer, the mass falling toward the the event horizon appears to decelerate toward zero when it is quite close, so everything inside will likely remain unknowable, to the point that some experts regard black holes as two dimensional = no volume = Surface area of the event horizon, but no volume. To me that is being extreme about the inside being unknowable. To me it seems reasonable that the in falling matter takes a reasonable length of time to travel from (in the reference frame of the in falling matter) the event horizon to the singularity, thus there is stuff throughout that volume. My guess it is very hot plasma for stellar mass black holes, but possibly habitable temperatures for black holes with the mass of a billion galaxies, if there were any that massive. So yes, low average density, unless you insist that there is no volume.
Possibly black holes with a mass of a billion galaxies would be detectable, by neither the outside nor the inside observer. I should think an outside observer could detect a powerful accretion disk, but possibly they would not conclude that it enclosed a black hole with a radius of several billion light years. I think I am correct that we could not see much past the Schwarzschild Horizon from either inside or outside. That spelling is likely wrong, as I hate honoring long ago scientists, instead of calling things by names which give a clue of what is being said. Neil

[Cougar]

Could these really large black holes exist?

Highly unlikely. Hai Fu at CalTech thinks there is evidence of a feedback mechanism that regulates both star formation and black hole growth. See his site and section on Black Hole Feedback & Extended Nebulae, along with linked papers on the subject.

[neilzero]

If the average density of the accretion disk is high collisions occur frequently, causing some of the stuff to be accreted, until the accretion disk density is low and collisions rare, so very little is accreted, unless new material is added to the accretion disk. Neil

[astromark]

I do not understand a limit on the mass of a Black Hole.

Its not unreasonable to assume that at the center of galaxies lies a BH.

That some have been calculated as being hundreds of times larger than this Galaxies giant BH.

Suggests there need not be any limit. I can imagine a several masses of a galaxy BH. Why not ?

[antoniseb]

... Suggests there need not be any limit. I can imagine a several masses of a galaxy BH. Why not ?

There is no reason why not... except practical reasons pertaining to how a black hole more than about 10 billion solar masses could have formed in our universe so far. When you read about a largest size, there is a presumption that the creation process blows away incoming material in a way that limits how much can actually join the black hole.

[astromark]

Now quietly digesting what I have just been told... Oh Wow... thats a nova event I would not want to see....Thanks for this.

[David Holland]

What I am imagining is not a something like the black hole at the center of a galaxy growing to a tremendous size. I'm thinking more along the lines of a cluster of galaxies dense enough that an event horizon forms around them. I suppose I should have asked what's the Schwartzchild radius of an object the mass of the Virgo Cluster, and would all the galaxies of the Virgo cluster fit in that volume. It seems like really big black holes would be easier to form than a small black hole because the densities required are less. There has to be a size of black hole where the density is the same as the average density of space. I'm guessing that the the Schwartzchild radius of such an object is less than the radius of the visible universe, if it was more we would be in a black hole and the universe would be closed, I think. I don't have the knowledge or math skills to figure it out.

[antoniseb]

... what's the Schwartzchild radius of an object the mass of the Virgo Cluster, and would all the galaxies of the Virgo cluster fit in that volume. ...

The mass of the Virgo cluster is about 10¹⁵ solar masses. The radius is about 3km/solar mass, so about 300 light years would be it for the mass of the Virgo cluster. I think this gives a density of billions (maybe a trillion) protons per cubic centimeter. I can't really imagine a process that would push/pull that much matter into such a small place, but my imagination doesn't prove anything either way.

[Grey]

What I am imagining is not a something like the black hole at the center of a galaxy growing to a tremendous size. I'm thinking more along the lines of a cluster of galaxies dense enough that an event horizon forms around them. I suppose I should have asked what's the Schwartzchild radius of an object the mass of the Virgo Cluster, and would all the galaxies of the Virgo cluster fit in that volume. It seems like really big black holes would be easier to form than a small black hole because the densities required are less. There has to be a size of black hole where the density is the same as the average density of space. I'm guessing that the the Schwartzchild radius of such an object is less than the radius of the visible universe, if it was more we would be in a black hole and the universe would be closed, I think. I don't have the knowledge or math skills to figure it out.

You can go here, and it will work out all the calculations for you! Antoniseb is right about the Virgo supercluster having a Schwarzschild radius of about 300 light years. That's a density of about 10 trillion protons per cubic centimeter. That's vastly higher than the mean density of the intergalactic medium, which is something like 10 to 100 protons per cubic meter.

[David Holland]

Thanks antoniseb. That pretty much answers my questions. The really big, low density black holes I was wondering about are probably too big to actually exist. Something the size of a galaxy cluster isn't even close. It also looks like the math wasn't as hairy as I had imagined.

[tommac]

low density ???

Arent the EH of a black hole defined as the max possible information density possible?

Thanks antoniseb. That pretty much answers my questions. The really big, low density black holes I was wondering about are probably too big to actually exist. Something the size of a galaxy cluster isn't even close. It also looks like the math wasn't as hairy as I had imagined.

[tommac]

From what I understand the volume of a black hole is defined by the area of its EH, which in turn is defined as the maximum information density ( max number of bits per volume ).

I have a question about really big black holes. As I understand it as the mass of a black hole increases the density decreases. (Thats the density an outside observer would get if measured the mass and divided by the volume enclosed by the event horizon.) For a large enough black hole the density would be the same as a galaxy. Larger ones would have the same density as galactic cluster. Even larger ones would have the same density as the universe itself. Could these really large black holes exist? Would we be able to detect them, perhapse by their gravity? What would it look like from the inside? Would the matter in black hole with the density of a galactic cluster necessarily have to collapse into a singularity? Could you see out of it? If we were in one of these things would we even know it?

EDIT: found this at: http://imagine.gsfc.nasa.gov/docs/as...rs/970808.html

What is the volume of a black hole?

Our intuitive sense of volume breaks down in the strong gravitational region in a black hole. So while the "size" of a black hole is given by the radius of its event horizon, it's volume is not determined by the usual 4/3*pi*r3. Instead, relativity makes it more complicated than that. As you pass the event horizon, the spatial direction 'inwards' becomes 'towards the future'-- you WILL reach the center, it's as inevitable as next Monday. The direction outsiders think of as their future becomes a spatial dimension once you are inside. The volume of a black hole, therefore, is its surface area times the length of time the hole exists (using the speed of light to convert from seconds to meters). Since a black hole last practically forever, the black hole's volume is almost infinite. (This is also a way of explaining the fact that you can pour stuff into a black hole forever and never fill it up. Another reason why black holes never fill up is that the radius of the event horizon increases as the mass of the black hole increases.)

[David Holland]

Tommac,
That's why I specified how I was measuring the density and volume. I knew it wasn't the the actual density and volume of the space inside the event horizon. What I didn't know was that the volume is the surface times the length of time the hole exists. Thanks for that information.

[Jeff Root]

Grey,

I take it that the figure of 10 to 100 protons per cubic meter includes
the matter in the galaxies. Is that correct? Meaning that if the matter
in the stars were spread out again into a uniform gas, it would be
astonishingly thin.

-- Jeff, in Minneapolis