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How Do Black Holes Grow?

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#26 Mister T

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Posted 27 July 2013 - 10:38 AM

>>I think the consensus is that eventually all the matter in the Universe will be collected in Black Holes. These Black Holes (if Hawking is right) will evaporate leaving a Universe filled with the background radiation shadow of our existence...<<

would it be possible that this radiation shadow would be the CMBR of the "next" universe?? :question:

#27 Jarad

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Posted 27 July 2013 - 11:29 AM

And 10+ billion years affords ample opportunity for BH's to form anew, grow, accumulate, and merge.



No question about there being plenty of time for them to form now. But some of the distant quasars we see require them to already be in the millions of solar masses much earlier. The question is how did those form so fast?

Some mechanism to funnel large amounts of gas to them relatively quickly, or perhaps in the very early universe some were able to form from a large cloud of collapsing gas before fusion ignition?

Jarad

#28 GlennLeDrew

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Posted 27 July 2013 - 01:58 PM

Ah yes, those earlier epoch quasars. They do suggest a pretty rapid building of super massive black holes. I imagine the rather higher overall mass density, the still-early phases of galaxy formation and the likely frenetic pace of star formation are contributing factors. And this obviously suggests at least some of 'today's' big BH's in galactic nuclei could well be ancient, now relatively quiet by contrast and not accreting mass at near the rate as in their youth...

#29 EJN

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Posted 27 July 2013 - 03:51 PM

At the July meeting of our astronomy club, the speaker was Dr. Laura Trouille,
a postdoctoral fellow at nearby Northwestern University. Her field of specialty
is active galactic nuclei & supermassive black holes.

http://faculty.wcas....stroResearch...

She said the current best theory of getting massive black holes in the early
universe is through galactic collisions & mergers. In the early universe
galaxies were on average much closer than today, and collisions and mergers
were more common. Collisions provided material to "feed" the black hole,
and galaxy mergers resulted in their central black holes eventually merging.


#30 Carl Coker

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Posted 05 August 2013 - 05:21 PM

Jarad,
Thanks for undertaking that calculation! It is worth pointing out that such a configuration, where a considerable volume of such mass can maintain such low and *uniform* density, is most unstable. Self gravity will have gone to work long beforehand, to first make a bunch of stars.


For a little perspective, Jarad's number turns out to be a sphere about the size of the orbit of Neptune.

If we take a neutron star (which is city-sized) very near the required mass, and toss in matter until an event horizon forms, does the neutron then star instantly 'squish' down to a singularity? I don't envisage a singularity, except in the way relativity describes it. If we could 'picture' the object in our non-relativistic framework, I think it would still be a city-sized object lurking just beyond the event horizon.


As soon as the neutron star gains enough mass to collapse within its Schwarzschild radius, it instantly loses pressure support and basically goes into freefall. Straight GR says it collapses to a point/singularity, but presumably quantum gravity steps in at some point and stops the collapse at some point.

The reason for this "instantaneous" collapse (for ~solar mass objects, it takes less than a second to go from the event horizon to the center in freefall) is that there is no mechanism which can provide enough pressure support to resist gravity at that point. You can envision space in a BH as an enormous river, rushing in towards the center. Inside the event horizon, this river is flowing faster than c. Therefore, to remain stationary, you must be moving outwards faster than light! This is impossible for any known object, so further collapse is inevitable.

Geometrically, what happens inside a black hole is that the radial dimension becomes timelike, while time itself is now a spacelike dimension. What this means is that all geodesics going forwards in time hit the singularity, since they head to inexorably smaller radii, just as we inexorably move forward in time. To get out of a black hole, you need either a time machine or a faster-than-light drive, which is functionally just a time machine by another name.

#31 GlennLeDrew

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Posted 05 August 2013 - 08:33 PM

Carl,
Useful further perspectives; thanks! There is one thing which you may be able to clarify...

Back to the neutron star to which we are tossing in mass. We know that the gravitational acceleration is highest at the surface, if we assume uniform density. This is because below the surface, only that mass interior to the sphere at a given radius is contributing to gravity. And so gravitation acceleration decreases as we approach the center, where it's zero.

At that moment the Schwartzchild radius passes beyond the surface of the neutron star, only in a thin region at and just barely below the surface is the gravitational acceleration faster than C. How does this 'discontinuity' affect the collapse?

#32 EJN

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Posted 06 August 2013 - 05:31 PM

By assimilating the physical and cultural uniqueness of the civilizations they encounter.

Wait, that's how the Borg gets bigger. Sorry!

Recent research has shown that black holes grow by eating
all their vegetables and taking their vitamins.

#33 Carl Coker

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Posted 07 August 2013 - 11:40 AM

Carl,
Useful further perspectives; thanks! There is one thing which you may be able to clarify...

Back to the neutron star to which we are tossing in mass. We know that the gravitational acceleration is highest at the surface, if we assume uniform density. This is because below the surface, only that mass interior to the sphere at a given radius is contributing to gravity. And so gravitation acceleration decreases as we approach the center, where it's zero.

At that moment the Schwartzchild radius passes beyond the surface of the neutron star, only in a thin region at and just barely below the surface is the gravitational acceleration faster than C. How does this 'discontinuity' affect the collapse?


I am not sure. I suspect that there is in fact no discontinuity, since GR does not behave like Newtonian gravity in this kind of situation.

If the discontinuity does occur, then I believe that the collapse will not be slowed by more than a factor of two or three. What would be happening there is that the pressure causing the collapse would come from the outer layers running into the material below, instead of the now irresistible gravitational pressure causing a simultaneous collapse of the entire star. But I'm honestly just guessing at this point.

#34 GlennLeDrew

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Posted 07 August 2013 - 03:16 PM

Carl,
I'm only guessing too! :grin: The notion of a 'discontinuity' is merely my own way of trying to picture the sequence of events. I've not delved into this to any real degree at all, and so am not in the least aware if my idea regarding the gravitational acceleration exceeding C commencing first in the outermost shell of the neutron star is at all valid. My own poorly conceived understanding of the 'weirdness' of even *near*-relativistic effects must be an impediment to a fuller understanding.

#35 Asbytec

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Posted 08 August 2013 - 08:03 AM

Perhaps someone could chime in with the result of this calculation:

For a *uniform density* of that of air (1kg/m^3), what is the radius of a spherical volume which has a surface gravity of C?


I cannot do the math, but it would seem anything with such an enormous amount of gravity would necessarily be very dense at any radius. If it were not dense, it would certainly collapse on itself until it was. Air simply cannot withstand such a pull of gravity, nor can iron for that matter. Basically, the singularity is about as dense as you can get squeezing degenerate matter (like a neutron star) to beyond degenerate.

Now, if you throw a warm cup of coffee in there, you've done something about entropy. Right?

I think it would still be a city-sized object lurking just beyond the event horizon.



I'll be honest, I'm not exactly sure why just adding a tiny bit more mass to a neutron star (pushing it into a black hole) will suddenly cause that neutron star to shrink to a singularity just because the mass edges up enough to create an event horizon.

Probably not, if we could envision a singularity it would probably be one. Strange things happen when you stretch space and time. Plus, the intense gravity pushes matter even closer together, as I understand it. A neutron star is held against it's own gravity by the exclusion principle (if memory serves.) A black hole is formed when even that irresistible force is overcome.

As soon as the neutron star gains enough mass to collapse within its Schwarzschild radius, it instantly loses pressure support and basically goes into freefall.


Sorry, reading and responding at the same time.






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