[paul.fr]

If there is, what happens when the point is reached? If not, why not?

[DoctorBeaver]

As far as I'm aware there is no upper limit. As more material is sucked into the blackhole, so its gravitational strength increases and it pulls in more material which increases its gravitational attraction... ad infinitum.

It has been conjectured that our entire universe is a black hole.

[ukmicky]

It has been conjectured that our entire universe is a black hole.

but if that were possible then it would mean.

1 Our universe has an edge.

2 It has a centre and someone possibly us could actually be residing at the center .

3 It hasn't always been expanding as they believe as there must have been times when it hasn't had anything to feed off.

4 The universe as a whole must be spinning as all black holes spin.

Also what happens to everything that the blackhole consumes ,wouldn't we see evidence of what it eats somehow and what would happen to light as it reached the edge, as it cant pass through what would happen to it.

Sorry Doc, i like being awkward []

[DoctorBeaver]

Actually, Micky, it doesn't mean that at all.

A black hole does not have to be super-dense to be a black hole. A sufficiently large amount of gas, or even milk, could form 1. Think of what a black hole actually is. It is an area of spacetime in which the escape velocity exceeds c. I'm sure 1 of our resident physics whizzos could supply the figures for a glob of milk or custard forming a black hole. It is the overall amount of matter, not just its density, that is important. The matter doesn't have to come from outside as it does in a conventional black hole, it exists within the horizon.

It's possible that there is enough matter in our universe to cause it to be a black hole. This means that there wouldn't be an edge as such because spacetime would curve around on itself. And space itself could still expand inside the horizon.

The universe could be spinning, (it could even be spinning in a higher dimension) but we have no external frame of reference to measure it against.

When I first read about this, I was a bit surprised as I'd always thought of a black hole in conventional terms.

[lyner]

A black hole does not have to be super-dense to be a black hole.

I don't understand that statement.
If it is not dense, then how can you get deep enough into the potential  well to reach an event horizon without being inside some of the mass? Once 'inside' a layer, your potential wrt all of that layer is zero so the potential gradient stops plunging so fast- it  bottoms out gracefully to zero before you get a black hole situation.
Nicht War?.

[another_someone]

A black hole does not have to be super-dense to be a black hole.

I don't understand that statement.
If it is not dense, then how can you get deep enough into the potential  well to reach an event horizon without being inside some of the mass? Once 'inside' a layer, your potential wrt all of that layer is zero so the potential gradient stops plunging so fast- it  bottoms out gracefully to zero before you get a black hole situation.
Nicht War?.

The average density of a black hole must be high, but all I think was being said there is that most of that mass is in the centre, and the region closer to the event horizon may have very little matter at all.

Then again, having reread Eth's comment, I am not actually sure if he meant that at all - maybe he could enlighten us as to what he did mean.

[DoctorBeaver]

I'll try to remember it. It was quite a while back that I read it & my understanding of the subject was even more limited in those days.

A black hole can be defined as matter, the mass of which lies within the Schwarzschild radius of that mass. The volume increases as the cube of the radius (double the radius is eight times the volume). This means that as the mass is proportional to the radius, the required density of the matter is lowered as the amount of matter rises.

The Schwarzschild radius can be as small or as big as you like, so long as there is enough mass within it. That means you can have black holes from smaller than the size of an atomic nucleus, right up to the size of the universe. If you look at how the density of the object changes with radius, you find the density goes down as the radius goes up. So by the time you get to a supermassive black hole the size of a solar system, its density is comparable to water. When you get up to the scale of the known universe then the theoretical Schwarzschild radius is actually larger than the universe.

I'll see if I can find the book I read it in (I've got a few text books here but I've got a feeling it was a library book)

[DoctorBeaver]

Incidentally, Schwarzschild translates from the German as Black Shield. Rather appropriate, methinks.

[lyner]

OK that seems reasonable. If the mass is big enough, the potential gradient,  beyond its radius can still be enough whatever the density.
In practical terms, though, what would be the distribution of mass / density inside such a body? In fact, this question applies inside any black hole. In the centre of a black hole, the gravitational potential would be zero.  Oh, but the pressure could still be v. high.

[DoctorBeaver]

Good question. I doubt very much that the mass would be evenly distributed.

I seem to remember reading somewhere that the Schwarzschild radius for unevenly distributed mass can be calculated. I think it was Kip Thorne who first solved the equations. I know he toyed with cylindrical & toroidal black holes. (Isn't it all tied up with his "hoop" test?)

My maths ability is way too basic to even think about trying to calculate anything like that. I'll see if I can find a reference anywhere.

[lyner]

I have a feeling that all this is very much related to whether our Universe is Open or Closed i.e  is there enough mass (GPE)  to use up all the KE of expansion. Once it started to collapse, you would definitely (I think!) get a black hole somewhere - or everywhere.

[DoctorBeaver]

I have a feeling that all this is very much related to whether our Universe is Open or Closed i.e  is there enough mass (GPE)  to use up all the KE of expansion. Once it started to collapse, you would definitely (I think!) get a black hole somewhere - or everywhere.

Whether the universe is open or closed could have a bearing on things. I mentioned that the Schwarzschild limit for the universe could be larger than the universe itself. As the universe expands it becomes ever closer to the Schwarzschild radius. If the universe continues to expand, it will eventually reach that radius.

What happens after that, I'm not sure. Could space continue to expand past the Schwarzschild radius?

[lyner]

I should have thought that masses couldn't escape, nor photons. But space itself? That could be another matter.
In any case, the idea of a diameter of something that is 'greater' than the space available to plot  it on, hurts my brain. It's like using graph paper stuck to a sphere and then  trying to plot a point that's further away than the circumference of the sphere. It would either overlay itself on some other point or you'd have to use complex numbers and disappear into another dimension.

[DoctorBeaver]

I should have thought that masses couldn't escape, nor photons. But space itself? That could be another matter.
In any case, the idea of a diameter of something that is 'greater' than the space available to plot  it on, hurts my brain. It's like using graph paper stuck to a sphere and then  trying to plot a point that's further away than the circumference of the sphere. It would either overlay itself on some other point or you'd have to use complex numbers and disappear into another dimension.

Your guess is as good as mine - probably better. But for what it's worth...

I'm a believer in the higher-dimensional models of the universe. I think it is too arrogant and anthropocentric to think that the only dimensions are those that we can perceive. So, let's assume that we exist on a 3-D brane in a higher-dimensional bulk.

If, as certainly seems to be the case, our universe is expanding, who is to say that it is only the perceivable 3 dimensions that are expanding? (Whether that is the case or not will probably never be known.) But let's assume that it is indeed only the 3 familiar dimensions that are expanding.

I don't have a problem with the concept of our universe expanding beyond its own Schwarzschild radius if there is a higher-dimensional bulk with our universe residing on a 3-D brane. Space, per se, has no mass, so therefore would not be constrained by gravity; it is only the matter & energy within the universe that would be susceptible to it.

So, any matter or energy that currently exists in our universe would remain within the Schwarzschild radius. Any space that had expanded beyond that limit would either be empty, or matter/energy would be transferred from the higher bulk (I think that is possibly where virtual particles come from anyway). Conservation of energy would not apply only to the 3 perceivable dimensions - it would encompass the whole of the higher-dimensional bulk. Therefore, matter/energy could be transferred either temporarily or permanently between the dimensions.

The question this raises in my mind is how would any transferred energy affect the Schwarzschild radius? Matter/energy would appear outside of the radius as a result of transfer from higher dimensions. Therefore, the mass of our 3-D universe would increase. I haven't figured out what the result of that would be  []

[syhprum]

There seems to be a limit to the size of the blackholes generated by the collapse of massive stars until recently the largest observed was 10 solar masses although one of 15 solar masses has just been reported

[syhprum]

I find it rather meaningless to talk of the universe spinning, one can only spin relative to something as far as the universe is concerned there is no 'something'.

[DoctorBeaver]

If we do, indeed, exist on a 3-D brane, then that could be rotating with respect to the higher dimensional bulk.

[Soul Surfer]

Syphrum that probably came from this week's new scientist  That applies to stellar mass black holes but the quasars that formed in the early universe are a different matter.  If there is very little turbulence and a very large mass manages to contract it can form a black hole without becoming a star these black holes could have many thousands or millions of solar masses in them.  The bigger they are the cooler they are when the horizons form.

The problem of rotation that you describe only applies to isolated simple rigid bodies.  For a complex gravitating fluid body like a galaxy. onr part of the body asks as a reference for othe parts. The differential rotation indicates the rotational energy of the body and requires no outside reference point.

[lyner]

I find it rather meaningless to talk of the universe spinning, one can only spin relative to something as far as the universe is concerned there is no 'something'.

Actually, if I were in a darkened room, in a vacuum, I could quite easily tell whether I was spinning or not because I would be aware of a centrifugal force and a Coriolis force ; geometry and mechanics would tell me I was spinning.  I would not need to refer to any other, outside frame.
You could imagine some measurement that might show the existence of similar effects in the Universe that could be explained  in terms of the whole thing rotating.

[syhprum]

I find it rather meaningless to talk of the universe spinning, one can only spin relative to something as far as the universe is concerned there is no 'something'.

Actually, if I were in a darkened room, in a vacuum, I could quite easily tell whether I was spinning or not because I would be aware of a centrifugal force and a Coriolis force ; geometry and mechanics would tell me I was spinning.  I would not need to refer to any other, outside frame.
You could imagine some measurement that might show the existence of similar effects in the Universe that could be explained  in terms of the whole thing rotating.

You experience these forces because you are rotating relative to the universe as a whole (acording to Mach) , rotation can only be relative