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If a black hole is an infinitesimally small point (point of singularity) then that would make its density infinite. If its density is infinite then its gravitational effect would also be infinite, essentially turning the whole universe into a point inside the event horizon of a black hole.

Another viewpoint is that the entire universe came out of an area no larger than a typical black hole. With all that stuff in such a small space, how did it not become a black hole?

I'm dramatically misunderstanding something here. Can someone fill me in? Beer in mind I'm a bit of a numpty when trying to explain stuff :confused:

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> If a black hole is an infinitesimally small point (point of singularity) then that would make its density infinite.

Correct.

>If its density is infinite then its gravitational effect would also be infinite

Not correct. Its mass is finite.

>Another viewpoint is that the entire universe came out of an area no larger than a typical black hole. With all that stuff in such a small space, how did it not become a black hole?

A black hole is a stationary solution of Einstein's field equations. The Big Bang is not a stationary solution. In the Big Bang scenario there is no problem with very high density creating collapse. As a simple way of thinking of it, the density was (about) the same everywhere, there wasn't some special region of colossal density compared with everywhere else. Imagining the universe recollapsing as a black hole comes from the mistake of thinking of the Big Bang as happening at some particular point, rather than "everywhere".

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If a black hole is an infinitesimally small point (point of singularity) then that would make its density infinite.

Correct, but black holes have a finite size. eg. A Schwarzschild black hole has a radius of double it's mass (in geometric units).

>Another viewpoint is that the entire universe came out of an area no larger than a typical black hole. With all that stuff in such a small space, how did it not become a black hole?

The very early universe is a bit of a grey area which remains to be satisfactorily explained.

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Oddly it is only possible to state 2 things about a black hole, it's mass and it's angular momentum.

Everything else is conjecture, or opinion, or guesswork or wishful thinking.

There is actually nothing that says a black hole is infintesimally small (zero).

Small yes, but how small is back to conjecture and speculation.

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  • 3 weeks later...

A black hole (with a Schwarzchild radius, charge, mass and spin) is one thing, a singularity (with no radius) is another. As for the "Big Bang" - why assume it happened at one point in simply space at a defined time? - if it happened "everywhere" that "was" we don't have the issue about it falling into its own black hole - there was simply no place to fall to. Or if it fell, its falling still, and there is plenty of room at the bottom.

P

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..think about it - if you are falling down an infinitely deep shaft - are you really falling or is the shaft simply getting bigger?

Why infinite?

Think about this: The star that went bang as a supernove was more massive then the black hole that it left behind. But the distortion of space/time by that more massive star was not infinite, therefore the "depth" distortion of space/time by the black hole should be less then that of the initial star.

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Is a black hole infinitely small? Surly it is just very small. It is made up of a finite amount of mass with a finite amount of gravity, just too much to allow light to escape! I would think a large black hole could be quite large - relatively speaking? Not sure how large - I'm not going to measure one!

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Is a black hole infinitely small? Surly it is just very small. It is made up of a finite amount of mass with a finite amount of gravity, just too much to allow light to escape! I would think a large black hole could be quite large - relatively speaking? Not sure how large - I'm not going to measure one!

In general relativity it's a point - a singularity - so has diameter zero, but is surrounded by a region called the event horizon with a size (called the Schwarzschild radius) that can be as big as you like. The black hole has finite mass hence the singularity has infinite density (as I said before).

In reality it's assumed that general relativity no longer applies at the centre of a black hole, and quantum gravity should be the way to understand it. In that case the "singularity" might have a size of the order of the Planck length (which is very small).

If someone would like to present a viable theory of quantum gravity then perhaps we can proceed further with this... :)

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Why infinite?

Think about this: The star that went bang as a supernove was more massive then the black hole that it left behind. But the distortion of space/time by that more massive star was not infinite, therefore the "depth" distortion of space/time by the black hole should be less then that of the initial star.

The mass of the initial star was spread over a much much larger volume than the mass of the subsequent black-hole. It is this super-concentration of matter which distorts space-time to such a high degree.

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In general relativity it's a point - a singularity - so has diameter zero, but is surrounded by a region called the event horizon with a size (called the Schwarzschild radius) that can be as big as you like. The black hole has finite mass hence the singularity has infinite density (as I said before).

In reality it's assumed that general relativity no longer applies at the centre of a black hole, and quantum gravity should be the way to understand it. In that case the "singularity" might have a size of the order of the Planck length (which is very small).

If someone would like to present a viable theory of quantum gravity then perhaps we can proceed further with this... :)

If general relativity says all matter and the structure of space and time breakdown at a singularity, wouldn't this also apply to force carrying particle(s) for gravity and mass, so how could a singularity have mass and gravity?

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