For a far better description than I could possibly type: http://cosmology.berkeley.edu/Education/BHfaq.html

If the entire universe is just massive enough to collapse eventually, what is the radius of the event horizon of the black hole that it would form?

''That depends on the real radius of the universe right now (not the visible radius). Can you measure it?''

No, but I'd be happy with an answer given as a function of that radius.

Radius of a BlackHole information: http://cosmology.berkeley.edu/Education/BHfaq.html#q2

A 1979 Disney movie that I remember rather enjoying (in 1979), starred a robot named V.I.N.CENT. (Vital Information Necessary CENTralized) http://us.imdb.com/Title?0078869 -- GarethCronin

''If the entire universe is just massive enough to collapse eventually, what is the radius of the event horizon of the black hole that it would form?''

If the universe collapses, it will collapse into a single singularity. There's nothing "outside" this singularity, so there is also no event horizon. To put it in another way: if the universe collapses, the entire universe is already inside the black hole, since no matter how fast you accelerate, you cannot escape this singularity. (Which is the definition of the internal of a black hole.) -- StephanHouben

Are you saying that once the various galaxies are approaching each other, there is already a distance beyond which no light ever reaches? 

''I certainly am not. That would definitely not be the case. -- sh''

''Let's put it this way: the Big Crunch singularity is not the type of singularity that is usually associated with a BH, but if we want to stretch the definition of the BH to include the Big Crunch, then the only reasonable conclusion is that the whole universe forms this BH. Everything inside a BH falls eventually in the BH singularity, and everything in the whole universe falls eventually in the Big Crunch singularity. -- sh''

Let me put the question this way - during the collapse, would an observer positioned with the outermost matter observe a black hole forming out of most or all matter? Alternatively, would the gravitational force arising from all matter eventually prevent direct 'escape' of radiation beyond some radius from within that radius?

''There is no "outermost" position in the universe, so the question is meaningless. An observer would probably see lots of BH formation and coalescing during the final phases of contraction. However, there is no formation of a universal black hole. -- sh''

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There is an article in the February 2002 issue of New Scientist postulating that BlackHole''''''s do not exist. That they are only a very problematic fiction of classical physics which disappears under quantum gravity. The end point of gravitational collapse according to Mottola and Mazur is an object they call a GravaStar. (http://www.cosmiverse.com/space01170204.html) (BrokenLink - use http://web.archive.org/web/20020603172310/http://www.cosmiverse.com/space01170204.html)

''a somewhat problematic assertion, since there is no 'finished' theory of quantum gravity yet. They might get there, but need to worry about much simpler falsifiable predictions before pretending to be able to assert anything like this''

Indubitably, but the idea of a gravastar seems to fit M-theory's explanation of black hole evaporation better than black holes themselves. Besides, it's a cool idea.

The article says that they published something in Physical Reviews Letters but I can't find anything on their website. Could someone with a membership please search it?

''the article says 'submitted to Phys. Rev.', and a search on Phys Rev. Lett. doesn't show it...''

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Also in 2002, researchers submitted proof of a supermassive (3.7 million solar masses) black hole at the center of our own galaxy, also known as the radio source Sagittarius A*. They have observed a star in a 15 year orbit around the black hole. At its closest approach in 2002, the star passed within 17 light-hours (3x the distance from the sun to Pluto) going 5000 km/s. Kepler's laws were used to determine the mass of the black hole from the area of the elliptical orbit.

* http://www.eso.org/public/news/eso0226/

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See also BlackHolesHaveNoHair

CategoryPhysics