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*From*: "Paul Camp" <pjcamp@coastal.edu>*Date*: Mon, 17 Feb 1997 13:29:19 EST

> > Regarding: > > >A black hole needs a "universe" to be in (spacetime and all that good > >stuff). It is not at all clear what the environment that preceded the > >big bang consisted of? Could it be characterized by a spacetime > >dimensionality? A temperature? A vacuum? > > Perhaps the Big Bang occurred everywhere simultaneously, so it wouldn't need > a location. Does that constitute too large a leap of faith? > JFK > It is not too large since that is exactly what happened. The universal expansion should not be thought of as stuff flying out into a previously existing spacetime. Rather, the spacetime has always been full of stuff, the spacetime geometry is what is doing the changing, and the stuff is largely just along for the ride. All this follows from the isotropy assumptions built into most Big Bang models. Regarding the black hole equivalence, that only works if the overall geometry of the universe is closed. All radial geodesics then form closed loops or terminate on a singularity -- i.e., you can't get out, which is really all that is required of the interior solution for a black hole (quantum phenomena excluded, of course). In particular, it does not necessarily have to be embedded in a larger geometry -- you don't have to have an outside in order to have an inside. And in both cases, the classical singularity lies in a timelike direction. None of this holds at all in flat or open geometries. It is not too hard to show that as the radius of a black hole's event horizon becomes larger and larger, corresponding to ever larger masses, the average density becomes less and less. You could then easily mock up a black hole solution with a density equal to that of the universe with the condition that it hasn't yet all fallen into a singularity. Of course, the main problem with all of this is that nobody really knows what the "correct" interior solution is for a black hole and in fact you can get a great deal of argument on exactly that point. However, in terms of the geodesic structure, it is pretty nearly the same as a closed universe so there is at least a logical equivalence. What got the expansion started is purely a matter of conjecture and, since all inlfationary models require the universe to be pretty much flat anyway (and therefore decidedly un-black-hole-like) it is probably a moot point. Paul J. Camp "The Beauty of the Universe Assistant Professor of Physics consists not only of unity Coastal Carolina University in variety but also of Conway, SC 29528 variety in unity. pjcamp@coastal.edu --Umberto Eco pjcamp@postoffice.worldnet.att.net The Name of the Rose (803)349-2227 fax: (803)349-2926

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