Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.2 9/18/84; site lanl.ARPA Path: utzoo!watmath!clyde!bonnie!akgua!sdcsvax!sdcrdcf!hplabs!hao!seismo!cmcl2!lanl!jlg From: jlg@lanl.ARPA Newsgroups: net.physics Subject: Re: Big Bang Impossible Message-ID: <17768@lanl.ARPA> Date: Mon, 10-Dec-84 16:07:19 EST Article-I.D.: lanl.17768 Posted: Mon Dec 10 16:07:19 1984 Date-Received: Thu, 13-Dec-84 01:41:16 EST References: <185@decwrl.UUCP> Sender: newsreader@lanl.ARPA Organization: Los Alamos National Laboratory Lines: 47 > If the whole universe was ina speck, said spec would have been a black hole > and the "big bang" could not happen. No one has yet disputed this. Why not? The whole universe was not (necessarily) in a spec. The density of the universe was very high. All of the universe that is presently visible (radius of about 15 billion light-years now) was once compressed into a VERY small space. However, high density does not cause black holes - high gravitational curvature does. Let's take a few examples: 1) Assume an infinite universe with uniform, but very high, density. Since the universe is infinite, it is radially symmetric around any point. Any radially symmertic region has a gravitational field of zero in the interior of the region (any physics 200 level course will force you to work out infinite variations on the proof of this fact for any inverse-square force ad nauseum). Therefore the gravitational curvature within this universe is zero everywhere - regardless of the density. (Now start stretching this universe uniformly in all directions and you'll have a 'big-bang' effect.) 2) Assume a finite, unbounded universe with uniform, but very high, density. Since it's hard for most people to visualize such a thing, consider the surface of a sphere. The surface of the sphere is radially symmetric around any point. A sphere's surface is a two dimensional object which is curved in the third dimension; the three dimensional analog of this curved in the fourth dimension is an example of a finite, unbounded space. As on the sphere, the 'hypershpere' is radially symmetric around every point, and by argument (1), the gravitational curvature is everywhere zero. (Now expand this universe uniformly in every direction, like increasing the diameter of the shere, and you again have a 'big-bang' effect.) 3) Assume either an infinite or finite universe that is at least VERY LARGE (this means at least large enough for the following scenario). Assume that the density is not uniform, but is VERY high in one region and low elsewhere. If the density of this universe declines gradually as function of the distance from the center of the high density region, so gradually that the gravitational curvature nowhere exceeds the Swartzchild limit, then no black hole will form. Since the properties of such VERY dense matter are not well known, this dense region may start to expand into the surrounding space (due perhaps to nothing more complicated than pressure). Well, there are three possibilities. None of these imply a primordial black hole, and all of them lead to the possibility of an expanding universe. Which of these (if any) is accurate is a subject for controversy, and there may be other explanations which are just as good.