ethan@utastro.UUCP (Ethan Vishniac) (05/17/84)
[If a bug never has a chance to happen, is it truly a bug?} I've been trying to make these notes short and to the point. D. Gwyn writes: >Why the night sky is dark (mistakenly called Olber's Paradox) is a >very interesting question that is by no means settled. There are >explanations besides the finite extent of the observable universe; >see Mandelbrot for one such (based on recursive clustering). In >any case, almost any sensible cosmological model is going to agree >with the Hubble effect so this is not an important point for >distinguishing between most cosmologies. Nigel Sharp has already written a nice note on Olber's paradox. I just want to add that Mandelbrot's picture is just a mathematical description of a suggestion that is somewhat older (see the references on page 8 of The Large-Scale Structure of the Universe by P.J.E. Peebles). It is based on an infinitely extended hierarchy of clustering, i.e. galaxies form clusters, which form superclusters (so far known to be true), which form superduper clusters (entirely unsupported by the evidence) etc. No *dynamical* model of the universe based on this suggestion has ever been constructed. The interest in building one sharply decreased after it became clear that the model was incompatible with the deep galaxy counts made by Brown (1978), Tyson and Jarvis (1979), Ellis (1980), Kron (1980). (This work has been continued up to the present with essentially similar results.) I am puzzled by the comment that the Hubble expansion is not a useful tool for distinguishing between cosmologies. Any model can have the Hubble law inserted by invoking ad hoc theories (tired light etc.), but only a restricted class will have it as a natural consequence of the physics used in building the model. "Just another Cosmic Cowboy" Ethan Vishniac {ut-sally,ut-ngp,kpno}!utastro!ethan Department of Astronomy University of Texas Austin, Texas 78712