Xref: utzoo sci.space:16261 sci.astro:5971 Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!cs.utexas.edu!ut-emx!ethan From: ethan@ut-emx.UUCP (Ethan Tecumseh Vishniac) Newsgroups: sci.space,sci.astro Subject: Re: Big Bang: Did it happen? Summary: a few comments on the BB Message-ID: <22574@ut-emx.UUCP> Date: 20 Dec 89 15:42:07 GMT References: <963@YaleVM.YCC.Yale.Edu> <263@cfa.HARVARD.EDU> <9364@hoptoad.uucp> Organization: The University of Texas at Austin, Austin, Texas Lines: 79 As someone who has spent some years playing with cosmology, I'd thought I'd kick in a few comments on the Big Bang. There are at least two senses in which people are using that phrase. In one sense it refers to the idea that are universe is currently expanding, has been doing so for some time, and at some finite time in the past (somewhere between 10 and 20 billion years ago) was extraordinarily hot and dense. In another sense it refers to the idea that the universe began with an initial singularity, and "before" this point neither time nor space existed. The first version of this idea can be analyzed using our current understanding of physics (like GR, particle physics etc.) and by choosing to approximate our universe as homogeneous and isotropic. This usually goes by the name of the "standard model". There is an impressive amount of evidence in favor of the first idea and no compelling evidence against it. Besides being consistent with physics as we know it, it can be tested by comparing the current blackbody background temperature (probably 2.75pm.05) with the estimates of the primordial abundance of certain light elements (H, He3, He4, D, Li7). Consistency between these figures can be used to constrain the average baryonic density of the universe (This does not represent a trivial prediction. It is difficult to get consistency and easy to imagine answers for the elemental abundances which would be inconsistent with this picture.) In addition, it has been used to predict the number of light, or massless weakly interacting particles (i.e. neutrinos). This prediction has just been verified. It is probably worth noting that the predicted abundance of baryonic matter is not inconsistent with dynamical estimates of the mass density of the universe. It has already been mentioned here that this model has the only consistent explanation for the blackbody background. The weakest aspect of this model is that it does not incorporate galaxy formation. There are a number of galaxy formation models that are consistent with the standard model, some of which cannot be ruled out at the present time. None of them is particularly compelling, although models using cold dark matter are probably ahead of the rest. These models may fail if current observations of large scale structure and motion hold up. One of the tightest constraints on all such models is the incredible isotropy of the blackbody background. The growth of gravitational structure in an expanding universe is slow enough that the perturbations that would have to grow into galaxies should have left some remnant disturbance in the blackbody background. Inflation is a way of explaining the features of the standard model by using particle physics, and appealing to the high temperatures present at *very* early times. It is a nice idea, but has problems and may not end up as part of the standard model. As someone else has noted it does seem to predict that the universe is very close to critical density. It does *not* predict that the universe is closed or open (although specific models of inflation do make such predictions). The bulk of the observational evidence seems to indicate that the universe has a much lower mean density. Inflation (at least in some versions) also predicts a power spectrum for temperature anisotropies in the blackbody background. The idea of the Big Bang as a real singularity in the fabric of space-time is another issue. This view results from an unreasonable extrapolation of classical GR into temperature regimes where quantum effects must be important, and there is no consensus view on how to make such corrections. There have been many speculative papers on how to do away with the initial singularity without changing the standard model appreciably. None of them command general assent, nor are they likely to in the near future. The bottom line is that the initial singularity and inflation could be disproved (in some sense) tomorrow and our view of the universe would not have changed a great deal. I certainly wouldn't regard it as the death of the "Big Bang Theory". -- I'm not afraid of dying Ethan Vishniac, Dept of Astronomy, Univ. of Texas I just don't want to be {charm,ut-sally,emx,noao}!utastro!ethan there when it happens. (arpanet) ethan@astro.AS.UTEXAS.EDU - Woody Allen (bitnet) ethan%astro.as.utexas.edu@CUNYVM.CUNY.EDU These must be my opinions. Who else would bother?