Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site utastro.UUCP Path: utzoo!watmath!clyde!akgua!mcnc!decvax!harpo!seismo!ut-sally!utastro!ethan From: ethan@utastro.UUCP (Ethan Vishniac) Newsgroups: net.physics,net.astro.expert Subject: very big bangs Message-ID: <275@utastro.UUCP> Date: Mon, 14-May-84 12:48:55 EDT Article-I.D.: utastro.275 Posted: Mon May 14 12:48:55 1984 Date-Received: Wed, 16-May-84 04:02:41 EDT Organization: UTexas Astronomy Dept., Austin, Texas Lines: 191 [But why not talk about the BIG picture?] I have been urged, privately and publicly, to continue this discussion as long as I can without repeating myself. I'm going to give this a shot. I assume that if I start repeating myself too much I'll also hear from my friends about it. Doubts about the standard model of cosmology arise from the following points (as near as I can tell): 1) Singularites (such as the t=0 point in the standard model) are estheticly repulsive and a sure sign that the theory has gone astray. 2) The general theory of relativity is based on dubious assumptions about physics and could easily be wrong when considering large scale phenomena. 3) The picture of an expanding universe is philosophically repugnant and the available facts can be otherwise explained. 4) The available facts can be otherwise explained so the history of the universe is essentially unknown, with many equally likely possibilities competing for attention. My short responses are as follows: 1) This is also my opinion. However, this does not have any bearing that I can see on the utility of the general relativity outside of the domain where we know that quantum gravity must be invoked. Many useful theories are incomplete in the sense that their domain of applicability is limited. General relativity is clearly one since it is not a quantum field theory. Nevertheless, after the first 10^-43 seconds (or rather at all times when the classical theory gives a larger number for the age of the universe) the question can be reduced to the question of what the correct classical theory of gravity is. 2) This is true. The only question is whether such an extended theory can explain the observed large scale evolution (or state) of the universe as well as GR. 3) I'm not interested in the first part of this one. The second part is equivalent to point 4. 4) If there are other equally viable theories which can explain the observed features of the universe I'm not aware of them (doesn't mean they don't exist). I'm going to list what I see as the main observational points. However I'd like to address a few theoretical points first. - The standard model of the universe is consistent with an infinite, or a finite universe. All we can say is that the part of the universe we can see is finite (approximately given by the speed of light times the age of the universe). This is why the night sky is dark. (Redshift of distant objects will not, by itself, accomplish this). - The vacuum may or may not have an associated energy density. That possibility can, by itself, be accommodated within GR. More complicated proposals mentioned by D. Gwyn and others require a different (extended?) theory. The following observational points must be explained by any satisfactory theory of cosmology. 1- Objects (like galaxies) are observed to have a redshift which is proportional to their distance. 2- In all directions there exists a population of unusual objects called quasars with large redshifts. At least some these objects show unmistakable signs of really being as far away as their redshifts indicate. These signs include the observation of distant galaxies and clusters of galaxies centered on the quasar, and the observation of quasars that are "lensed" by the gravitational fields of galaxies that are themselves very distant. Also, quasars have a large space density at large distances and a very small one nearby. 3- There is an isotropic background of microwave radiation in space. It has a spectrum which is a perfect blackbody with a temperature of 2.7 K. The upper limits on anisotropy are very small (depending on the angular scale a few times 10^-5 to 10^-4). The only exception to this is a dipole variation in the temperature which can be explained as due to the motion of our galaxy through space (relative to the rest frame of the radiation background) at 600 km/sec. 4- Stars have a metallicity (an abundance of elements heavier than Hydrogen and Helium) which is related to their ages. The oldest stars are the most nearly pure Hydrogen and Helium. This is clearly proof that our galaxy is changing as it gets older. Nearby galaxies show similar trends. The oldest stars appear to be the same age, i.e. we do not see any star clusters that are clearly older than the oldest stars in our own galaxy. 5- The chemical abundances of elements of low atomic number in nearly unprocessed material, in our own galaxy or in nearby galaxies appear to reflect a primordial set of abundances. These abundances are what one would expect from the standard model. They are inconsistent with many proposed alternatives. 6- On the largest scales (certainly no greater than a 100 megaparsecs or so) the distribution of galaxies in space appears to be relatively uniform. That is to say that variations in density are no greater than a factor of two in any particular region and possibly a whole lot less. Now for some detailed comments on these points. First, I don't think that point number one is in dispute here. However, the explanation for it in the standard theory needs elaborating. The redshift effect can be explained in the following three ways - all of which are equivalent descriptions in the standard model: 1) the universe is expanding and as each galaxy moves away from its neighbors the light that reaches its neighbors is redshifted due to a Doppler effect; 2) the universe is expanding and as light propagates through an expanding spacetime its wavelength is stretched; 3) the universe is expanding and so light that reaches us from a distant galaxy was emitted when the gravitational potential of the universe was larger (in magnitude, the sign is negative) than it is now so that the light we see has climbed out of a potential well and so is redshifted by a gravitational effect. In the standard model all these are equivalent descriptions (although some are more unsatisfactory than others due to the picture that they evoke). Models that produce this effect without expansion rely on "tired light" schemes that are used (to my mind) as a kind of deus ex machina, invoked with no supporting evidence. The second point has been disputed by Doug Gwyn. I think that reasonable doubt could be said to exist during the 60's and (to a lesser extent) during the 70's. Our observations have improved and I don't think any reasonable doubt remains. The discovery of the microwave background was one of the turning points of modern cosmology. The perfect isotropy of the background was recognized immediately as a strong argument for its extragalactic origin. The fact that it adheres so closely to blackbody shape shows that it must have been produced in a gas in thermal equilibrium. The present matter density of the universe is much too small to produce such a background. The standard explanation is that we live a "hot" universe with about 10^9 photons per baryon. As the universe expanded the gas eventually cools to the point where the loose ions in the gas collect electrons and becomes neutral. When this happens the universe changes from an opaque gas (due to electron scattering) to a transparent gas. This happens when the temperature is about 4000 K. Subsequently the light from this redshifts as it streams through the universe so that the present observed temperature is less than 3K. Less orthodox explanations involve generating the background from an early epoch of star formation. Such an event would have to have happened when the universe was much more dense than it is now. The large number of photons per baryon in the universe is thought to be related to the matter/antimatter asymmetry in the universe through processes that occurred at *very* large temperatures (about 10^28 K). I am not aware of any explanation for the background in a steady state universe. This is the major reason such theories have died within the astronomical community. The metallicity versus age relationship for stars is reasonable proof that our galaxy, and those near us have evolved from nearly pure hydrogen and helium (at some early time) to their present states. As near as can be determined, the time this has taken has been the same for all galaxies observed. The fact that the universe seems to have contained a "primordial" set of abundances that agree with the standard picture is an relatively unappreciated strength of the model. These abundances were predicted at a time when they were relatively uncertain, and so constitute a prediction of amazing accuracy. These abundances do not follow automatically from "any cataclysmic event". Of course, other models might be found to produce this result. However, all of those I am aware of fall into the category of "I don't know what will result so I will assume that my model *could* give agreement with observation". The nearly uniform distribution of matter on large scales is simply mentioned as a confirmation (along with the microwave background) that the central assumptions of homogeneity and isotropy used in the standard model are reasonably well confirmed by observation. Any alternative to the standard model of cosmology has to explain these points. It should be clear that any theory which takes the "perfect cosmological principal" (homogeneity in space *and* time) as its starting point is going to have a tough time of it. One possible fix (from that point of view) is to assume that the visible, finite universe is a kind of fluctuation away from a set of stable conditions which existed within our universe at temperatures comparable to the Planck temperature (at which gravity must be quantized ~ 10^32 K). The "big bang" is the beginning of that fluctuation. This suggestion has the "advantage (!?!)" of being so divorced from observation as to be uncheckable. We could wait for the regions exterior to our universe to restore equilibrium :-). However, the boundary is probably advancing upon us at the speed of light. Life as we know it will be impossible on the other side. (Could this be the end of the net?). "Just another Cosmic Cowboy" Ethan Vishniac {ut-sally,ut-ngp,kpno}!utastro!ethan Department of Astronomy University of Texas Austin, Texas 78712