ethan@utastro.UUCP (05/08/84)
[Redshift this line] This article is *very* long. It is basically a response to Doug Gwyn's article. This seems much more interesting than thermostats to me also. For those who are monumentally bored.. what is an "n" key for anyhow? If this discussion is only between Doug and I, then I may end up resorting to mail in the future. I have quoted at length from Doug's article so that third parties can follow the discussion easily. >The Hubble effect does not have to be Doppler (involving relative >longitudinal velocities) in nature. The same spectral line shifts >etc. can be explained in other ways, e.g. gravitational red-shift, >tiring photons, and others harder to explain simply. Agreed. However, "tiring photons" is not an explanation. We have no theory of physics in which photons "tire", nor any other observation which might lend credence to the idea that this phenomenon occurs. Gravitational effects which produce a systematic redshift would either be based on local effects (i.e. local to the distant object) or global (i.e. cosmological). Since rather ordinary looking galaxies can have large redshifts local effects are an unlikely explanation. Global effects would have to be explained in terms of an alternative model of the universe. All such models that I know of (that do not resort to the conventional explanation for redshifts) are strongly inhomogeneous with the Earth as a privileged point. They are models that are isotropic (around us) but not homogeneous. It is impossible to address this point directly (until someone travels to, or communicates with, another galaxy!) but nothing else in our observations suggests such an interpretation. >The idea that quasars are ancient objects is entirely based on the >idea that they are far away and that there is some form of global >time frame. I don't know whether the Burbidges ever changed their >minds about the matter, but they used to think that quasars are >relatively close (in which case their energy output is no longer >so hard to explain, nor is the periodicity of the pulsars). G. Burbidge has never changed his mind. However, I don't think he has a case anymore. Many of the *relatively* nearby quasars have been observed to be surrounded by "fuzz" with a density profile and spectrum which leaves little doubt that these objects are distant galaxies with quasars (whatever that may be) in their centers. Burbidge has continued to base his case on the alleged clustering of distant quasars around nearby bright galaxies. This effect would be impossible to understand if quasars were at cosmological distances. However this effect is probably due to the fact that people observe nearby galaxies a lot and so have accidentally discovered many quasars near them on the sky. Uniformly selected samples of quasars do not show this effect. I don't understand your comment about pulsars at all. These objects are in our galaxy a cosmology would appear to have no bearing on them. >The idea of a global time frame is so repugnant to the relativist that >it must be seriously challenged. >This matter of the 3-degree K blackbody background radiation is >interesting. The blackbody spectrum does not appear to be invariant >even under Lorentz transformation (I would appreciate any proof to >the contrary), so if it really is (a) blackbody-spectral and (b) >isotropic as observed from Earth, then it singles out the Earth as >a privileged reference frame. Not impossible, but highly suspicious. I have combined these points because I think this is a single issue. A blackbody spectrum is not Lorentz invariant. However the effect that uniform motion produces is quite simple. The spectrum continues to be blackbody, but the apparent temperature changes depending on the angle between the observer's motion and the direction of the source. The 3-K (actually 2.7) background is omnidirectional. Therefore a moving observer (moving with respect to the radiation field rest frame) will see the temperature of the background acquire a dipole variation. Such a variation is observed and indicates that our galaxy is moving (along with several nearby galaxies) at about 600 km/sec. All of which indicates that the universe has a preferred "rest" frame (and a preferred time slicing). On the average all galaxies are at rest with respect to this rest frame (which expands with the mean Hubble flow). However individual galaxies show "peculiar" (local) velocities comparable to our galaxy's motion. Our planet is no more a "preferred" place than any other place in the universe. >Similarly, I don't think that only the "big bang" is capable of >explaining the current abundances of the elements. It seems likely >that any cosmic cataclysm would produce similar results (I would >like more discussion on this since I don't know that much about the >current ideas on this topic). This is wrong. Actually, depending on the parameters one feeds into a model of the big bang (allowing inhomogeneities and anisotropies) one can make a universe which is pure hydrogen or pure helium. Present observations severely limit one's freedom. The most important point is that the big bang model (here I mean specifically the homogeneous, isotropic version) implies that the primordial abundances must fall within a narrow range and that they must not depend on which galaxy one examines. Present observations are consistent with this picture. Nucleosynthesis in the big bang produces no atoms as heavy as carbon (or heavier). All subsequent nucleosynthesis must be done in stars. Supernovae (or similar things) produce lots of heavier elements. The earliest stars we see in dwarf galaxies have very very small amounts of heavier elements. (However, not none at all. People worry about this) This is getting very long. Doug's next paragraph is a detailed justification for giving the vacuum an energy density which is self-gravitating (the infamous "cosmological constant"). This includes a criticism of my claiming that relativity is probably safe after the first 10^-43 seconds. I'm going to surprise him by saying that I think the possibility is so open that I don't consider its inclusion a significant modification of general relativity. As I mentioned in a previous article, the addition of a cosmological constant does not, in any way, change our interpretation of the universe as an expanding system. Einstein abandoned his original scheme to use one to "stabilize" the universe at a given size for two reasons. First, the universe is observed to be expanding. Second, such a constant will *not* succeed in stabilizing the universe. Instead, nonuniformities in the distribution of matter will produce exponentially expanding regions and exponentially shrinking regions. As for a natural value for the cosmological constant... there is none. Any value, including zero, is quite possible within our present understanding of physics. The only firm limits come from demanding that gravity work as observed on the scale of galaxies. This is too weak a limit to be of much use when discussing the later evolution of the universe (the early evolution of the universe is not affected). Therefore, the value of the cosmological constant is an open question. I might add that the "natural" values suggested by current particle physics are so ludicrously large as to demand that its value be at least partly canceled by some, as yet unknown, symmetry of the full quantum theory of gravity. "Just another Cosmic Cowboy" Ethan Vishniac {ut-sally,ut-ngp,kpno}!utastro!ethan Department of Astronomy University of Texas Austin, Texas 78712
gwyn@brl-vgr.UUCP (05/09/84)
On the off-chance that no one else is really interested in this topic, I will try to keep future additions brief. (Come on! Let's hear somebody else at least say whether he cares about any of this stuff!) I doubt that Ethan Vishniac & I are going to eventually agree on the answers to the various issues. Partly this is because it is hard to get unambiguous answers to cosmological questions through experiment and observation. For example, it is NOT known that the observed universe is expanding in any real sense. What IS known is that there is an observed Hubble phenomenon, namely, the red shift of galaxies whose distance has been determined by non-Hubble means is roughly proportional to their distance from us. The conclusion that these galaxies are receding from us proportionally to their distance, which is an expansion, is simply the usual INTERPRETATION of the Hubble phenomemon based on the assumption that the red shift is caused by a Doppler effect (relative velocity of light emitter and receiver). This is not the only possible interpretation of this effect; for example, E. A. Milne (who accepted special relativity but not the general theory) explained the effect via his "kinematic relativity" (actually, I am prepared to yield that Milne's idea was really expansion) and various non-mainstream cosmological models predict this effect directly from path integrals of the "photons". (The "tiring photon" idea has been seriously proposed by reputable theoreticians in the past.) I don't think Ethan fully understood what I was saying about the natural appearance of the cosmological constant in the Einstein-Schr"odinger unified field theory. The best reference for this particular point is "Space-Time Structure" by Erwin Schr"odinger (1950, Cambridge University Press). The same author also had a marvelous treatise on cosmology entitled "Expanding Universes" (1956, Cambridge) which I wish I had a copy of (can anybody supply me with one?). I redeveloped the E-S unified field theory from first principles in my Master's thesis, although the cosmological constant receives only brief mention there. The important thing about this generalization of general relativity is that the way in which the cosmological constant is produced forces one to acknowledge that (a) it is non-zero, and (b) its particular value is determined by the measurement standard (e.g. meter), somewhat the same as the way the speed of light depends on the relation between the time and space measurement standards (although the analogy is not exact). In terms of distance units comparable with the "radius of the universe" (which is a meaningful concept in any natural cosmology engendered by this alternative theory), the cosmological constant is around 1; in terms of laboratory units (e.g. cm^-2), it is quite small, < 10^-50. Until we have "measured" the radius of curvature of the universe in lab units, the value of this constant in cm^-2 will not be accurately known. The first reference I have found to the "self-gauging" universe is A. S. Eddington, "The Mathematical Theory of Relativity" (2nd Ed., 1924, Cambridge). Another interesting point is that the deSitter cosmology is related to (invariant with respect to, I think, but am not entirely sure) the deSitter group that at one point attracted some interest from the elementary particle theoreticians. The E-S theory raises the idea that there is an inherent relation between the large-scale structure of the universe and the behavior of elementary particles; of course these days this idea is in vogue, but it was pooh-poohed when originally suggested. One more note: The E-S unified field theory is NOT just classical general relativity "with an additional term". Its formulation reaches deeply into the fundamental concepts that lead to the idea of metric and geodesic, so the older ideas of these do not necessarily apply to the extended theory. The E-S theory under the usual interpretation of the meanings of the quantities appearing in it leads to nonlinear electromagnetic field equations of Born-Infeld type (C. R. Johnson showed this for the similar Einstein-Straus- Kaufman theory in Physical review D, 1971-1973). Sciama and Corben suggest that the non-zero torsion of the extended theory is related to spin and isospin, and it is easy to see how the E-S theory includes a "classical two-valuedness" due to an internal symmetry of the theory. These ideas cannot be handled by standard general relativity alone, as is well known. I happen to think that Einstein knew what he was doing when he went off to work on the unified field theory. I realize this is not the consensus viewpoint of the physics community, but on reading nearly all the available criticism of Einstein's program I have found little evidence of understanding of his reasons for pursuing this approach. One gets the impression that the majority of physicists thought that he was getting senile or something. "I may be crazy but if so at least I am in good company!"
rh@mit-eddie.UUCP (Randy Haskins) (05/12/84)
Since some of the people submitting want to hear feedback on whether or not people want to see more, here's some: keep it up. I'm not in the field, just an interested science-type. While I don't have the background to understand of lot of the things discussed, I enjoy reading the articles for what things I do understand. That's the reason I read this group. As if there isn't enough going on, anyone care to talk about what significance is placed on the possibility that the entire Universe might qualify as a black hole (i.e., light can't escape, time slowed to zero at the perimeter, etc.)? -- Randwulf (Randy Haskins); Path= genrad!mit-eddie!rh
gwyn@brl-vgr.ARPA (Doug Gwyn ) (05/14/84)
I doubt that even the black hole advocates can claim that the universe (or our part of it, at any rate) is inside the critical radius of a humongous black hole. But who knows? If they want to believe it enough I am sure they'll work up a theory to make it so. Here is my favorite cosmology: (1) Space isotropic (same, on a large scale, in all directions). (2) Distant light red-shifted `a la Hubble. (3) Therefore, light is "infinitely" red-shifted in the limit that it originates from a certain large distance (call it the "radius of the universe" for definiteness). (4) The local average density of matter etc. is static (does not change in time). (5) The above all apply to every point in the universe, at every time. It is kind of cute that the distance horizon follows you around as you fly across the universe. No danger of ever falling off the edge. If anyone has a more appealing cosmology than this I would like to hear it.
wls@astrovax.UUCP (William L. Sebok) (05/16/84)
> I doubt that even the black hole advocates can claim that the > universe (or our part of it, at any rate) is inside the critical > radius of a humongous black hole. But who knows? If they want > to believe it enough I am sure they'll work up a theory to make > it so. I seem to recall that in the standard cosmology the closed Friedmann universe is essentially equivalent to the whole universe being a big black hole. -- Bill Sebok Princeton University, Astrophysics {allegra,akgua,burl,cbosgd,decvax,ihnp4,kpno,princeton,vax135}!astrovax!wls