ethan@utastro.UUCP (Ethan Vishniac) (05/18/84)
One more note on cosmology, this one on the microwave background. D. Gwyn notes: >The last point of this posting is that THE 2.7oK ISOTROPIC BLACK- >BODY BACKGROUND RADIATION ESTABLISHES A PARTICULAR FRAME OF >REFERENCE AS "DISTINGUISHED". This is contrary even to the Special >Theory of Relativity and needs a VERY skeptical examination. >Unfortunately, even from the first detection of this phenomenon >(which is known ONLY for this solar system) there was little >adverse reaction to this suggestion. The microwave background does indeed establish a particular frame of reference as distinguished. This is not a violation of special or general relativity. This is just an example of reality failing to show the symmetries implicit in the physical laws which govern our universe, hardly a novel situation. As for there being little adverse reaction to this discovery, well this was before my time, but it's not how I heard it. Perhaps the reason Mr. Gwyn and I disagree is that we inhabit different universes :-)? The reason the opposition failed is that no one could produce an alternative explanation of the background which was even moderately convincing. He then continues: >To give one VERY SIMPLISTIC alternative explanation >of the 2.7oK radiation, just to show that alternatives to the >conventional idea that it is a remnant of the Big Bang are possible, >consider that this region of space may be immersed in a gas (which >it is) that has some source of energy being supplied to it by >local objects (galactic magnetic field, for example). Then the gas >would be expected to have a black-body spectrum and it would be >expected to be stationary (relatively) with respect to the solar >system. Naturally this point was considered immediately. The reason it is regarded as impossible is straightforward. A gas will not produce black body radiation unless it is optically thick at the relevant wavelengths. If we were immersed in local cloud of gas at 3K which was optically thick then we wouldn't be able to see any point sources of radiation at the relevant wavelengths. We can see the plane of our galaxy at these wavelengths (I'm not sure which extragalactic objects are visible in this range. I think one can detect galaxies at these wavelength but I'm not sure.). Therefore the radiation must be uniform on the scale of our galaxy. There is no way to have a 3K gas distributed uniformly on this scale. Being so cold it would collapse to the plane of our galaxy. If contained in discrete clouds then it would not be nearly so isotropic around the sky as is observed. Finally, the dipole observed is large enough to imply a velocity which is too large for motion within our galaxy. It is however, very close to the motion of our galaxy relative to a distant shell of galaxies, which implies that the frame in which it is stationary is also that in which the average peculiar motions of the galaxies are zero. The idea of making the microwave background out of discrete sources of radiation has been kicking around for a while. It is much smoother than the distribution of galaxies. One suggestion, which is however advanced in the spirit of big bang cosmologies, is that at least part of the background is due to the thermalized radiation of stars at some very early epoch. It needs to be an early epoch so that the radiation field can be effectively thermalized by some high density intergalactic medium. (The densities required are enough to rule out any recent epoch.) This is a minority view that survives only by moving to progressively earlier epochs as the data improves. In the standard model the radiation is a remnant from an epoch when the matter density and temperature were high enough (the latter being equal, at the time, to the temperature of the radiation) so that the universe was opaque, due to electron scattering. As the temperature dropped below about 4000K the radiation became free-streaming because the matter became electrically neutral. The fact that the present temperature is about 2.7 K is due to the subsequent redshifting of the photons as the universe expanded. We see from this that the universe has expanded by a factor of about 1500 since then. I'm still unsure why D. Gwyn resists the idea of a frame which is "privileged" in the sense that the microwave background is isotropic (i.e. does not even possess a dipole). The existence of such a frame does not imply the existence of a frame which is "privileged" in the sense of having different physical laws. On every smaller scale we encounter such frames anyhow: the reference frame in which your office is stationary, the reference frame in which the solar system is stationary, the reference frame in which the motions of the nearby stars averages to zero, the reference frame in which our galaxy is motionless, the reference frame in which the local group of galaxies is stationary, the reference frame in which the motions of galaxies in the Virgo supercluster average to zero, and the reference frame in which the peculiar motions of distant galaxies average to zero. In fact, that last frame is (within the errors) the same as the frame in which the microwave background is isotropic. "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.ARPA (Doug Gwyn ) (05/21/84)
It may well happen that a cosmologically uniform 2.7oK black-body background is possible, although I would not attribute it to the remnants of a big bang. I suppose one could argue for privileged local frames in a particular cosmology on the basis of asymmetry of cosmological red-shifts for a fast-moving (locally) observer, so the asymmetry of the background radiation would be a similar situation. Let me withdraw my objection to this particular idea. The existence of a preferred local spatial frame (or family of frames) does also imply a preferred local time frame (family of), namely a time frame for which the background radiation is symmetric. This is not terribly upsetting so long as the preferred frame is different for different points in cosmological space-time. The "big bang" goes even further in that it has an absolute time origin (singularity at T=0).