willner@cfa.harvard.edu (Steve Willner) (11/16/86)
> In article <3422@sdcrdcf.UUCP> markb@sdcrdcf.UUCP (Mark Biggar) writes: > >... In fact, there is an > >observed doppler shift in the background radiation as seen from earth. > >This has been used to get a fairly good estimate of the real velocity > >of the earth, sun, milky way, etc. with respect to the rest of the universe > Then in article <5397@brl-smoke.ARPA>, gwyn@brl-smoke.ARPA (Doug Gwyn ) writes: > The original interpretation of the "isotropic background > black-body radiation" as being cosmic rather than local > was largely due to its isotropic nature (so far as had > been measured initially). > > Now that the phenomenon is taken for granted to be cosmic > and not local, the anisotropy that newer measurements have > turned up is taken to indicate absolute motion with > respect to the cosmos. > The measured anisotropy is equivalent to an Earth motion of 600-800 kilometers per second with respect to the microwave background. (I'm not in my office right now, but I'll be glad to look up the exact value later if anyone wants to know.) This means that the background is isotropic to rather better than one part in 300. The direction of the anisotropy is such that it implies that the Milky Way Galaxy is falling toward the nearest large cluster of galaxies (the Virgo Cluster) at a speed of a few hundred km/s. These results are not surprising if the background is cosmological, and any other theory of the background must explain them. I am aware of at least one other theory, due (I believe) to Hannes Alfven. He suggested that the background is due to radiation from cool dust particles in galaxies at redshifts of about 2 or 3. It is certainly true that if dust were present in these galaxies with the same abundance as in our own, the universe would become opaque at about these redshifts and we would not see any putative "cosmological" background (i.e. a background arising at redshift about 300). The problem is that one needs heavy elements to have dust particles, and most astronomers believe that the heavy elements were created in stars in the relatively recent past, say at redshifts of 0.5 or less. The creation time of heavy elements is less than firmly established (to put it mildly), so dust is not ruled out as an explanation of the microwave background. But I know of no explanation that places the origin of the background any closer than this one, which is hardly local. Incidentally, the Cosmic Background Explorer (COBE) satellite should return extremely valuable data on the microwave background. The problem is getting it launched. It was supposed to be launched into polar orbit by the Space Shuttle; now that this is impossible, COBE is undergoing drastic reduction in weight so it can be launched by a Delta. There are also some difficult ground-based observations that might help determine the origin of the microwave background. Steve Willner Phone 617-495-7123 Bitnet: willner@cfa1 60 Garden St. FTS: 830-7123 UUCP: willner@cfa Cambridge, MA 02138 USA Telex: 921428 satellite cam