willner@cfa.HARVARD.EDU (Steve Willner, OIR) (01/18/90)
From article <9364@hoptoad.uucp>, by tim@hoptoad.uucp (Tim Maroney): > So many scientists seem to take the dark matter for granted, I'n not sure this is true, but press reports may make it seem so. > but I've yet to see any clear reason for > postulating it other than a sort of religious dogma that the universe > will eventually recollapse. Is there really any stronger basis for > believing that we only see ten percent of the universe, or are people > letting their aesthetics guide their modeling? There are actually two separate questions that are often confused. The first is best called the "Hidden Mass Problem." Gravitational evidence, including rotation curves, the stability of spiral galaxy disks, and the dynamics of clusters of galaxies imply that galaxies have about 3-10 times more mass than estimated for the visible stellar population. The "hidden mass" is distributed in roughly the same way as the visible mass; i.e., it accompanies the observed galaxies. The debate now is over the form of this matter, not its existence. Possibilities include "brown dwarfs" or almost anything else (black holes, "planets") in the mass range between 0.05 and 0.005 solar mass. There are other possibilities as well. The second problem is usually called the "Missing Mass Problem." Even accounting for the hidden mass, observable galaxies add up to only 0.3 or less of the "critical mass," i.e., the maximum mass allowed where the Universe expands forever (not recollapses). As far as I can tell, there is indeed no observational evidence for existence of the "missing mass;" indeed, there is considerable evidence that the "missing" mass, if it exists, cannot be in the form of baryons. The "missing mass" must also _not_ accompany the visible galaxies but rather must be more uniformly distributed through the Universe. Theorists often (always?) "let their aesthetics guide their modeling." What's wrong with that? The relevant questions are whether models are ruled out by existing data and whether simpler models would explain the data just as well. The answer to the first is "maybe" and to the second "no". Models with mass lower than critical invoke either coincidence or some other component just as arbitrary as non-baryonic dark matter. Both the "hidden mass" and the "missing mass" questions are at the forefront of research these days, which means that nobody knows the right answers. ------------------------------------------------------------------------- Steve Willner Phone 617-495-7123 Bitnet: willner@cfa 60 Garden St. FTS: 830-7123 UUCP: willner@cfa Cambridge, MA 02138 USA Internet: willner@cfa.harvard.edu
terry@utastro.UUCP (Terry Hancock) (01/18/90)
In article <284@cfa.HARVARD.EDU> willner@cfa.HARVARD.EDU (Steve Willner, OIR) writes: >From article <9364@hoptoad.uucp>, by tim@hoptoad.uucp (Tim Maroney): >> So many scientists seem to take the dark matter for granted, > > >> but I've yet to see any clear reason for >> postulating it [a Closed Universe] other than a sort of >> religious dogma that the universe >> will eventually recollapse. Is there really any stronger basis for >> believing that we only see ten percent of the universe, or are people >> letting their aesthetics guide their modeling? > > >The second problem is usually called the "Missing Mass Problem." Even >accounting for the hidden mass, observable galaxies add up to only >0.3 or less of the "critical mass," i.e., the maximum mass allowed >where the Universe expands forever (not recollapses). As far as I can >tell, there is indeed no observational evidence for existence of the >"missing mass;" indeed, there is considerable evidence that the >"missing" mass, if it exists, cannot be in the form of baryons. The >"missing mass" must also _not_ accompany the visible galaxies but rather >must be more uniformly distributed through the Universe. > Well, there is ONE sort of evidence that I've seen -- the redshift curve (i.e. the curve relating distance to redshift) has a different shape for the open, flat, and closed models. Unfortunately, this is a difficult measurement, since it requires an alternate form of distance measurement -- AND the difference between the curves is only significant towards extreme distances. The only distance method I have heard of being applied to this problem involves the measurement of relative brightness and the assumption of statistically homogenous galaxy-brightness both over space and over the history of the universe. Needless to say this is a risky assumption. HOWEVER, despite all these caveats, the resulting curves that I have seen loosely support a closed universe. This (presumably) implies that we are missing some of the mass which is responsible for the closure. Of course, the observation could be prey to a number of systematic errors, and I wasn't all that convinced. >Theorists often (always?) "let their aesthetics guide their modeling." >What's wrong with that? The relevant questions are whether models are >ruled out by existing data and whether simpler models would explain the >data just as well. The answer to the first is "maybe" and to the second >"no". Models with mass lower than critical invoke either coincidence >or some other component just as arbitrary as non-baryonic dark >matter. Both the "hidden mass" and the "missing mass" questions are at >the forefront of research these days, which means that nobody knows the >right answers. The whole problem with this area is that there's too little "existing data" to support the theories. I'm taking all of it with a grain (maybe a tonne) of salt until a LOT more observations are made (and possible). I don't honestly expect to live long enough to see a day when I take any cosmological theories seriously. Just remember that Ptolemy's universe was not "ruled out by existing data" for a very long time. Which, of course, is not all bad -- a lot of good astronomy was done with the Ptolemaic assumptions. And, of course, cosmology can be fun, if not correct -- which is probably why there's so much interest in it (aside from the desire to obtain a "God's Eye" view of the world, perhaps). >------------------------------------------------------------------------- >Steve Willner Phone 617-495-7123 Bitnet: willner@cfa >60 Garden St. FTS: 830-7123 UUCP: willner@cfa >Cambridge, MA 02138 USA Internet: willner@cfa.harvard.edu ********************************* Terry Hancock terry@astro.as.utexas.edu *********************************