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P.L. Schechter (MIT)
When astronomers talk about Lutz-Kelker corrections, metallicity dependent zeropoints, statistical parallaxes, Tully-Fisher relations, "fundamental" planes, light curve decline rates and, worst of all, Malmquist bias, physicists begin heading for the exits, showing signs of severe allergic reaction. They respond less violently to so-called "direct" methods of measuring distances which bypass the traditional distance ladder. Two of these, gravitational lens time delay measurements (Refsdal's method) and the Sunyaev-Zeldovich (S-Z) effect, give distance measurements to objects at high redshift which appear to rival more traditional approaches. Present, model mediated interpretations of such measurements give low values for the Hubble constant. But as is often the case with new techniques, initial enthusiasm is followed by increasing concern about systematic errors connected with messy astrophysical details. The single largest source of error in modelling lenses is the difficulty in constraining the degree of central concentration of the lensing galaxy. Sources of systematic error in S-Z distances include the clumpiness of intracluster gas, temperature variations within that gas and a bias toward selecting clusters that are elongated along the line of sight. Present best estimates of the Hubble constant, along with best estimates of the systematic uncertainties, and the prospects for improving upon these, will be presented.
Support from NSF grant AST96-16866 is gratefully acknowledged.
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