The Cosmological Parameters H_0 and Omega_0

R. Brent Tully

Published 1998-02-03Version 1

The first section discusses a recalibration of the luminosity-linewidth technique and its use in a determination of H_0. The recalibration introduces (i) new cluster calibration data, (ii) new corrections for reddening as a function of inclination, and (iii) a new zero-point calibration using 13 galaxies with distances determined via the cepheid period-luminosity method. It is found that H_0=82+/-16 km/s/Mpc (95% confidence). The second section concerns a dynamic measurement of Omega_0. The non-linear Least Action method can be used to reconstruct the orbits galaxies have followed to reach their current positions. The three free parameters of the models are M/L, age of the Universe, and a measure of the possible extended nature of dark halos compared with the distribution of light. Models are constrained by 900 observed distances. Best chi^2 fits are for models with M/L values consistent with Omega_0=0.25+/-0.2. The concept of `antibiasing' is introduced in an attempt to reconcile this low estimate of Omega_0 with the results from linear analyses. It is argued that M/L values in elliptical-rich regions are a factor of 7 larger than those in spiral-rich regions. Hence, whereas it is commonly thought that there is the bias that mass is more extended than light, there may be a cosmologically important regime where there is the antibias of mass more concentrated than light. This possibility makes the inferrance of Omega_0 from beta unclear.