GAIA and the extragalactic distance scale

The local expansion field (v(220) < 1200 km s(-1)) and the cosmic expansion field out to 30 000 km s(-1) are characterized by H-0 = 58 [km s(-1) Mpc(-1)]. While the random error of this determination is small (+/- 2 units), it may still be affected by systematic errors as large as +/- 10%>. The local expansion is outlined by Cepheids and by Cepheid-calibrated TF distances of a complete sample of field galaxies and by nearby groups and clusters; the cosmic expansion is defined by Cepheid-calibrated SNe Ia. The main source of systematic errors are therefore the shape and the zero point of the P-L relation of Cepheids and its possible dependence on metallicity. GAIA will essentially eliminate these systematic error sources. Another source of systematic error is due to the homogenization of SNe Ia as to decline rate Deltam(15) and color (B - V). GAIA will discover most of the 1100 SNe Ia within 10 000 km s(-1), which will occur during its four-year lifetime. If their photometric parameters can be determined from the ground, they will fix the dependence of the SNe Ia luminosity on Deltam(15) and (B - V) with high accuracy. At the same time they will yield exquisite distances to an equal number of field galaxies. - GAIA will also revolutionize the very local distance scale by determining fundamental distances of the companion galaxies of the Milky Way and even of some spirals in- and possibly outside the Local Group from their rotation curves seen in radial velocities and proper motions. Moreover, GAIA will obtain trigonometric parallaxes of RR Lyrae stars, of red giants defining the TRGB, of stars on the ZAMS, of White Dwarf defining their cooling sequence, and of globular clusters, and determine the metallicity dependence of these distance indicators. It will thus establish a self-controlling network of distance indicators within the Local Group and beyond.