Ultimate goals

Extragalactic distances depend directly on distance determinations lower on the ladder. One might simplistically describe the ladder sequence as follows: (i) the astronomical unit, (ii) parallax and open clusters, (iii) cepheid distances, and finally (iv) extragalactic standard candles. In fact, the numerous methods of the actual ladder link and overlap in a variety of ways. A version of the ladder is shown in Fig. 9. We do not attempt to explain each of the methods in the figure although many of them have been discussed or alluded to in this chapter.

There are two other approaches to determining the Hubble constant that do not depend directly on the ladder, though they are less well developed. One is based on gravitational lensing of distant quasars and another is based on fluctuations in the cosmic microwave background due to photon scattering in the hot plasmas of clusters of galaxies, the Sunyaev-Zeldovich effect.

A prime objective of these several methods is to obtain absolute distances to the most distant galaxies. This directly provides the luminosity of any such galaxy and thus provides the absolutely calibrated luminosity functions for the several types of galaxies. In other words, it yields the power output of these galaxies, a fundamental quantity indeed. The distances to the galaxies together with their measured recessional speeds (from spectral redshifts) yield H0, the Hubble constant at the current epoch.

Deviations of the distances from those predicted by the linear Hubble law, r = v/H0 (29), indicate there are motions superimposed on the "Hubble flow". These are

Hubble constant, H and Galaxy luminosity function

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