The Lya forest absorption lines in the spectra of quasars are interpreted as caused by the crossings of the light beam with the walls of a bubble structure (expanding with the Hubble flow only). Then, the typical separation between the absorption lines is proportional to the mean size of the bubbles. The variable factor is the expansion rate H [ z ] . T h e Friedmann regression analysis of the observed line separations determines the density parameter 0 0 and the normalized cosmological term A0 = A c 2 / 3 H i of the appropriate cosmological model: 00 = 0.014 f 0.002, Xo = 1:OSO f 0.006.Depending on the Hubble parameter this method reveals the values of the present mean matter density p~, o = 2.6 h Z . kg m-' and of the cosmological constant A = 3.77 hZ m-' (with h = Ho/(100 km/sMpc)). According to our analysis all models with A = 0 must be excluded. The curvature of space is positive. The curvature radius Ro is 3.3 times the Hubble radius ( c / H o ) . The age t o is 2.8 t,imes the Hubble age (IT;'). A A A subject classification: 161Recently, Hoell and Priester (1991b) ([HP91] hereafter) showed that the Lycr forest in quasar spectra can be understood as the result of a homogeneous bubble structure at least up to a redshift of z = 4.4 if the universe is represented by a Friedmann-Lemaitre model with an actual expansion rate Ho = 90 km/(s.Mpc) and an age of about 30. lo9 years. In the present paper we include data from further spectra, partly new, partly omitted in the first paper because of a too cautious estimation of their sensitivity. The analysis is now based on published spectra of 21 quasars with a total of 1320 Lya absorption lines and supports our former result (Liebscher, Priester, Hoell (1992) ([LPH92] hereafter)). The apparent increase in scatter is balanced by the increase in number. Hence, the estimated variance of the parameters does not change appreciably. The Friedmann regression analysis yields the values of the density parameter Q , and of the normalized cosmological term A 0 = A c 2 / 3 H i . The generalized density parameter Of = Qo + A0 turns out to exceed 1, i.e. the space is closed and the curvature index is k = $1.The method is based on the assumption that the bubble structure in the large scale distribution of matter, which is observed in our galactic neighbourhood up to a redshift of 0.05 (deLapparent et al. 1986) was at rest in comoving coordinates at least since the emission of the quasar light, and that the L y a forest in the quasar spectra is due to the cuts of the light beam through hydrogen filaments within the walls of the bubble structure. For a homogeneous and comoving bubble structure the size parameter X of the voids is independent of time. The mean spacing 2 between the absorption lines is measured as a function of the redshift z itself, and we replace the time t by the corresponding value of the redshift z . If we denote the typical bubble size in comoving radial distance x by X = Ax and the corresponding spacing of the redshifts by 2 = Az, we obtain I8 Astron. Nachr. 313 ...

A new theoretical approach is applied to the cloud model as an explanation for the Lyman a forest. It is based on the complete Friedmann equations including a A-term. In the data sets of Lu et al. 1991 and Roser 1993 the lines are constrained by a lower limit (0.36 A, 0.32 k ) of the intrinsic equivalent width. This makes them suitable for a cloud model analysis, supplementing the bubble wall models which also make use of the weaker lines. Both data sets can be represented satisfactorily by a Friedmann-Lernaitre model with XO = 1.08 f 0.02 and RO = 0.014 f 0.006, although the regression formulae differ by an essential factor depending on the redshift. The age of the universe according to this model is 2.8.H:'. It is not necessary to invoke any evolutionary effects in the number densities and in the physical parameters of the clouds.

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Wolfgang Priester was one of Germany's most versatile and quixotic astrophysicists, re-inventing himself successively as a radio astronomer, space physicist and cosmologist, and making a lasting impact on each field. We focus in this personal account on his contributions to cosmology, where he will be most remembered for his association with quasars, his promotion of the idea of a nonsingular "big bounce" at the beginning of the current expansionary phase, and his recognition of the importance of dark energy (Einstein's cosmological constant Λ) well before this became the standard paradigm in cosmology.

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