Weak interacting particles (WIPs): neutrinos (νe, νμ, ντ), the hypothetical photino (), the gravitino (), etc., may have nonzero rest mass. This fact is extremely important for cosmology. WIPs do not annihilate in the very early Universe and their number is preserved. If they have a rest mass, their mass density may dominate in the Universe (1).

To date observations have not yet unequivocally determined whether the universe is open or closed. Although for the luminous matter Ω≲0.1, the possibility of existence of non-luminous matter (especially if the neutrino has a non-zero mass) leaves ground for considering that the universe may indeed be closed. In this case the universe is expected to recollapse and become again, at a time to, radiation dominated. Hence R(t) ∼ (tf−t)1/2 ∼ 1/T (t), where tf is the time of collapse to the singularity and T is the temperature. During this new radiation dominated era, a black hole of mass Mo will accrete at a rate

where ρ(t) ⋍ αT4 (t) and r(t) = 2GM(t)/c2. Eq (1) upon integration yields

(All subscript zero quantities are taken at t = to). Eq (2) shows that the mass of the black hole diverges when τo/τ ∼ Mo/4πro2 ρo cρo where τo = tf − to.

In this presentation we show how the study of the isotropy of the X-ray sky contributes to our understanding of the structure of the universe at moderate redshifts (1≲z<<zrecombination). Actually, the anisotropy of the sky flux provides the information, much as the microwave sky anisotropy does for earlier epochs. [See reports in this volume.] Though we are currently unable to make measurements with the precision and small solid angles typically achieved in the microwave, comparatively crude limits from the X-ray fluctuations place limits on the largest scale structure of the universe. We first outline the measurements of the X-ray sky and its anisotropies made with the HEAO 1 A-2 experiment. Detailed presentations are found elsewhere [Shafer 1982; Marshall et al. 1980; Piccinotti et. al. 1982; Iwan et al. 1982; Shafer et al. in prep.]. We then show how the anisotropies place limits on the origin of the X-ray sky and on any large scale structure of the universe, following the example of previous analyses which used earlier anisotropy estimates [see e.g. Fabian and Rees 1978; Rees 1980; Fabian 1981].

The origin of the high energy (X-ray and gamma-ray) background may be attributed to discrete sources, which are usually thought to be active galactic nuclei (AGN) (cf. Rothschild et al. 1982, Bignami et al. 1979). At X-rays a lot of information has been obtained with HEAO-1 in the spectral range 2–165 keV. At gamma-rays the background has been estimated from the Apollo 15 and 16 (Trombka et al. 1977) and SAS-2 (Bignami et al. 1979) observations. A summary of some of the observations (Rothschild et al. 1982) is shown in Figure 1. The contribution of AGN to the diffuse high energy background is uncertain at X-rays although it is generally estimated to be in the 20–30% range (Rothschild et al. 1982). At gamma-rays, in the range 1–150 MeV, AGN (specifically Seyfert galaxies) could account for all the emission.

HEAO 1 A2 and Einstein SSS spectral observations of Seyfert galaxies and BL Lac objects suggest that in both cases, the X-ray emission is due to relativistic particles. The five BL Lac objects have very soft spectra and at higher energies (above 10 keV) may have hard tails. Combining our X-ray data with radio, infrared, optical, and ultraviolet observations, we can fit the BL Lac spectra with the familiar synchrotron self-Compton model if we allow for relativistic beaming (Urry and Mushotzky 1982, Urry et al. 1982). We show that Doppler beaming of an underlying (Seyfert-like) source population flattens the observed luminosity function, and we emphasize that the relative numbers of BL Lacs and quasars in given spectral intervals are strong functions of selection effects, the degree of Doppler beaming, and the form of the intrinsic luminosity function.

The evidence for the evolution of the intergalactic clouds that produce the Lyman-alpha absorption lines in the spectra of QSOs is reviewed. The recent detection of OVI in clouds that produce both Lyman-alpha and Lyman-beta absorption lines is discussed. The identification of molecular hydrogen in the spectrum of 1442+101 (OQ172) is not confirmed by high resolution spectra.

The origin of the narrow metal absorption lines in the spectra of QSOs remains uncertain despite the large amount of high-quality data obtained at high resolution over the past decade. Recently, statistical tests of a uniform sample of C IV absorption systems have shown that their redshift distribution is consistent with the view that these lines arise in randomly distributed intervening galaxies in a Friedmann Universe (Young, Sargent and Boksenberg 1982, hereafter YSB). In a complementary approach we consider in detail the physical properties of the absorbing material in cases of particular interest. We have chosen the high-redshift BL Lac object 0215+015 (Blades et al. 1982, hereafter BHMP) for detailed spectroscopic study because it is currently in a bright phase (V ∼ 14.5–16.5) and because it has several strong absorption systems with differing ionization structure. From our medium-resolution spectra of 0215+015 (BHMP) it appears that the density of C IV systems per unit redshift in this object is somewhat higher than, but not inconsistent with, the average density in the YSB sample.

The optical spectra of distant quasars (OQ 172, PHL 957, PKS 0237–233 and 11 others) were reanalysed with the purpose of searching molecular lines /2, 4/.

The statistic studies of the absorbing forest in the spectra of high redshift QSOs will provide an important approach to the understanding of the physical properties of the absorbing materials. For this purpose a collection of uniform samples would be very important and urgent. It is very time consuming of the large telescopes to take the high resolution, high ratio-to-noise and wide coverage of wavelength of the QSO's spectra, therefore, an international collaboration in a uniform way would be very desirable.

Close pairs of QSOs (separations ≲ 1–5 arcmin) provide a powerful approach to the study of narrow absorption lines in QSO spectra. By looking for absorption in the spectrum of the higher-redshift QSO at the redshift of the other (“associated absorption”), or absorption in both spectra at the same redshift (“common absorption”), one may address several issues: the cosmological nature of QSO redshifts, the origin of the narrow absorption lines of high redshift and excitation (intrinsic or intervening), the clustering of absorbing systems (with each other, and with QSOs), the sizes of the absorbing regions (for metal absorption lines and for Lyα lines; absorption cross-sections of individual galaxies and of clusters of galaxies), and the presence and nature of gaseous halos around QSOs.

Many years ago M. Schmidt established the presence of a strong evolutionary effect in quasistellar objects (QSOs). Though this important problem is not yet fully resolved, there can be no doubt that in earlier epochs of the Universe evolution of the spatial density of QSOs (in the comoving coordinates) was much higher than it is now. Thus, for instance, the spatial density of the optically selected QSO is:

according to Schmidt (1977). Here τ = z/(l+z), whence, for instance, if z = 3, ρ ∼ 1000 ρ0, where ρ0 is the local density.

There are now four known cases of multiply imaged QSOs, one with a detected foreground object at roughly half the affine distance to the QSO: 0957+561 (17 mag, z=1.4, separation =6″), 1115+080 (17 mag, z=1.7, sl=1.8″, s2=2.3″), 2345+007 (19 mag, z=2.1, s=7″) and 1635+267 (19 mag z=2, s=5″). In addition, 1548+115 (19 mag, z=1.9) is a probable lens event with a foreground QSO, but no secondary image has been found. Perhaps 500 candidate QSOs have been surveyed optically for multiple images by all observers. 0957+561 is the only catalogued QSO shown to be multiple. Of the remaining 1548 QSOs currently catalogued, any secondary image is masked by atmospheric scattering of the QSO light. Typically, this sets detection limits of ≳ 3 mag fainter and < 2 arcsec separation from the bright component, for any secondary image. Objective prism and grism surveys look directly for multiple QSOs with identical emission lines and have surveyed 1500 QSOs. The remaining three lensed QSOs come from these more efficient surveys. Although the exciting search for multiply imaged QSOs has only begun, sufficient data already exist to test two hypotheses: (A) QSOs are intrinsically luminous and occasionally are multiply imaged through a chance alignment with a foreground galaxy of sufficient mass gradient and (B) all QSOs are the result of gravitational lens magnification of a distant Seyfert nucleus by foreground galaxy(s). I will first address hypothesis B, then A. I assume that mass (seen and unseen) clusters with galaxies and/or clusters of galaxies.

There has been a good deal of discussion in recent literature about the hypothesis, first put forward by Barnothy and Barnothy (1968), that quasars could be gravitationally lensed nuclei of Seyfert's galaxies (Turner, 1980; Tyson, 1981). Large amplifications (> 3–4 magnitudes) are needed to account for the widespread distribution in the intrinsic optical luminosities of quasars. A direct verification of this hypothesis is difficult to achieve due to the limited angular resolution of ground based telescopes. However, this hypothesis may be tested in a global sense by referring to radiation properties of Sy 1 nuclei which must be preserved, or must change in any predictable way, through the lensing process.

Nieto (1979) found an excess of optically variable QSOs (OV) near bright galaxies (m < 15.7): 6 observed versus 1.6 expected for r < 5′. The probability involved was p = 5 × 10−3. Because of the small number of OV QSOs in this sample (N = 41, sample 1), this result needed a confirmation. So the same analysis was repeated with a sample of 112 QSOs (sample 3) from Hewitt and Burbidge (1980). Eleven objects were observed at r < 5′ versus 4.4 expected, so p = 4 × 10−3, the sample made up with 71 objects (sample 2) supporting slightly the result found with the first 41 objects. A notable difference between these two samples 1 and 2 is that the objects included in sample 2 are fainter than the objects included in sample 1. Repeating then the same analysis on samples of QSOs at different brightness levels suggests that the excess is related to the apparent brightness of the QSOs.

It is suggested that unseen matter in a galactic envelope or in a group of galaxies may consist of substellar bodies originating as the first permanent ‘stars’ in the formation of a very massive galaxy according to a model previously proposed by the author.

Properties of the large-scale distribution of galaxies are considered. Particular attention is paid to properties of the large-scale structures such as anisotropy of superclusters and the existence of large regions practically devoid of galaxies. Another question discussed in detail is the link between superclusters and formation of a network or cellular structure. An explanation of the latter is proposed in the frame of the fragmentation scenario. The role of the neutrino rest mass is discussed.

The nature of the distribution of galaxies poses a challenging problem for theorists. It seems reasonable, as a start, to suppose that galaxies and clusters arose from small perturbations by gravitational instability. However, one still has the problem of the choice of initial conditions, for example, the shape of the fluctuation spectrum and the cosmological density parameter Ω. A considerable simplification is to assume that the clustering pattern obeys some simple similarity scaling, so that the clustering at some early time, apart from a change in length scale, is statistically indistinguishable from the pattern observed today. The power-law shape of the two-point correlation function and the simple forms of higher order correlation functions (Peebles, 1980) have provided some evidence that such a simplifying assumption may be relevant - just how relevant is the subject of this article.

Simulations of static (non-evolving) fields of galaxies are carried out by means of a Monte-Carlo computer technique which models all physical effects acting on the light from distant galaxies, from emission at the source up to the final detection and measurement on the photographic plate by means of a high-speed measuring machine. Parameters for the model galaxies are computed in order to match the main types of measurement made on astronomical photographs (e.g. image centroids, isophotal magnitudes and colours, orientations and shapes).

Recently finished redshift surveys make it possible to study the large-scale environment of superclusters and their mutual relationship.

Figure 1 shows the distribution of nearby clusters in the sky in supergalactic coordinates at two redshift intervals. Nearby clusters in the distance interval 75 to 150 Mpc form a belt around us which is close to the supergalactic equator; its inclination is only 20°. The following superclusters belong to this belt: Ursa Major-Lynx (Giovanelli and Haynes 1982), Coma, Hydra-Centaurus, Pavo-Corona Australes, and Perseus-Pisces. Coordinates and redshifts for a number of previously unknown southern clusters have been derived by Dr. H. Corwin and Dr. M. Tarenghi (Einasto et al. 1982).

A study of the way observable clustering depends on expansion history is reported. Observable shapes that result from evolving otherwise identical systems are intercompared to show differences due to different expansion histories. Four cases are compared: non-expanding, Ω =1, and two open universes with 0.10 and 0.03 as final values of Ω There is remarkably little difference in observable forms for the expanding cases. The 0.03 universe expanded by a factor 500 during the experiment. This study is an example of the way numerical experiments can be used is studies of galaxy clustering.