Recently it has been shown that many of the puzzling features of conventional cosmological models (such as the horizon and flatness problems) could be explained by invoking inflationary models of the early universe with an exponential expansion phase at very early epochs. These models have the added advantage that they are able to make a definite prediction about the present matter density in the universe, i.e. they require that the density be exactly equal to the closure density which in turn can be easily estimated from the Hubble constant now known to within a factor of two. Now if one goes back to an earlier idea that explored the possibility of unusual clustering of quasar redshifts around z = 2 or 3, we get an example of another cosmological model with a definite prediction for the present overall matter density. This is a modified version of the Eddington-Lemaitre type of model which naturally accommodates such features as a clustering of quasars at certain epochs. From these models one can get a prediction for the present matter density which would be an involved function of the Hubble constant and the redshifts at which such clustering occurs. It can be shown that if such clustering had occurred at any z, the present matter density predicted would be substantially smaller than the corresponding closure density. The conclusion is that any clustering of quasar redshifts is incompatiable with inflationary universe models, indirectly providing observational support for these new theories.

We have investigated the strong concentration of high redshift radio quasars in a large area of the sky near M33 reported by Arp. Selection effects appear to be important in determining the significance of the inhomogeneity.

The gravitational lens phenomenon is shown to be not uncommon. A search is now underway to find a larger number of examples, using the VLA to search for candidates, following up with optical observations to establish equality of redshifts in the images. Most of the present lensing examples exhibit anomalous behavior that is most easily understood as evidence for large quantities of intervening non-luminous matter.

Qualitative features of gravitational lensing are discussed in terms of a scalar framework based on Fermat's principle. The lensing action of galaxy-like models with spherical and elliptical mass distributions are described. The elliptical model has three distinct regimes of lensing, of which two correspond to lensing with three images and one with five images. One of the three-image geometries has been frequently explored in the past. Models proposed for 0957+561 correspond to this. The five-image geometry has been invoked for 1115+080. Some general model-independent properties of gravitational lensing are listed. If image parities were available, it might be possible to make statements about the lensing mass even when it is made up of dark matter.

The generalisation from circular to elliptical mass distributions acting as gravitational lenses produces a number of interesting effects. These effects include an extra indeterminacy in predictions of the time delay between the two brightest images, if only the relative brightness and separation of these images is known. This indeterminacy is removed if further information about the system (such as a measurement of the astigmatism of the lens) is available.

The gravitational lens models involving extended mass distribution generally predict an odd number of images with one of the images close to the centre of the principal lensing galaxy. In all the observed lens systems only an even number of images have been unambigously detected so far. It is demonstrated that the presence of a compact nucleus at the centre of lensing galaxies would dim the “odd” image significantly without affecting the rest of the image configuration.

Studies of the characteristic properties of gravitational lensing by clusters of galaxies suggest that the dark matter in them is probably smoothly distributed on the scale of the cluster itself, rather than being clumped into halos around individual galaxies.

We present the results of a prelim minary observational run of the FRV sample (Fringant et al, 1983) of close pairs of UVX objects at high galactic latitude. The extragalactic physical pairs are ∼30% and most of them do appear as “interactivating” AGNs. The method seems equally adapted to find new gravitational mirages as well as really double QSOs.

The gravitationally lensed QSO 0957+561A,B has been monitored from 1979 to the present with CCD cameras. We believe that the arrival time difference is 1.2 ± 0.2 years.

CCD images of the lensed QSO candidate 2237+0305 in the blue and near-infrared are examined. At least two blue images of the QSO, separated by about 1 arcsec, are found. The inferred velocity dispersion of the lensing galaxy core of 150 km/sec implies that no dark matter is required for this case. Limits on the mass distribution of the lensing galaxy core and bar can be obtained from the data. Since the galaxy is so nearby, this lensed QSO is a good candidate for assisted lensing by individual stars in the galaxy core.

The absorption lines in QSO spectra may be produced in material surrounding the QSO, in intergalactic clouds and in the interstellar gas of galaxies along the line of sight to the QSO. The intergalactic clouds produce weak Lyα absorption lines. The intervening galaxies produce absorption line systems with heavy element abundances and ionizations similar to the H i clouds in the halo of our galaxy. The material surrounding the QSO produces broad absorption troughs. Evidence suggests a continuity in spatial correlation and heavy element abundances for the Lyα clouds and intervening galaxies.

It is now widely accepted that a population of intergalactic hydrogen clouds is responsible for the plethora of absorption lines seen at wavelengths shorter than Ly α emission in all high-z QSOs. The question of whether the comoving number density of such clouds evolves with redshift has been somewhat controversial. It is usually assumed that

The forest of absorption lines observed in all high-redshift QSOs at wavelengths shorter than Ly α emission is generally accepted as being due to a population of intergalactic hydrogen clouds. In order to study the physical properties of these clouds and possible changes in the properties with redshift it is first necessary to establish the true continuum level. The difficulty in defining continuum levels in the Ly α forest region is readily acknowledged in the literature but only rarely do published spectra show the adopted continuum.

We describe the various absorption systems in 0215+015, and present results from our new, high-resolution studies at 10 km/sec (FWHM).

The extent and physical conditions of diffuse gas in the outer regions of galaxies are currently the subject of considerable interest. A very sensitive way to probe the gas is by observing the absorption lines it produces in the spectra of background objects. However, a detailed investigation of the interstellar medium associated with external galaxies requires the availability, in the field of the galaxy under study, of several probes (QSOs, Active Galactic Nuclei, supernovae) which are: (a) sufficiently bright for high-resolution spectroscopy (B ≤ 17.5) and, (b) located over a range of projected distances from the galaxy, say from 10 to 200 kpc. As there are very few QSOs in the literature which meet these requirements, we have been carrying out a search of nearby galaxy fields for the specific purpose of finding a number of suitable background probes.

Absorption lines at the redshifts of cosmic voids in Perseus-Pisces and Böotes have been detected in the ultraviolet spectra of background quasars. The detection of Si IV and C IV lines besides Ly α suggests chemical enrichment of gas in the voids. Additional observations of three sight-lines through the void in the Böotes suggests that the gas in the void is clumped in large clouds.