Those of us who were around a few decades ago, frequently became initiated into cosmology by a very clear and concise small book, written by Herman Bondi, entitled “Cosmology” (1952/1960). It is interesting to see the list of chapter headings:

The first astronomical background firmly established was that of cosmic rays. Photons at various wavelengths came later; and in some bands we have not year clearly peeled away all the sources to see a true background, if one even exists. All known backgrounds are astrophysically important and at least several cosmologically so. The path by which the standard hot big bang came to be generally regarded as standard is littered with the detritus of mistaken impressions, misunderstandings, and missed opportunities.

The Cosmic Background Explorer (COBE) was developed by NASA Goddard Space Flight Center to measure the diffuse infrared and microwave radiation from the early universe. It also measured emission from nearby sources such as the stars, dust, molecules, atoms, ions, and electrons in the Milky Way, and dust and comets in the Solar System. It was launched 18 November 1989 on a Delta rocket, carrying one microwave instrument and two cryogenically cooled infrared instruments. The Far Infrared Absolute Spectrophotometer (FIRAS) mapped the sky at wavelengths from 0.01 to 1 cm, and compared the CMBR to a precise blackbody. The spectrum of the CMBR differs from a blackbody by less than 0.03%. The Differential Microwave Radiometers (DMR) measured the fluctuations in the CMBR originating in the Big Bang, with a total amplitude of 11 parts per million on a 10° scale. These fluctuations are consistent with scale-invariant primordial fluctuations. The Diffuse Infrared Background Experiment (DIRBE) spanned the wavelength range from 1.2 to 240 μm and mapped the sky at a wide range of solar elongation angles to distinguish foreground sources from a possible extragalactic Cosmic Infrared Background Radiation (CIBR). In this paper we summarize the COBE mission and describe the results from the FIRAS instrument. The results from the DMR and DIRBE were described by Smoot and Hauser at this Symposium.

Observations of the Cosmic Microwave Background (CMB) Radiation have put the standard model of cosmology, the Big Bang, on firm footing and provide tests of various ideas of large scale structure formation. CMB observations now let us test the role of gravity and General Relativity in cosmology including the geometry, topology, and dynamics of the Universe. Foreground galactic emissions, dust thermal emission and emission from energetic electrons, provide a serious limit to observations. Nevertheless, observations may determine if the evolution of the Universe can be understood from fundamental physical principles.

We review the main results obtained in the first cosmic experiment for study of the large scale anisotropy of CBR at 8 mm RELICT-1 and compare them with data of COBE mission.

This paper reviews the great progress recently made in searches for and the characterization of anisotropies in the cosmic microwave background. We now have secure detections on some angular scales and improved upper limits on others. As the sensitivity of such searches increases, understanding sources of foreground noise (e.g., Galactic and extragalactic radio emission) becomes more important. Also reviewed are the contributions aperture synthesis (interferometric) observations can make in characterizing cosmic background fluctuations and foreground sources of noise. Some recent results from the Very Large Array at λ = 3.6 cm are given; these set a limit δT/T ≲ 1.4 × 10−5 on fluctuations at θ ~ 80′. Recent work on the Sunyaev-Zel'dovich effect is summarized.

Various techniques and methods that have been developed for using extragalactic radio sources as cosmological probes of the universe are listed. The discussion is mainly confined to the cosmological tests employing extended radio sizes of powerful radio galaxies and quasars as standard metric rods (in a statistical sense) to figure out the geometry of the universe. Some comments are made on the recent use of the milliarcsec scale sizes of compact radio sources for angular size–redshift tests. It is further pointed out that the estimates of clustering for quasars selected at centimetre wavelengths could be seriously affected by the relativistic beaming.

The potentially important role of jet-cloud interactions in determining the appearance of high-redshift radio galaxies is discussed and investigated via new 3-dimensional simulations of off-axis jet-cloud collisions. The results indicate that the most powerful radio sources are likely to be observed during or shortly after a jet-cloud interaction, and that such interactions can explain both the radio structures and the spatial association between optical and radio light found in powerful radio galaxies at high redshift. It is argued that, due to the radio-power dependence of such complicating effects, the optical-infrared colours and morphologies of very radio-luminous high-redshift galaxies can tell us essentially nothing about their evolutionary state. Either one must study much less radio-luminous sources in which the AGN-induced contamination is minimised, or one must attempt to determine what fraction of the baryonic mass of the radio galaxy has been converted into stars at the epoch of observation. Recent observations aimed at performing the latter experiment on two well-known high-redshift radio galaxies (4C 41.17 & B2 0902+34) are described. It is concluded that at present there exists no clear evidence that either of these famous galaxies is ‘primæval’; on the contrary, the continued low-dispersion of the infrared Hubble diagram at z > 2 points toward a much higher redshift of formation for elliptical galaxies.

Dust, H I and molecular gas must be important components of distant radio galaxies. Recent observations to detect these constituents are discussed.

We have searched for gravitational milli–lens systems by examining VLBI maps of ~ 300 flat–spectrum radio sources. So far we have followed up 7 candidates, with separations in the range 2–20 mas. None have been confirmed as lenses but several of them can not yet be definitively ruled out. If there are no milli-lenses in this sample then uniformly–distributed black holes of 106 to 108 M⊙ cannot contribute more than ~ 1% of the closure density.

The Diffuse Infrared Background Experiment (DIRBE) on the Cosmic Background Explorer (COBE) satellite is designed to conduct a sensitive search for isotropic cosmic infrared background radiation over the spectral range from 1.25 to 240 μm. The cumulative emissions of pregalactic, protogalactic, and evolving galactic systems are expected to be recorded in this background. The DIRBE instrument has mapped the full sky with high redundancy at solar elongation angles ranging from 64°to 124°to facilitate separation of interplanetary, Galactic, and extragalactic sources of emission. Conservative limits on the isotropic infrared background are given by the minimum observed sky brightnesses in each DIRBE spectral band during the 10 months of cryogenic operation. Extensive modeling of the foregrounds is under way to isolate or strongly limit the extragalactic infrared component. The current approach to these modeling efforts is described and representative present residuals are reported.

Detailed observations in the optical do not allow to discriminate between the following scenarii of galaxy formation:

- synchronous formation at large redshift (z > 7).

- hierarchical merging, down to z ≤ 1, of proto-galactic lumps formed at very high redshift (≈ 10).

- galaxy formation at low z (≈ 2-5), but partially obscured by dust.

More than 1500 galaxies in a field of 2.6′ × 4.6′ were detected by Tyson (1988) with CCD images in the B band. These deep counts of galaxies revealed the presence of an unexpectedly large population of blue galaxies, corresponding to a number excess of 3 to 5 times more galaxies at B > 23 than one can expect from the local luminosity function. These objects lie apparently at moderate redshift (z = 0.3 for B = 23–24) as shown by spectroscopy of B-selected galaxies (Colless et al. 1990).

Diffuse background radiation is integrated light which is consisted of various components of interplanetary, stellar, interstellar, galactic and intergalactic origins as well as cosmic background radiation, the remnant of the pre-galactic phenomena in the early history of the universe.

The Cosmic Background Radiation gives us one of the few probes into the density perturbations in the early universe that should later lead to the formation of structure we now observe. Recent advances in degree scale anisotropy measurements have allowed us to begin critically testing cosmological models. Combined with the larger scale measurements from COBE we are now able to directly compare data and theory. These measurements promise future progress in understanding structure formation. Because of the extreme sensitivities needed (1-10 ppm) and the difficulties of foreground sources, these measurements require not only technological advances in detector and measurement techniques, but multi spectral measurements and careful attention to low level systematic errors. This field is advancing rapidly and is in a true discovery mode. Our own group has been involved in a series of eleven experiments over the last six years using the ACME (Advanced Cosmic Microwave Explorer) payload which has made measurements at angular scales from 0.3 to 3 degrees and over a wavelength range from 1 to 10 mm. The recent data from these and other measurements will be reviewed as well as some of the challenges and potential involved in these and future measurements.

In this paper we review the history of the search for and study of the motions of nearby galaxies with respect to the Hubble Flow. The current status of the field is that (1) convincing infall has been detected into dense clusters, especially the Virgo cluster, (2) the microwave background direction is moderately well aligned with the measured flow nearby but not apparently on larger scales, and (3) there is good but not perfect consistency between the nearby density fields and velocity fields. Particular problems exist in the different Ω's required to fit the density field derived from optically selected and IRAS (60μ) selected galaxy samples.

New observations obtained during the last two years have slightly shifted the balance of evidence towards larger values of Ho. A compilation of recent determinations shows reasonable agreement and suggests that Ho ≳ 75 km s−1 Mpc−1.

The methods are reviewed that give a distance modulus to the core of the Virgo cluster of m - M = 31.64 ± 0.08 (D = 21.3 ± 0.8 Mpc). It is shown that the cosmic velocity of the cluster core is 1179 ± 17 km s−1, which, when combined with the distance gives Ho = 55 ± 3 km s−1Mpc−1 from the Virgo cluster data alone. Nine independent methods are reviewed that confirm that Ho = 50 ± 2. Discussion is made why all methods that are said to give the short distance scale (Ho ~ 85) are incorrect.

Peculiar velocity measurements of 2500 southern spiral galaxies show large-scale flows in the direction of the Hydra-Centaurus clusters which fully participate in the flow themselves. The flow is not uniform over this region and seems to be associated with the denser regions which participate in the flow of amplitude about 400km/s. In the less dense regions the flow is small or non-existent. This makes the flow quite asymmetric and inconsistent with that expected from large-scale, parallel streaming flow that includes all galaxies out to 6000km/s as previously thought. The flow cannot be modelled by a Great Attractor at 4300km/s or the Centaurus clusters at 3500km/s. Indeed, from the density maps derived from the redshift surveys of “optical” and IRAS galaxies, it is difficult to see how the mass concentrations can be responsible particularly as they themselves participate in the flow. These results bring into question the generally accepted reason for the peculiar velocities of galaxies that they arise solely as a consequence of infall into the dense regions of the universe. To the N. of the Great Attractor region, the flow increases and shows no sign of diminishing out to the redshift limit of 8000km/s in this direction. We may have detected flow in the nearest section of the Great Wall.