IN cosmological models based on the standard Friedmann–Robertson–Walker geometry, the apparent flux density or angular size of standard candles or standard rods varies with redshift in a way that depends on the deceleration parameter q0. (Open universes have q0 0.5.) At low redshift, however, observational errors are much greater than the differences in q0 expected for different cosmological models, while at high redshift observational uncertainties, particularly at optical wavelengths, and apparent systematic evolutionary changes in sources obscure the expected geometrical effects. Here I show that measurements by very-long-baseline interferometry (VLBI) of compact radio sources associated with active galaxies and quasars may be largely free of evolutionary effects even at substantial redshifts. The relation between angular size and redshift for a sample of these sources indicates a value of q0 close to 0.5, corresponding to cosmological density near the critical value.

The cosmological deceleration parameter estimated from the angular-size/redshift relation for compact radio sources

Radio galaxies and feedback from AGN jets

Part I. Stars and Stellar Evolution up to the Second World War: 1. The legacy of the nineteenth century 2. The classification of stellar spectra 3. Stellar structure and evolution 4. The end points of stellar evolution Part II. The Large-Scale Structure of the Universe, 1900-39: 5. The Galaxy and the nature of spiral nebulae 6. The origins of astrophysical cosmology Part III. The Opening up of the Electromagnetic Spectrum: 7. The opening up of the electromagnetic spectrum and the new astronomies Part IV. The Astrophysics of Stars and Galaxies since 1945: 8. Stars and stellar evolution 9. The physics of the interstellar medium 10. The physics of galaxies and clusters of galaxies 11. High-energy astrophysics Part V. Astrophysical Cosmology since 1945: 12. Astrophysical cosmology 13. The determination of cosmological parameters 14. The evolution of galaxies and active galaxies with cosmic epoch 15. The origin of galaxies and the large-scale structure of the Universe 16. The very early Universe References Name index Object index Subject index.

The Cosmic Century

Analysis of new high-resolution radio observations of quasars with redshifts >1.5 shows that these powerful extragalactic radio sources have epoch-dependent physical properties. Distant quasars have a more bent, distorted appearance and are smaller than those nearby. The ambient medium, interacting with the radio jets, is postulated as playing an important role in producing these epoch-dependent effects. Optical absorption line studies are important in this respect. These observations provide an important new probe of young galaxies and their formation.

Evolution of radio structure in quasars: a new probe of protogalaxies?

The main foundations of the standard $$\Lambda $$
 CDM model of cosmology are that: (1) the redshifts of the galaxies are due to the expansion of the Universe plus peculiar motions; (2) the cosmic microwave background radiation and its anisotropies derive from the high energy primordial Universe when matter and radiation became decoupled; (3) the abundance pattern of the light elements is explained in terms of primordial nucleosynthesis; and (4) the formation and evolution of galaxies can be explained only in terms of gravitation within a inflation + dark matter + dark energy scenario. Numerous tests have been carried out on these ideas and, although the standard model works pretty well in fitting many observations, there are also many data that present apparent caveats to be understood with it. In this paper, I offer a review of these tests and problems, as well as some examples of alternative models.

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Tests and problems of the standard model in Cosmology

The angular size — redshift (θ − z) relation can in principle be used to discriminate between world models because the angular size subtended by a rigid rod is quite a sensitive function of cosmology, specially at z ≳ 0.5. The test is simpler to apply to objects for which a metric diameter is measured than to objects with isophotal diameters (Sandage 1961). It was first suggested by Hoyle (1938) at the Paris symposium on Radio Astronomy, that the separation between the two lobes of extragalactic radio sources such as Cyg-A, could be used for performing such a test. In an Einstein-de Sitter Universe sources like Cyg-A cannot have angular sizes ≲ 15 arcsec (the minimum occuring at z = 1.25) whereas in the Steady State Universe their sizes should asymptotically approach a value near 4 arcsec at large redshifts. It was not until the early seventies that the test was actually applied to samples of radio quasars with redshifts of upto ~ 2 (Legg 1970; Wiley 1971; Wardle & Wiley 1974). The angular sizes were found to show a large scatter due to a wide distribution of physical sizes and the projection effects associated with the essentially linear radio structures. The upper envelope to the θ -values (which would be expected to show much less scatter) nevertheless appeared to fall off monotonically with increasing z, more or less like the Euclidean relation θ ∝ z-1. The θ − z test thus appeared to be incompatible with the predictions of uniform world models in which the linear sizes of quasars are independent of epoch.

V. K. Kapahi

Papers

The Angular Size — Redshift Relation as a Cosmological Tool