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Item Comparison of cosmological models using Bayesian theory(American Physical Society, 2002-01-15) John, Moncy V.; Narlikar, J. V.Using the Bayesian theory of model comparison, a new cosmological model due to John and Joseph @M. V. John and K. Babu Joseph, Phys. Rev. D 61, 087304 ~2000!# is compared with the standard VLÞ0 cosmologi- cal model. Their analysis based on the recent apparent magnitude-redshift data of type Ia supernovas found evidence against the new model; our more careful analysis finds instead that this evidence is not strong. On the other hand, we find that the angular size-redshift data from compact ~milliarcsecond! radio sources do not discriminate between the two models. Our analysis serves as an example of how to compare the relative merits of cosmological models in general, using the Bayesian approach.Item quasi-steady state cosmology(Wydawnictwa Uczelniane, 2001-06-20) Narlikar, J. V.This paper begin with a discussion of the shortcomings of standard cosmology, and then outlines an alternative cosmology called Quasi-Steady State Cosmology (QSSC). The theoretical framework and observational tests of the QSSC are next described. Finally some tests which can distinguish between the standard big bang cosmology and the QSSC are briefly mentioned.Item Counting of radio sources: A personal perspective(Kluwer, Dordrecht, 2000-03-25) Narlikar, J. V.This article gives the author's personal perspective on the continuing efforts by radio astronomers to determine the nature of the cosmological model by counting radio sources in the universe out to different levels of faintness. Although initially the source counts were expected to reveal the underlying geometry of space and time, subsequent experience showed that the issue is mixed up with the physical properties of the sources and their evolution with epoch. It is shown, how the earlier claims of disproof of the steady state model through source sounts, turned out to rest on very uncertain evidence.Item Quasi-steady state cosmology(IOP Publishing, 1995-03-14) Hoyle, F.; Burbidge, G.; Narlikar, J. V.Because of a number of unsatisfactory features of the standard hot big bang cosmology, it is argued that there is a case for exploring alternative approaches to cosmology. The approach described here called the quasi steady state cosmology (QSSC, uses a field theoretic description of matter creation within the framework of general relativity. A cosmological solutions with the universe expanding exponentially along with cycles of expansion and contraction arises from mini-creation events taking place near the event horizons of highly collapsed massive objects. The now familiar phenomena like QSOs, AGN, radio sources etc. are the manifestations of matter creation in such events. In this way cosmology is seen to be related to high energy astrophysics in a very direct way. The quasi can explain the abundances of light nuclei and the microwave background, observed large scale features of the universe like the m-z relation, the source count, the angular size-redshift relation, as well as observed distribution of the ages of galaxies.Item Singularity and matter creation in cosmological models(Nature Publishing Group, 1973-04-30) Narlikar, J. V.Item Cosmological models in a conformally invariant gravitational theory-II : A new model(Wiley-Blackwell, 1972-01-27) Hoyle, F.; Narlikar, J. V.Item Cosmological models in a conformally invariant gravitational theory-I : The Friedmann model(Wiley-Blackwell, 1972-01-27) Hoyle, F.; Narlikar, J. V.The present paper discusses the formulation of the Friedmann cosmological models in terms of a conformally invariant ravitational theory. This theory is Machian in the sense that the mass of a particle arises from the interaction of the particle with a mass field m(X) generated by other particles. In cosmology the mass field m(X) at any particular space-time point X arises predominantly from particles at great distance from X. The Friedmann models are usually discussed in terms of the RobertsonWalker line element. It is known that this line element is conformal to the Minkowski line element ds² = dr²-d² -r² (d0 ² +sin² 0 dᵩ²). Cosmological space-time can therefore be transformed to Minkowski space-time by a suitable conformal transformation. It is not possible in the usual expositions to take advantage of this geometrical simplification because Einstein's gravitational equations are not conformally invariant. However, the present theory is conformally invariant so that transformation to Minkowski space is possible not only for the geometry but also for the physics. The three Friedmann cases k = 0, ± I are discussed in detail from this point of view. Although the cases k = ± I are spatially homogeneous in the Robertson-Walker frame they are not similarly homogeneous in the Minkowski frame, where they can be seen to represent only local clouds that happen to be symmetrically distributed with respect to an observer at r = o. This lack of homogeneity is not shared by the k = 0 case, which emerges from the analysis as the only model consistent with homogeneity in both frames, Robertson-Walker and Minkowski. The conformal transformation function between these two frames is singular at T = o. It is this mathematical breakdown of the transformation function which introduces the well-known singularity of the Friedmann models with respect to the Robertson-Walker frame--the singularity usually referred to as the origin of the Universe. From the present point of view this so-called origin does not arise physically at all. It turns out that the Universe possesses an opposite half, T < 0 in the Minkowski frame, which connects smoothly with ' our' half, T > o. Both halves of the Universe contribute to the mass function m(X), and are therefore connected physically. Indeed the appropriate form for m(X) appears to demand that both halves of the Universe be present. The half T < 0 is missed when the Robertson-Walker frame is used.Item Cosmological models and their observational validation(IOP Science, 1966-04-15) Davidson, W.; Narlikar, J. V.The observational data of cosmology are presented in considerable detail and their interpretation in terms of various cosmological models is described. The data survey the nature and cosmic distribution of luminous matter (galaxies), radio sources, quasars, intergalactic matter, the radio and optical background radiation, x rays, y rays and cosmic rays. The theoretical basis of cosmological models is described in its general aspects and in particular cases. Topics discussed include the homogeneous anisotropic models, isotropic models, RIach’s principle, relativistic cosmology, Newtonian cosmology and the various versions of the steady-state theory and creation of matter. Theoretical observable relations are derived or described for the isotropic models of relativistic cosmology and the steady-state theory. The theoretical relations between observables are compared with the data from various observational tests. These include the red-shift-apparent-magnitude relation, counts of radio sources, the radiation background at radio and optical frequencies and the x-ray and y-ray background. The possibility of intergalactic cosmic rays is discussed and an account is given of recent observational tests for intergalactic hydrogen. The relevance to particular cosmologies of neutrino degeneracy, the absorber theory of radiation, the age distribution of galaxies and the origin of the chemical elements is analysed in some detail. The most important conclusion is that several tests, among them the measurements of red shift and apparent magnitude, the counts of radio sources, the radio background at high frequency and the present He/H abundance ratio, all support a universe that was denser in the past. Furthermore, recent tests suggest that the density of hydrogen in intergalactic space is below the mean density of luminous matter (galaxies) by several orders of magnitude. Thus the evidence is now extremely strong against the steady-state cosmology in its original simple form, while relativistic cosmology to the above extent finds support. On the other hand, there is the difficulty that the evolutionary ages of some star clusters are estimated to exceed the predicted ages of galaxies in both cosmologies, while the absorber theory of radiation gives consistent retarded solutions in the steady-state theory but not in relativistic cosmology. Although contrary arguments exist, there is strong theoretical evidence that a singularity in the relativistic cosmology is inevitable, and no way has yet been found in conventional physics either to prevent it or to describe it. However, the discovery of new force fields in physics at high density, on the lines of the negative energy C field of the steady-state theory, may show how a contracting universe may he reversed into expansion without singularity. This might imply a finitely oscillating universe, for which there are special difficulties. The theoretical and observational study of the recently discovered quasars may throw light on this issue. But from the point of view of present-day physics the evidence points to a fundamental singularity of the observable Universe that occurred about ten thousand million years ago. There is also considerable evidence that this was followed by a hyperbolic expansion.Item Cosmology Today: Models and constraints(Indian Academy of Sciences, 1995-03-12) Padmanabhan, T.Cosmological models for structure formation are severely constrained by several of the recent observational results. we now have observations which probe the power spectrum of fluctuations from about 0.5h-1 Mpc. these probes and the constraints they imply on models for structure formation are reviewed.Item Attempt to explain the smallness of the cosmological constant(World Scientific Publication Company, 1987-11-09) Singh, T. P.; Padmanabhan, T.Fields which couple directly to the cosmological constant (Λ) may provide a scenario for explaining the smallness of Λ at the present epoch. In this paper we postulate the existence of a scalar field which couples universally to the trace of energy—momentum tensor of matter. Various possibilities for the explicit form of the coupling function are considered. The field equations in such a theory are derived, and the cosmological models with such a scalar field are analyzed. The proposed coupling makes the effective cosmological constant a dynamically evolving quantity, which can be driven to zero by allowing the scalar field to grow to sufficiently large values. For the case of linear coupling, however, it does not seem to be possible to attain sufficient growth during the age of the universe (~1017 s). A quadratic coupling to the trace can evolve Λ to a value consistent with today’s observations, but the universe is dominated by the scalar field, rather than by radiation, at late times. The evolution is singular for couplings through a higher power law, in that the scalar field blows up at a finite time. The model is not very sensitive to initial conditions and the problems encountered can be avoided only by a severe fine-tuning of the parameters in the basic theory.