Research Publications
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Item Parasectones in astronomy 26 - Dark matter in the niverse(Khagol, 1999-05-01) Naralikar, J.V.Item Quasi-steady state cosmology: A study of angular size against redshift(Wiley-Blackwell, 1999-02-25) Banerjee, Shyamal K.; Narlikar, J. V.The data on angular sizes and redshifts of ultracompact radio sources used by Jackson & Dodgson in a recent paper have been applied to the various theoretical models in the framework of the quasi-steady-state cosmology proposed by Hoyle, Burbidge & Narlikar. It is found that although acceptable fits to the data are available for the flat models, those with a negative curvature of spatial sections provide a better fit. These latter models require low densities of matter and as such do not demand too high a proportion of dark matter. A comparison is made with the work of Jackson & Dodgson and theoretical implications of the test are discussed.Item Alternative cosmologies(Kluwer, Dordrecht, 1997-11-14) Narlikar, J. V.Item Contributions of astronomy to our understanding of physics INSA - Vainu Bappu Award Lecture at Madurai(Indian National Science Academy, 1989-01-06) Narlikar, J. V.Item Constraints on the shape of the density spectrum from COBE and galaxy surveys(Wiley-Blackwell, 1992-10-28) Padmanabhan, T.; Narasimha, D.Item Modelling the nonlinear gravitational clustering in the expanding universe(Wiley-Blackwell, 1995-11-02) Padmanabhan, T.The gravitational clustering of collisionless particles in an expanding universe is modelled using some simple physical ideas. I show that it is indeed possible to understand the nonlinear clustering in terms of three well defined regimes: (1) linear regime (2) quasilinear regime which is dominated by scale-invariant radial infall and (3) nonlinear regime dominated by nonradial motions and mergers. Modelling each of these regimes separately I show how the nonlinear two point correlation function can be related to the linear correlation function in heirarchical models. This analysis leads to results which are in good agreement with numerical simulations thereby providing an explanation for numerical results. The ideas presented here will also serve as a powerful anlytical tool to investigate nonlinear clustering in different models. Several implications of the result are discussed.Item Transfer of power in nonlinear gravitational clustering(Wiley-Blackwell, 1996-12-15) Bagla, J. S.; Padmanabhan, T.We investigate the transfer of power between different scales and the coupling of modes during the non-linear evolution of gravitational clustering in an expanding universe. We start with a power spectrum of density fluctuations that is exponentially damped outside a narrow range of scales, and use numerical simulations to study the evolution of this power spectrum. Non-linear effects generate power at other scales, with most power flowing from larger to smaller scales. The ‘cascade’ of power leads to equipartition of energy at smaller scales, implying a power spectrum with index n ~ - 1. We find that such a spectrum is produced in the range 1 < ð < 200 for density contrast ð. This result continues to hold even when small-scale power is added to the initial power spectrum. Semi-analytic models for gravitational clustering suggest a tendency for the effective index to move towards a critical index Nc ~-1. We find that such a spectrum is produced in the range 1< ð<200 for density contrast ð. This result continues to hold even when small-scale power is added to the initial power spectrum. Semi-analytic models for gravitational clustering suggest a tendency for the effective index to move towards a critical index Nc ~-1 in this range. For n< Nc , power in this range grows faster than linear rate, while if n> Nc , it grows at a slower rate- thereby changing the index closer to Nc. At scales larger than the narrow range of scales with initial power, a k⁴ tail is produced. We demonstrate that non-linear small scales do not affect the growth of perturbations at larger scales.Item Effects of anticorrelation on gravitational clustering(Wiley-Blackwell, 2001-01-28) Kanekar, Nissim; Padmanabhan, T.We use non-linear scaling relations (NSRs) to investigate the effects arising from the existence of negative correlations on the evolution of gravitational clustering in an expanding universe. It turns out that such anti-correlated regions have important dynamical effects on ıt all scales. In particular, the mere existence of negative values for the linear two-point correlation function ξbL over some range of scales starting from l = Lo, implies that the non-linear correlation function is bounded from above at ıt all scales x < Lo. This also results in the relation ξb ∝ x-3, at these scales, at late times, independent of the original form of the correlation function. Current observations do not rule out the existence of negative ξb for 200 h-1 Mpc ła ξb ła 1000 h-1 Mpc; the present work may thus have relevance for the real Universe. The only assumption made in the analysis is the ıt existence of the NSR; the results are independent of the form of the NSR as well as of the stable clustering hypothesis.Item Gravitational perturbation of homogeneous collisionless dark matter(Indian Academy of Sciences, 1985-08-12) Padmanabhan, T.; Vasanthi, M. M.The effect of a perturbing mass on a homogeneous collisionless cloud of dark matter is considered in the linear approximation. It is shown by that gravitational potential can have turning points, in sharp contrast with gravitating system of finite extent. The model offers a reasonable explanation for the observed secondary maxima in the destiny distribution of rich clusters . The relevance of the model to the flatness of the rotation curves of galaxies is also discussed.Item Gravitational collapse in an expanding universe: scaling relations for two-dimensional collapse revisited(Wiley-Blackwell, 2005-03-21) Ray, Suryadeep; Bagla, J. S.; Padmanabhan, T.We investigate non-linear scaling relations for two-dimensional (2D) gravitational collapse in an expanding background using a 2D TreePM code, and study the strongly non-linear regime ( ¯ξ 200) for power-law models. Evolution of these models is found to be scale invariant in all our simulations. We find that the stable clustering limit is not reached, but there is a model independent non-linear scaling relation in the asymptotic regime. This confirms results from an earlier study that only probed the mildly non-linear regime( ¯ξ 40). The correlation function in the extremely non-linear regime is a less steep function of scale than reported in earlier studies. We show that this is due to coherent transverse motions in massive haloes. We also study density profiles, and find that the scatter in the inner and outer slopes is large and that there is no single universal profile that fits all cases. We find that the difference in typical density profiles for different models is smaller than expected from similarity solutions for halo profiles, and transverse motions induced by substructure are a likely reason for this difference being small.