Browsing by Author "Vasanthi, M. M."
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Item Can curvature effects be neglected in the early universe?(Elsevier Science Publishers, 1982-05-31) Padmanabhan, T.; Vasanthi, M. M.In the discussion of GUTS in the early universe it is assumed that there exists a locally inertial space-time region large enough to contain a sufficient number of particles justifying the use of fiat space statistical mechanics. We show that this assumption is false.Item Constraints on unstable heavy neutrinos from cosmology(Indian Academy of Sciences, 1985-12-11) Vasanthi, M. M.; Padmanabhan, T.Cosmological scenario with massive unstable neutrinos are discussed. Restrictions on the mass and lifetime of the unstable neutrino are derived from (a) age and mass density of the universe and (b) the growth of primordial fluctuations. It will not be possible to accommodate unstable neutrinos with masses above ~ `1 keV in standard cosmology unless they have exceedingly small lifetimeItem Dynamical scenarios with unstable neutrinos(American Astronomical Society, 1987-04-15) Padmanabhan, T.; Vasanthi, M. M.We describe a cosmological scenario with an unstable warm dark matter candidate (a heavy neutrino νH with mass of ~120 eV) which decays into a “hot” particle (a light nuetrino νL with mass of ~6 eV) and a relativistic boson. Theoretical and observational constraints on such a model are discussed. We note that (1) decay of νH disrupts the condensates made of primordial νL, lowering their mass to acceptable values of ~1012 M⊙. (2) The relativistic boson can contribute nearly 0.25 to the total density in a “recent” decay. The model predicts two prominent scales in dark matter distribution: (i) a mass of about ~4 × 1012M⊙ around the galaxies, distributed over about ~200 kpc, and (ii)a smoother density of ~10-27 g cm-3 distributed over ~1 Mpc. We argued that the model agrees with observations at all scales.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 Nature and distribution of dark matter: 1. Milky way and dwarf spheroids(Indian Academy of Sciences, 1985-09-20) Padmanabhan, T.; Vasanthi, M. M.We argue that observations on Milky Way and dwarf spheroidals imply existence of individual haloes around dwarf spheroidals. If neutrions (or any other 'hot' particle) provide the dark matter then we show that: (i) Embedding of visible matter inside large (few Mpc) dark matter haloes of about 10 kpc radius around them, and have an (M/L) radio of about 100 kpc in radius. if 'cold' dark matter makes up the haloes, then no significant constraints are obtained. We discuss briefly the effect of these constraints on larger scales.Item Nature and distribution of dark matter: 2. Groups and clusters(Indian Academy of Sciences, 1989-08-11) Vasanthi, M. M.; Padmanabhan, T.We study the mass-radius relationship for aggregates of galaxies, viz. binaries, small groups and clusters. The data are subjected to a simple best-fit analysis similar to the one carried out earlier for individual field galaxies. The analysis shows that: (i) The data on binary galaxies are consistent with the assumption that binaries are just two galaxies, each with an individual isothermal (M∝R) dark matter halo, moving under the mutual gravitational attraction, (ii) The data on the groups of galaxies are too scattered to obey a single power-law relation of the form M = kRᶯ with any degree of reliability, (iii) The data on groups and clusters fit better with a law of the form M = AR³ +BR. This form suggests the existence of two components in dark matter-one which is clustered around the galaxies (M∝R) and another which is distributed smoothly (M∝R3). The smooth distributions becomes significant only at scales ≥ 1 Mpc and hence does not affect binaries significantly. We briefly discuss the theoretical implications of this analysis.Item Possible cosmological scenario with an unstable 17 Kev neutrino(Nature Publishing Group, 1985-09-26) Padmanabhan, T.; Vasanthi, M. M.