2003 (IPP)

Permanent URI for this collectionhttp://localhost:4000/handle/11007/626

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Has files: YesSubject: search.filters.subject.Cosmology: theory

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    Size of the longest filaments in the Universe
    (2011-07-05) Bharadwaj, Somnath; Bhavsar, Suketu P.; Sheth, Jatush V.
    We analyze the filamentarity in the Las Campanas redshift survey (LCRS) and determine the length scale at which filaments are statistically significant. The largest length-scale at which filaments are statistically significant, real objects, is between 70 to 80 h−1 Mpc, for the LCRS −3o slice. Filamentary features longer than 80 h−1 Mpc, though identified, are not statistically significant; they arise from chance alignments. For the five other LCRS slices, filaments of lengths 50 h−1 Mpc to 70 h−1 Mpc are statistically significant, but not beyond. These results indicate that while individual filaments up to 80 h−1 Mpc are statistically significant, the impression of structure on larger scales is a visual effect. On scales larger than 80 h−1 Mpc the filaments interconnect by statistical chance to form the the filament-void network. The reality of the 80 h−1 Mpc features in the −3o slice make them the longest coherent features in the LCRS. While filaments are a natural outcome of gravitational instability, any numerical model attempting to describe the formation of large scale structure in the universe must produce coherent structures on scales that match these observations.
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    Morphology of the supercluster-void network in /\ CDM cosmology
    (2011-07-05) Shandarin, S.F.; Sheth, Jatush V.; Sahni, Varun
    We report here the first systematic study of the supercluster-void network in the ΛCDM concordance cosmology in which voids and superclusters are treated on an equal footing. Superclusters are defined as individual members of an over-dense excur- sion set and voids are defined as individual members of a complementary under-dense excursion set at the same density threshold. We determine the geometric, topological and morphological properties of the cosmic web at a large set of density levels by computing Minkowski functionals for every supercluster and void using SURFGEN (Sheth et al. 2003). The properties of the largest (percolating) supercluster and the complementary void are found to be very different from properties of individual su- perclusters and voids. Individual superclusters totally occupy no more than about 5% of the total volume and contain no more than 20% of mass if the largest supercluster is excluded. Likewise, individual voids totally occupy no more than 14% of volume and contain no more than 4% of mass if the largest void is excluded. Although super- clusters are more massive and voids are more voluminous the difference in maximum volumes is not greater than by an order of magnitude. The genus value of individual superclusters can be ∼ 5 while the genus of individual voids can reach ∼ 40, implying significant amount of substructure in superclusters and especially in voids. One of our main results is that large voids, as defined through the density field (read dark matter distribution) can be distinctly non-spherical.
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    Morphology of Mock SDSS Catalogues
    (2011-07-05) Sheth, Jatush V.
    ABSTRACT We measure the geometry, topology and morphology of the superclusters in mock SDSS catalogues prepared and reported by Cole et al.(1998). The mock catalogues refer to τCDM and ΛCDM flat cosmological models and are populated by galaxies so that these act as biased tracers of mass, conforming with the observed two-point correlation function measured using APM catalogue on scales between 1 to 10 h−1Mpc. We compute the Minkowski Functionals (hereafter, MFs) for the cosmic density fields using SURFGEN (Sheth et al.2003) and use the available 10 realizations of τCDM to study the effect of cosmic variance in estimation ofMFs and Shapefinders; the statistics derived from MFs, and used to study the sizes and shapes of the superclusters. The MFs and Shapefinders are found to be extremely well constrained statistics, useful in assessing the effect of higher order correlation functions on the clustering of galaxy- distribution. We show that though all the mock catalogues of galaxies have the same two-point correlation function and similar clustering amplitude, the global MFs due to τCDM show systematically lower amplitude compared to those due to ΛCDM, an indirect, but detectable effect due to nonzero, higher order correlation functions. This enables us to successfully distinguish the two models of structure formation. We further measure the characteristic thickness (T), breadth (B) and length (L) of the superclusters using the available 10 realizations of τCDM. While T6B and T, B∈[1,17] h−1Mpc, we find the top 10 superclusters to be as long as 90 h−1Mpc, with the longest superclusters identified at percolation to be rare objects with their length as large as 150 h−1Mpc. The dominant morphology of the large superclusters is found to be filamentary. The thickness, breadth and planarity of the superclusters follow well- defined distributions which are different for the two models. Thus, these are found to be sensitive to the cosmological parameter-set and are noted to be candidate statis- tics which can compare the rival models of structure formation. Further, the longest structures of τCDM are found to be significantly longer than those in ΛCDM. Finally, mass and volume-weighted dimensionless Shapefinders – Planarity and Filamentarity – are found to be well-constrained statistics useful to discriminate the two models. We note some interesting effects of bias and stress the importance of incorporating realistic treatment of bias in preparing and analysing the mock catalogues.
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    Is the present expansion of the universe really accelerating?
    (2011-07-05) Vishwakarma, R. G.
    The current observations are usually explained by an accelerating ex- pansion of the present universe. However, with the present quality of the supernovae Ia data, the allowed parameter space is wide enough to accommodate the decelerating models as well. This is shown by considering a particular example of the dark energy equation-of-state wφ ≡ pφ/ρφ = −1/3, which is equivalent to modifying the geometrical curvature index k of the standard cosmology by shifting it to (k − α) where α is a constant. The resulting decelerating model is consistent with the recent CMB observations made by WMAP, as well as, with the high redshift supernovae Ia data including SN 1997ff at z = 1.755. It is also consistent with the newly discovered supernovae SN 2002dc at z = 0.475 and SN 2002dd at z = 0.95 which have a general tendency to improve the fit.
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    Exploring the expanding universe and dark energy using the statefinder diagnostic
    (2011-07-05) Ujjaini, Alam; Sahni, Varun; Saini, Tarun Deep; et al.
    The coming few years are likely to witness a dramatic increase in high quality Sn data as current surveys add more high redshift supernovae to their inventory and as newer and deeper supernova experiments become operational. Given the current variety in dark energy models and the expected improvement in observational data, an accurate and versatile diagnostic of dark energy is the need of the hour. This paper examines the Statefinder diagnostic in the light of the proposed SNAP satellite which is expected to observe about 2000 supernovae per year. We show that the Statefinder is versatile enough to differentiate between dark energy models as varied as the cosmological constant on the one hand, and quintessence, the Chaplygin gas and braneworld models, on the other. Using SNAP data, the Statefinder can distinguish a cosmological constant (w = −1) from quintessence models with w > −0.9 and Chaplygin gas models with κ 6 15 at the 3σ level if the value of Ωm is known exactly. The Statefinder gives reasonable results even when the value of Ωm is known to only ∼ 20% accuracy. In this case, marginalizing over Ωm and assuming a fiducial LCDM model allows us to rule out quintessence with w > −0.85 and the Chaplygin gas with κ 6 7 (both at 3σ). These constraints can be made even tighter if we use the Statefinders in conjunction with the deceleration parameter. The Statefinder is very sensitive to the total pressure exerted by all forms of matter and radiation in the universe. It can therefore differentiate between dark energy models at moderately high redshifts of z < 10.