2006 (IPP)

Permanent URI for this collectionhttp://localhost:4000/handle/11007/335

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

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Now showing 1 - 10 of 15
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    Smoothing supernova data to reconstruct the expansion history of the universe
    (2006-01-10) Shafieloo, Arman; Alam, Ujjaini; Sahni, Varun
    We propose a non-parametric method of smoothing supernova data over redshift using a Gaussian kernel in order to reconstruct important cosmological quantities including H(z) and w(z) in a model independent manner. This method is shown to be successful in discriminating between different models of dark energy when the quality of data is commensurate with that expected from the future SuperNova Acceleration Probe (SNAP). We find that the Hubble parameter is especially well-determined and useful for this purpose. The look back time of the universe may also be determined to a very high degree of accuracy ( < ∼ 0.2%) in this method. By refining the method, it is also possible to obtain reasonable bounds on the equation of state of dark energy. We explore a new diagnostic of dark energy– the ‘w-probe’– which can be calculated from the first derivative of the data. We find that this diagnostic is reconstructed extremely accurately for different reconstruction methods even if Ω0m is marginalized over. The w-probe can be used to successfully distinguish between ΛCDM and other models of dark energy to a high degree of accuracy.
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    Velocity-metallicity correlation for high-z DLA galaxies: Evidence for a mass-metallicity relation?
    (2006-06-08) Ledoux, C.; Petitjean, Patrick; Fynbo, J. P. U.; et al.
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    Universe with time dependent deceleration parameter and /\ term in general relativity
    (2006-07-28) Pradhan, Anirudh; Otarod, Saeed
    A new class of exact solutions of Einstein’s field equations with perfect fluid for an LRS Bianchi type-I spacetime is obtained by using a time dependent deceleration parameter. We have obtained a general solution of the field equations from which three models of the universe are derived: exponential, polynomial and sinusoidal form respectively. The behaviour of these models of the universe are also discussed in the frame of reference of recent supernovae Ia observations.
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    Re-analysis of the three-year wilkinson microwave anisotropy probe temperature power spectrum and likelihood
    (2006-10-02) Eriksen, H. K.; Huey, Greg; Saha, Rajib; et al.
    We analyze the three-year WMAP temperature anisotropy data seeking to confirm the power spectrum and likelihoods published by the WMAP team. We apply five independent implementations of four algorithms to the power spectrum estimation and two implementations to the parameter estimation. Our single most important result is that we broadly confirm the WMAP power spectrum and analysis. Still, we do find two small but potentially important discrepancies: On large angular scales there is a small power excess in the WMAP spectrum (5–10% at ℓ . 30) primarily due to likelihood approximation issues between 13 ≤ ℓ . 30. On small angular scales there is a systematic difference between the V- and W-band spectra (few percent at ℓ & 300). Recently, the latter discrepancy was explained by Huffenberger et al. (2006) in terms of over-subtraction of unresolved point sources. As far as the low-ℓ bias is concerned, most parameters are affected by a few tenths of a sigma. The most important effect is seen in ns. For the combination of WMAP, Acbar and BOOMERanG, the significance of ns = 1 drops from ∼ 2.7σ to ∼ 2.3σ when correcting for this bias. We propose a few simple improvements to the low-ℓ WMAP likelihood code, and introduce two important extensions to the Gibbs sampling method that allows for proper sampling of the low signal-to-noise regime. Finally, we make the products from the Gibbs sampling analysis publically available, thereby providing a fast and simple route to the exact likelihood without the need of expensive matrix inversions.
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    Quintessentially geometric model
    (2006-10-16) Gumjudpai, Burin; Naskar, Tapan; Ward, John
    We consider string inspired cosmology on a solitary D3 brane moving in the background of a ring of branes located on a circle of radius R. The motion of the D3 brane transverse to the plane of the ring gives rise to a radion field which can be mapped to a massive non-BPS Born-Infeld type field with a cosh potential. For certain bounds of the brane tension we find an inflationary phase is possible, with the string scale relatively close to the Planck scale. The relevant perturbations and spectral indices are all well within the expected observational bounds. The evolution of the universe eventually comes to be dominated by dark energy, which we show is a late time attractor of the model. However we also find that the equation of state is time dependent, and will lead to late time Quintessence.
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    Primordial magnetic fields and CMB anisotropies
    (2006-01-25) Subramanian, Kandaswamy
    Possible signatures of primordial magnetic fields on the Cosmic Microwave Background (CMB) temperature and polarization anisotropies are reviewed. The signals that could be searched for include excess temperature anisotropies particularly at small angular scales below the Silk damping scale, B-mode polarization, and non-Gaussian statistics. A field at a few nG level produces temperature anisotropies at the 5µK level, and B-mode polarization anisotropies 10 times smaller, and is therefore potentially detectable via the CMB anisotropies. An even smaller field, with B0 < 0.1 nG, could lead to structure formation at high redshift z > 15, and hence naturally explain an early re-ionization of the Universe.
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    On searches for gravitational waves from mini creation events by laser interferometric detectors
    (2006-01-10) Sarmah, B. P.; Banerjee, S.K.; Dhurandhar, Sanjeev
    As an alternative view to the standard big bang cosmology the quasi-steady state cosmol- ogy(QSSC) argues that the universe was not created in a single great explosion; it neither had a beginning nor will it ever come to an end. The creation of new matter in the universe is a regular feature occurring through nite explosive events. Each creation event is called a mini-bang or, a mini creation event(MCE). Gravitational waves are expected to be generated due to any anisotropy present in this process of creation. Mini creation event ejecting matter in two oppositely directed jets is thus a source of gravitational waves which can in principle be detected by laser interferometric detectors. In the present work we consider the gravitational waveforms propagated by linear jets and then estimate the response of laser interferometric detectors like LIGO and LISA.
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    Non-circular beam correction to the CMB power spectrum
    (2006-08-24) Souradeep, Tarun; Mitra, Sanjit; Sengupta, Anand; et al.
    In the era of high precision CMB measurements, systematic effects are beginning to limit the ability to extract subtler cosmological information. The non-circularity of the experimental beam has become progressively important as CMB experiments strive to attain higher angular resolution and sensitivity. The effect of non-circular beam on the power spectrum is important at multipoles larger than the beam-width. For recent experiments with high angular resolution, optimal methods of power spectrum estimation are computationally prohibitive and sub-optimal approaches, such as the Pseudo-Cl method, are used. We provide an analytic framework for correcting the power spectrum for the effect of beam non-circularity and non-uniform sky coverage (including incomplete/masked sky maps). The approach is perturbative in the distortion of the beam from non-circularity allowing for rapid computations when the beam is mildly non-circular. When non-circular beam effect is important, we advocate that it is computationally advantageous to employ ‘soft’ azimuthally apodized masks whose spherical harmonic transform die down fast with m.
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    Measuring statistical isotropy of CMB anisotropy
    (2006-07-25) Souradeep, Tarun; Hajian, Amir; Basak, Soumen
    The statistical expectation values of the temperature fluctuations and polarization of cosmic microwave background (CMB) are assumed to be preserved under rotations of the sky. We investigate the statistical isotropy (SI) of the CMB maps recently measured by the Wilkinson Microwave Anisotropy Probe (WMAP) using the bipolar spherical harmonic formalism proposed in Hajian & Souradeep 2003 for CMB temperature anisotropy and extended to CMB polarization in Basak, Hajian & Souradeep 2006. The Bipolar Power Spectrum (BiPS) had been measured for the full sky CMB anisotropy maps of the first year WMAP data and now for the recently released three years of WMAP data. We also introduce and measure directional sensitive reduced Bipolar coefficients on the three year WMAP ILC map. Consistent with our published results from first year WMAP data we have no evidence for violation of statistical isotropy on large angular scales. Preliminary analysis of the recently released firstWMAP polarization maps, however, indicate significant viola- tion of SI even when the foreground contaminated regions are masked out. Further work is required to confirm a possible cosmic origin and rule out the (more likely) origin in observational artifact such as foreground residuals at high galactic latitude.
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    Dark energy cosmology from higher-order, string-inspired gravity, and its reconstruction
    (2006-04-10) Nojiri, Shin'ichi; Odintsov, Sergi D.; Sami, M.
    In this paper we investigate the cosmological effects of modified gravity with string curvature corrections added to the Einstein-Hilbert action in the presence of a dynamically evolving scalar field coupled to Riemann invariants. The scenario exhibits several features of cosmological interest for the late universe. We show that higher-order stringy corrections can lead to a class of dark energy models consistent with recent observations. The models can give rise to quintessence without recourse to a scalar field potential. The detailed treatment of the reconstruction program for general scalar-Gauss-Bonnet gravity is presented for any given cosmology. The explicit examples of reconstructed scalar potentials are given for an accelerated (quintessence, cosmological constant, or phantom) universe. Finally, the relation with modified F G gravity is established at the classical level and is extended to include third order terms on the curvature.