18 research outputs found

    Current Status Of Velocity Field Surveys: A Consistency Check

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    We present an analysis comparing the bulk--flow measurements for six recent peculiar velocity surveys, namely, ENEAR, SFI, RFGC, SBF and the Mark III singles and group catalogs. We study whether the direction of the bulk--flow estimates are consistent with each other and construct the full three dimensional bulk--flow vectors for each survey. We show that although the surveys differ in their geometry, galaxy morphologies, distance measures and measurement errors, their bulk flow vectors are expected to be highly correlated and in fact show impressive agreement in all cases. We found a combined weighted mean bulk motion of 330 km s−1^{-1} ±101\pm 101 km s−1^{-1} toward l=234°±11°l= 234^{\degree}\pm 11^{\degree} and b=12°±9°b=12^{\degree}\pm 9^{\degree} in a sphere with an effective depth of ∼4000\sim4000 km s−1^{-1}.Comment: 16 pages, 2 figures 2 tables, minor changes, reflects published versio

    Lensing and Supernovae: Quantifying The Bias on the Dark Energy Equation of State

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    The gravitational magnification and demagnification of Type Ia supernovae (SNe) modify their positions on the Hubble diagram, shifting the distance estimates from the underlying luminosity-distance relation. This can introduce a systematic uncertainty in the dark energy equation of state (EOS) estimated from SNe, although this systematic is expected to average away for sufficiently large data sets. Using mock SN samples over the redshift range 0<z≤1.70 < z \leq 1.7 we quantify the lensing bias. We find that the bias on the dark energy EOS is less than half a percent for large datasets (≳\gtrsim 2,000 SNe). However, if highly magnified events (SNe deviating by more than 2.5σ\sigma) are systematically removed from the analysis, the bias increases to ∼\sim 0.8%. Given that the EOS parameters measured from such a sample have a 1σ\sigma uncertainty of 10%, the systematic bias related to lensing in SN data out to z∼1.7z \sim 1.7 can be safely ignored in future cosmological measurements.Comment: 5 pages, 4 figures; one figure and references added; minor modifications to text; reflects version accepted for publication in Ap

    Beyond Two Dark Energy Parameters

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    Our ignorance of the dark energy is generally described by a two-parameter equation of state. In these approaches a particular {\it ad hoc} functional form is assumed, and only two independent parameters are incorporated. We propose a model-independent, multi-parameter approach to fitting the dark energy, and show that next-generation surveys will constrain the equation of state in three or more independent redshift bins to better than 10%. Future knowledge of the dark energy will surpass two numbers (e.g., [w0w_0,w1w_1] or [w0w_0,waw_a]), and we propose a more flexible approach to the analysis of present and future data.Comment: 4 pages, 1 figure; Discussion expanded to include next-generation BAO surveys and possible systematics in SN surveys; reflects version accepted for publication in Phys. Rev. Let

    No Evidence for Dark Energy Dynamics from a Global Analysis of Cosmological Data

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    We use a variant of principal component analysis to investigate the possible temporal evolution of the dark energy equation of state, w(z). We constrain w(z) in multiple redshift bins, utilizing the most recent data from Type Ia supernovae, the cosmic microwave background, baryon acoustic oscillations, the integrated Sachs-Wolfe effect, galaxy clustering, and weak lensing data. Unlike other recent analyses, we find no significant evidence for evolving dark energy; the data remains completely consistent with a cosmological constant. We also study the extent to which the time-evolution of the equation of state would be constrained by a combination of current- and future-generation surveys, such as Planck and the Joint Dark Energy Mission.Comment: 6 pages, 5 figure

    MISSING POWER VS LOW-ALIGNMENTS IN THE COSMIC MICROWAVE BACKGROUND: NO CORRELATION IN THE STANDARD COSMOLOGICAL MODEL

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    ABSTRACT On large angular scales ( 60 • ), the two-point angular correlation function of the temperature of the cosmic microwave background (CMB), as measured (outside of the plane of the Galaxy) by the Wilkinson Microwave Anisotropy Probe, shows significantly lower large-angle correlations than expected from the standard inflationary cosmological model. Furthermore, when derived from the full CMB sky, the two lowest cosmologically interesting multipoles, the quadrupole ( = 2) and the octopole ( = 3), are unexpectedly aligned with each other. Using randomly generated full-sky and cut-sky maps, we investigate whether these anomalies are correlated at a statistically significant level. We conclusively demonstrate that, assuming Gaussian random and statistically isotropic CMB anisotropies, there is no statistically significant correlation between the missing power on large angular scales in the CMB and the alignment of the = 2 and = 3 multipoles. The chance to measure the sky with both such a lack of large-angle correlation and such an alignment of the low multipoles is thus quantified to be below 10 −6

    Weak Lensing of the Primary CMB Bispectrum

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    The cosmic microwave background (CMB) bispectrum is a well-known probe of the non-Gaussianity of primordial perturbations. Just as the intervening large-scale structure modifies the CMB angular power spectrum through weak gravitational lensing, the CMB primary bispectrum generated at the last scattering surface is also modified by lensing. We discuss the lensing modification to the CMB bispectrum and show that lensing leads to an overall decrease in the amplitude of the primary bispectrum at multipoles of interest between 100 and 2000 through additional smoothing introduced by lensing. Since weak lensing is not accounted for in current estimators of the primordial non-Gaussianity parameter, the existing measurements of fNLf_{\rm NL} of the local model with WMAP out to lmax∼750l_{\rm max} \sim 750 is biased low by about 6%. For a high resolution experiment such as Planck, the lensing modification to the bispectrum must be properly included when attempting to estimate the primordial non-Gaussianity or the bias will be at the level of 30%. For Planck, weak lensing increases the minimum detectable value for the non-Gaussianity parameter of the local type fNLf_{\rm NL} to 7 from the previous estimate of about 5 without lensing. The minimum detectable value of fNLf_{\rm NL} for a cosmic variance limited experiment is also increased from less than 3 to ∼\sim 5.Comment: 9 Pages, 9 Figures, Submitted to PR

    Cosmic Shear from Scalar-Induced Gravitational Waves

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    Weak gravitational lensing by foreground density perturbations generates a gradient mode in the shear of background images. In contrast, cosmological tensor perturbations induce a non-zero curl mode associated with image rotations. In this note, we study the lensing signatures of both primordial gravitational waves from inflation and second-order gravitational waves generated from the observed spectrum of primordial density fluctuations. We derive the curl mode for galaxy lensing surveys at redshifts of 1 to 3 and for lensing of the cosmic microwave background (CMB) at a redshift of 1100. We find that the curl mode angular power spectrum associated with secondary tensor modes for galaxy lensing surveys dominates over the corresponding signal generated by primary gravitational waves from inflation. However, both tensor contributions to the shear curl mode spectrum are below the projected noise levels of upcoming galaxy and CMB lensing surveys and therefore are unlikely to be detectable.Comment: 5 Pages, 4 Figures, Submitted to PR

    Missing Power vs low-l Alignments in the Cosmic Microwave Background: No Correlation in the Standard Cosmological Model

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    On large angular scales (greater than about 60 degrees), the two-point angular correlation function of the temperature of the cosmic microwave background (CMB), as measured (outside of the plane of the Galaxy) by the Wilkinson Microwave Anisotropy Probe, shows significantly lower large-angle correlations than expected from the standard inflationary cosmological model. Furthermore, when derived from the full CMB sky, the two lowest cosmologically interesting multipoles, the quadrupole (l=2) and the octopole (l=3), are unexpectedly aligned with each other. Using randomly generated full-sky and cut-sky maps, we investigate whether these anomalies are correlated at a statistically significant level. We conclusively demonstrate that, assuming Gaussian random and statistically isotropic CMB anisotropies, there is no statistically significant correlation between the missing power on large angular scales in the CMB and the alignment of the l=2 and l=3 multipoles. The chance to measure the sky with both such a lack of large-angle correlation and such an alignment of the low multipoles is thus quantified to be below 10^{-6}.Comment: 4 Pages, 1 Figur

    Implications of Two Type Ia Supernova Populations for Cosmological Measurements

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    Recent work suggests that Type Ia supernovae (SNe) are composed of two distinct populations: prompt and delayed. By explicitly incorporating properties of host galaxies, it may be possible to target and eliminate systematic differences between these two putative populations. However, any resulting {\em post}-calibration shift in luminosity between the components will cause a redshift-dependent systematic shift in the Hubble diagram. Utilizing an existing sample of 192 SNe Ia, we find that the average luminosity difference between prompt and delayed SNe is constrained to be (4.5±8.9)(4.5 \pm 8.9)%. If the absolute difference between the two populations is 0.025 mag, and this is ignored when fitting for cosmological parameters, then the dark energy equation of state (EOS) determined from a sample of 2300 SNe Ia is biased at ∼1σ\sim1\sigma. By incorporating the possibility of a two-population systematic, this bias can be eliminated. However, assuming no prior on the strength of the two-population effect, the uncertainty in the best-fit EOS is increased by a factor of 2.5, when compared to the equivalent sample with no underlying two-population systematic. To avoid introducing a bias in the EOS parameters, or significantly degrading the measurement accuracy, it is necessary to control the post-calibration luminosity difference between prompt and delayed SN populations to better than 0.025 mag.Comment: 4 pages, 4 figures; New figures added, some old figures removed; The effect of the uncertainty in the two population model on parameter estimation discussed; Reflects version accepted for publication in Astrophys. J. Let

    A Redetermination of the Hubble Constant with the Hubble Space Telescope from a Differential Distance Ladder

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    We report observations of 240 Cepheid variables obtained with the Near Infrared Camera (NICMOS) through the F160W filter on the Hubble Space Telescope (HST). The Cepheids are distributed across six recent hosts of Type Ia supernovae (SNe Ia) and the "maser galaxy" NGC 4258, allowing us to directly calibrate the peak luminosities of the SNe Ia from the precise, geometric distance measurements provided by the masers. New features of our measurement include the use of the same instrument for all Cepheid measurements across the distance ladder and homogeneity of the Cepheid periods and metallicities thus necessitating only a differential measurement of Cepheid fluxes and reducing the largest systematic uncertainties in the determination of the fiducial SN Ia luminosity. The NICMOS measurements reduce differential extinction in the host galaxies by a factor of 5 over past optical data. Combined with an expanded of 240 SNe Ia at z<0.1 which define their magnitude-redshift relation, we find H_0=74.2 +/-3.6, a 4.8% uncertainty including both statistical and systematic errors. We show that the factor of 2.2 improvement in the precision of H_0 is a significant aid to the determination of the equation-of-state of dark energy, w = P/(rho c^2). Combined with the WMAP 5-year measurement of Omega_M h^2, we find w= -1.12 +/- 0.12 independent of high-redshift SNe Ia or baryon acoustic oscillations (BAO). This result is also consistent with analyses based on the combination of high-z SNe Ia and BAO. The constraints on w(z) now with high-z SNe Ia and BAO are consistent with a cosmological constant and improved by a factor of 3 from the refinement in H_0 alone. We show future improvements in H_0 are likely and will further contribute to multi-technique studies of dark energy.Comment: 60 pages, 15 figures Accepted for Publication, ApJ. This is the second of two papers reporting results from a program to determine the Hubble constant to 5% precision from a refurbished distance ladder based on extensive use of differential measurement
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