18 research outputs found
Current Status Of Velocity Field Surveys: A Consistency Check
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 km s
toward and
in a sphere with an effective depth of km s.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
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
we quantify the lensing bias. We find that the bias on the dark energy EOS is
less than half a percent for large datasets ( 2,000 SNe). However, if
highly magnified events (SNe deviating by more than 2.5) are
systematically removed from the analysis, the bias increases to 0.8%.
Given that the EOS parameters measured from such a sample have a 1
uncertainty of 10%, the systematic bias related to lensing in SN data out to 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
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., [,] or
[,]), 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
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
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
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 of the
local model with WMAP out to 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 to 7 from the previous estimate of
about 5 without lensing. The minimum detectable value of for a
cosmic variance limited experiment is also increased from less than 3 to
5.Comment: 9 Pages, 9 Figures, Submitted to PR
Cosmic Shear from Scalar-Induced Gravitational Waves
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
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
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 . 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 . 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
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