673 research outputs found
Analytic model for galaxy and dark matter clustering
We investigate an analytic model to compute nonlinear power spectrum of dark
matter, galaxies and their cross-correlation. The model is based on
Press-Schechter halos, which cluster and have realistic dark matter profiles.
The total power spectrum is a sum of two contributions, one from correlations
betwen the halos and one from correlations within the same halo. We show that
such a model can give dark matter power spectra which match well with the
results of N-body simulations, provided that concentration parameter decreases
with the halo mass.
Galaxy power spectrum differs from dark matter power spectrum because pair
weighted number of galaxies increases less rapidly than the halo mass, as
predicted by theoretical models and observed in clusters. In this case the
resulting power spectrum becomes a power law with the slope closed to the
observed. Such a model also predicts a later onset of nonlinear clustering
compared to the dark matter, which is needed to reconcile the CDM models with
the data. Generic prediction of this model is that bias is scale dependent and
nonmonotonic. For red or elliptical galaxies bias in power spectrum may be
scale dependent even on very large scales.
Our predictions for galaxy-dark matter correlations, which can be observed
through the galaxy-galaxy lensing, show that these cannot be interpreted simply
as an average halo profile of a typical galaxy, because different halo masses
dominate at different scales and because larger halos host more than one
galaxy. We discuss the prospects of using cross-correlations in combination
with galaxy clustering to determine the dark matter power spectrum (ABRIDGED).Comment: 16 pages, 7 figures, submitted to Phys. Rev.
Early reionization by decaying particles and cosmic microwave background radiation
We study the reionization scenario in which ionizing UV photons emitted from
decaying particle, in addition to usual contributions from stars and quasars,
ionize the universe. It is found that the scenario is consistent with both the
first year data of the Wilkinson Microwave Anisotropy Probe and the fact that
the universe is not fully ionized until z \sim 6 as observed by Sloan Digital
Sky Survey. Likelihood analysis revealed that rather broad parameter space can
be chosen. This scenario will be discriminated by future observations,
especially by the EE polarization power spectrum of cosmic microwave background
radiation.Comment: 5 pages, 5 figures, fig 2, table 1, and some typos are correcte
Gravitational lensing as a contaminant of the gravity wave signal in CMB
Gravity waves (GW) in the early universe generate B-type polarization in the
cosmic microwave background (CMB), which can be used as a direct way to measure
the energy scale of inflation. Gravitational lensing contaminates the GW signal
by converting the dominant E polarization into B polarization. By
reconstructing the lensing potential from CMB itself one can decontaminate the
B mode induced by lensing. We present results of numerical simulations of B
mode delensing using quadratic and iterative maximum-likelihood lensing
reconstruction methods as a function of detector noise and beam. In our
simulations we find the quadratic method can reduce the lensing B noise power
by up to a factor of 7, close to the no noise limit. In contrast, the iterative
method shows significant improvements even at the lowest noise levels we
tested. We demonstrate explicitly that with this method at least a factor of 40
noise power reduction in lensing induced B power is possible, suggesting that
T/S=10^-6 may be achievable in the absence of sky cuts, foregrounds, and
instrumental systematics. While we do not find any fundamental lower limit due
to lensing, we find that for high-sensitivity detectors residual lensing noise
dominates over the detector noise.Comment: 6 pages, 2 figures, submitted to PR
Power Spectra in Global Defect Theories of Cosmic Structure Formation
An efficient technique for computing perturbation power spectra in field
ordering theories of cosmic structure formation is introduced, enabling
computations to be carried out with unprecedented precision. Large scale
simulations are used to measure unequal time correlators of the source stress
energy, taking advantage of scaling during matter and radiation domination, and
causality, to make optimal use of the available dynamic range. The correlators
are then re-expressed in terms of a sum of eigenvector products, a
representation which we argue is optimal, enabling the computation of the final
power spectra to be performed at high accuracy. Microwave anisotropy and matter
perturbation power spectra for global strings, monopoles, textures and
non-topological textures are presented and compared with recent observations.Comment: 4 pages, compressed and uuencoded RevTex file and postscript figure
Mimicking transPlanckian effects in the CMB with conventional physics
We investigate the possibility that fields coupled to the inflaton can
influence the primordial spectrum of density perturbations through their
coherent motion. For example, the second field in hybrid inflation might be
oscillating at the beginning of inflation rather than at the minimum of its
potential. Although this effect is washed out if inflation lasts long enough,
we note that there can be up to 30 e-foldings of inflation prior to horizon
crossing of COBE fluctuations while still giving a potentially visible
distortion. Such pumping of the inflaton fluctuations by purely conventional
physics can resemble transPlanckian effects which have been widely discussed.
The distortions which they make to the CMB could leave a distinctive signature
which differs from generic effects like tilting of the spectrum.Comment: 3 pages, 4 figures; presented at PASCOS 03, TIFR, Mumbai, Indi
Detection of large scale intrinsic ellipticity-density correlation from the Sloan Digital Sky Survey and implications for weak lensing surveys
The power spectrum of weak lensing shear caused by large-scale structure is
an emerging tool for precision cosmology, in particular for measuring the
effects of dark energy on the growth of structure at low redshift. One
potential source of systematic error is intrinsic alignments of ellipticities
of neighbouring galaxies (II correlation) that could mimic the correlations due
to lensing. A related possibility pointed out by Hirata and Seljak (2004) is
correlation between the intrinsic ellipticities of galaxies and the density
field responsible for gravitational lensing shear (GI correlation). We present
constraints on both the II and GI correlations using 265 908 spectroscopic
galaxies from the SDSS, and using galaxies as tracers of the mass in the case
of the GI analysis. The availability of redshifts in the SDSS allows us to
select galaxies at small radial separations, which both reduces noise in the
intrinsic alignment measurement and suppresses galaxy- galaxy lensing (which
otherwise swamps the GI correlation). While we find no detection of the II
correlation, our results are nonetheless statistically consistent with recent
detections found using the SuperCOSMOS survey. In contrast, we have a clear
detection of GI correlation in galaxies brighter than L* that persists to the
largest scales probed (60 Mpc/h) and with a sign predicted by theoretical
models. This correlation could cause the existing lensing surveys at z~1 to
underestimate the linear amplitude of fluctuations by as much as 20% depending
on the source sample used, while for surveys at z~0.5 the underestimation may
reach 30%. (Abridged.)Comment: 16 pages, matches version published in MNRAS (only minor changes in
presentation from original version
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