896 research outputs found
Initial conditions of the universe: A sign of the sine mode
In the standard big bang model the universe starts in a radiation dominated
era, where the gravitational perturbations are described by second order
differential equations, which will generally have two orthogonal set of
solutions. One is the so called {\it growing(cosine)} mode and the other is the
{\it decaying(sine)} mode, where the nomenclature is derived from their
behaviour on super-horizon(sub-horizon) scales. The decaying mode is
qualitatively different to the growing mode of adiabatic perturbations as it
evolves with time on \emph{super-horizon} scales. The time dependence of this
mode on super-horizon scales is analysed in both the synchronous gauge and the
Newtonian gauge to understand the true gauge invariant behaviour of these
modes. We then explore constraints on the amplitude of this mode on scales
between Mpc and Mpc using the
temperature and polarization anisotropies from the cosmic microwave background,
by computing the Fisher information. Binning the primordial power
non-parametrically into 100 bins, we find that the decaying modes are
constrained at comparable variance as the growing modes on scales smaller than
the horizon today using temperature anisotropies. Adding polrisation data makes
the decaying mode more constrained. The decaying mode amplitude is thus
constrained by of the growing mode. On super-horizon scales, the
growing mode is poorly constrained, while the decaying mode cannot
substantially exceed the scale-invariant amplitude. This interpretation differs
substantially from the past literature, where the constraints were quoted in
gauge-dependent variables, and resulted in illusionary tight super-horizon
decaying mode constraints. The results presented here can generally be used to
non-parametrically constrain any model of the early universe.Comment: Fixed typo in figure 6. Previously the noise curves were labelled
incorrectly. New figure fixes that issue - main results are unchange
Cosmic Shear from Galaxy Spins
We discuss the origin of galactic angular momentum, and the statistics of the
present day spin distribution. It is expected that the galaxy spin axes are
correlated with the intermediate principal axis of the gravitational shear
tensor. This allows one to reconstruct the shear field and thereby the full
gravitational potential from the observed galaxy spin fields. We use the
direction of the angular momentum vector without any information of its
magnitude, which requires a measurement of the position angle and inclination
on the sky of each disk galaxy. We present the maximum likelihood shear
inversion procedure, which involves a constrained linear minimization. The
theory is tested against numerical simulations. We find the correlation
strength of nonlinear structures with the initial shear field, and show that
accurate large scale density reconstructions are possible at the expected noise
level.Comment: Accepted by the ApJL, revised discussion, minor changes, LaTex file,
8 pages, 1 ps figur
The Nonlinear Evolution of Galaxy Intrinsic Alignments
The non-Gaussian contribution to the intrinsic halo spin alignments is
analytically modeled and numerically detected. Assuming that the growth of
non-Gaussianity in the density fluctuations caused the tidal field to have
nonlinear-order effect on the orientations of the halo angular momentum, we
model the intrinsic halo spin alignments as a linear scaling of the density
correlations on large scales, which is different from the previous
quadratic-scaling model based on the linear tidal torque theory. Then, we
analyze the halo catalogs from the recent high-resolution Millennium Run
simulation at four different redshifts (z=0,0.5,1 and 2) and measure
quantitatively the degree of the nonlinear effect on the halo spin alignments
and its changes with redshifts. A clear signal of spin correlations is found on
scales as large as 10 Mpc/h at z=0, which marks a detection of the nonlinear
tidal effect on the intrinsic halo alignments. We also investigate how the
nonlinear effect depends on the intrinsic properties of the halos. It is found
that the degree of the nonlinear tidal effect increases as the halo mass scale
decreases, the halo specific angular momentum increases, and the halo peculiar
velocity decreases. We discuss implication of our result on the weak
gravitational lensing.Comment: ApJ in press, revised version, mistakes and typos corrected,
discussion improved, 29 pages, 11 figure
Normalizing the Temperature Function of Clusters of Galaxies
We re-examine the constraints which can be robustly obtained from the
observed temperature function of X-ray cluster of galaxies. The cluster mass
function has been thoroughly studied in simulations and analytically, but a
direct simulation of the temperature function is presented here for the first
time. Adaptive hydrodynamic simulations using the cosmological Moving Mesh
Hydro code of Pen (1997a) are used to calibrate the temperature function for
different popular cosmologies. Applying the new normalizations to the
present-day cluster abundances, we find for a hyperbolic universe, and for a spatially flat universe with a cosmological constant.
The simulations followed the gravitational shock heating of the gas and dark
matter, and used a crude model for potential energy injection by supernova
heating. The error bars are dominated by uncertainties in the heating/cooling
models. We present fitting formulae for the mass-temperature conversions and
cluster abundances based on these simulations.Comment: 20 pages incl 5 figures, final version for ApJ, corrected open
universe \gamma relation, results unchange
Initial conditions of the universe: Decaying tensor modes
Many models of the early universe predict that there should be primordial
tensor perturbations. These leave an imprint into the temperature and
polarisation anisotropies of the cosmic microwave background (CMB). The
differential equation describing the primordial tensor perturbations is a
second order differential equation and thus has two solutions. Canonically, the
decaying solution of this equation in radiation domination is dropped as it
diverges at early times and on superhorizon scales while it is then suppressed
at late times. Furthermore, if there is an inflationary phase prior to the
radiation domination phase, the amplitude of the decaying mode will also be
highly suppressed as it enters the radiation phase, thus its effect will be
negligible. In this study we remain agnostic to the early universe models
describing pre-radiation domination physics and allow this mode to be present
and see what effect it has on the CMB anisotropies. We find that the decaying
mode, if normalised at the same time on subhorizon scales as the growing mode
leaves an imprint on the CMB anisotropies that is identical to the growing
mode. Contrary to expectation, on large scales both modes are poorly
constrained for a scale invariant spectrum, and the apparent divergence of the
decaying mode does not lead to a divergent physical observable. Quantitatively,
the decaying mode can be more constrained both from temperature and
polarisation anisotropies. We use a model independent, non-parametric, approach
to constrain both of these primordial tensor perturbations using the
temperature and polarisation anisotropies. We find that both modes are best
constrained at the reionisation and recombination bumps and crucially, at the
reionisation bump the decaying mode can be distinguished from the growing mode.Comment: 10 pages, comments welcome
Mapping dark matter with cosmic magnification
We develop a new tool to generate statistically precise dark matter maps from
the cosmic magnification of galaxies with distance estimates. We show how to
overcome the intrinsic clustering problem using the slope of the luminosity
function, because magnificability changes strongly over the luminosity
function, while intrinsic clustering only changes weakly. This may allow
precision cosmology beyond most current systematic limitations. SKA is able to
reconstruct projected matter density map at smoothing scale with
S/N, at the rate of 200-4000 deg per year, depending on the
abundance and evolution of 21cm emitting galaxies. This power of mapping dark
matter is comparable to, or even better than that of cosmic shear from deep
optical surveys or 21cm surveys.Comment: 4 pages, 1 figures. Discussions added. PRL accepte
The Kinetic Sunyaev-Zel'dovich Effect from Radiative Transfer Simulations of Patchy Reionization
We present the first calculation of the kinetic Sunyaev-Zel'dovich (kSZ)
effect due to the inhomogeneous reionization of the universe based on detailed
large-scale radiative transfer simulations of reionization. The resulting sky
power spectra peak at l=2000-8000 with maximum values of
l^2C_l~1\times10^{-12}. The peak scale is determined by the typical size of the
ionized regions and roughly corresponds to the ionized bubble sizes observed in
our simulations, ~5-20 Mpc. The kSZ anisotropy signal from reionization
dominates the primary CMB signal above l=3000. This predicted kSZ signal at
arcminute scales is sufficiently strong to be detectable by upcoming
experiments, like the Atacama Cosmology Telescope and South Pole Telescope
which are expected to have ~1' resolution and ~muK sensitivity. The extended
and patchy nature of the reionization process results in a boost of the peak
signal in power by approximately one order of magnitude compared to a uniform
reionization scenario, while roughly tripling the signal compared with that
based upon the assumption of gradual but spatially uniform reionization. At
large scales the patchy kSZ signal depends largely on the ionizing source
efficiencies and the large-scale velocity fields: sources which produce photons
more efficiently yield correspondingly higher signals. The introduction of
sub-grid gas clumping in the radiative transfer simulations produces
significantly more power at small scales, and more non-Gaussian features, but
has little effect at large scales. The patchy nature of the reionization
process roughly doubles the total observed kSZ signal for l~3000-10^4 compared
to non-patchy scenarios with the same total electron-scattering optical depth.Comment: 14 pages, 13 figures (some in color), submitted to Ap
Tentative Detection of Galaxy Spin Correlations in the Tully Catalogue
We report a tentative detection of spin correlations in the Tully catalogue
of nearby galaxies. We define a simple but nontrivial spin correlation
function, and find an analytic estimate of it in the frame of the linear
perturbation theory. Then, we present the observed spin correlation signal from
the Tully galaxies with error bars. The three dimensional spin correlation
turns out to be significant at the 97% confidence level, detected out to a few
Mpc. This observed correlation is consistent with the theoretical
prediction based on the gravitational instability picture of galaxy formation.
An analysis of systematic errors is also presented. The observed strength of
correlation may be sufficient to significantly affect blank field of weak
lensing searches.Comment: Accepted by the ApJ Letters, revised Physical Analysis, LaTex file,
10 pages, 1 ps figur
Cosmology in a String-Dominated Universe
The string-dominated universe locally resembles an open universe, and fits
dynamical measures of power spectra, cluster abundances, redshift distortions,
lensing constraints, luminosity and angular diameter distance relations and
microwave background observations. We show examples of networks which might
give rise to recent string-domination without requiring any fine-tuned
parameters. We discuss how future observations can distinguish this model from
other cosmologies.Comment: 17 pages including 4 figures, of which one is in colo
Reionization: Characteristic Scales, Topology and Observability
Recently the numerical simulations of the process of reionization of the
universe at z>6 have made a qualitative leap forward, reaching sufficient sizes
and dynamic range to determine the characteristic scales of this process. This
allowed making the first realistic predictions for a variety of observational
signatures. We discuss recent results from large-scale radiative transfer and
structure formation simulations on the observability of high-redshift Ly-alpha
sources. We also briefly discuss the dependence of the characteristic scales
and topology of the ionized and neutral patches on the reionization parameters.Comment: 4 pages, 5 figures (4 in color), to appear in Astronomy and Space
Science special issue "Space Astronomy: The UV window to the Universe",
proceedings of 1st NUVA Conference ``Space Astronomy: The UV window to the
Universe'' in El Escorial (Spain
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