118 research outputs found
Beyond the plane-parallel approximation for redshift surveys
Redshift space distortions privilege the location of the observer in
cosmological redshift surveys, breaking the translational symmetry of the
underlying theory. This violation of statistical homogeneity has consequences
for the modeling of clustering observables, leading to what are frequently
called `wide angle effects'. We study these effects analytically, computing
their signature in the clustering of the multipoles in configuration and
Fourier space. We take into account both physical wide angle contributions as
well as the terms generated by the galaxy selection function. Similar
considerations also affect the way power spectrum estimators are constructed.
We quantify, in an analytical way the biases which enter and clarify the
relation between what we measure and the underlying theoretical modeling. The
presence of an angular window function is also discussed. Motivated by this
analysis we present new estimators for the three dimensional Cartesian power
spectrum and bispectrum multipoles written in terms of spherical Fourier-Bessel
coefficients. We show how the latter have several interesting properties,
allowing in particular a clear separation between angular and radial modes.Comment: 16 pages, 5 figure
The Zeldovich approximation and wide-angle redshift-space distortions
The contribution of line-of-sight peculiar velocities to the observed
redshift of objects breaks the translational symmetry of the underlying theory,
modifying the predicted 2-point functions. These `wide angle effects' have
mostly been studied using linear perturbation theory in the context of the
multipoles of the correlation function and power spectrum. In this work we
present the first calculation of wide angle terms in the Zeldovich
approximation, which is known to be more accurate than linear theory on scales
probed by the next generation of galaxy surveys. We present the exact result
for dark matter and perturbatively biased tracers as well as the small angle
expansion of the configuration- and Fourier-space two-point functions and the
connection to the multi-frequency angular power spectrum. We compare different
definitions of the line-of-sight direction and discuss how to translate between
them. We show that wide angle terms can reach tens of percent of the total
signal in a measurement at low redshift in some approximations, and that a
generic feature of wide angle effects is to slightly shift the Baryon Acoustic
Oscillation scale.Comment: 13 pages, 7 figures, matches published versio
Stochastic bias in multi-dimensional excursion set approaches
We describe a simple fully analytic model of the excursion set approach
associated with two Gaussian random walks: the first walk represents the
initial overdensity around a protohalo, and the second is a crude way of
allowing for other factors which might influence halo formation. This model is
richer than that based on a single walk, because it yields a distribution of
heights at first crossing. We provide explicit expressions for the
unconditional first crossing distribution which is usually used to model the
halo mass function, the progenitor distributions, and the conditional
distributions from which correlations with environment are usually estimated.
These latter exhibit perhaps the simplest form of what is often called nonlocal
bias, and which we prefer to call stochastic bias, since the new bias effects
arise from `hidden-variables' other than density, but these may still be
defined locally. We provide explicit expressions for these new bias factors. We
also provide formulae for the distribution of heights at first crossing in the
unconditional and conditional cases. In contrast to the first crossing
distribution, these are exact, even for moving barriers, and for walks with
correlated steps. The conditional distributions yield predictions for the
distribution of halo concentrations at fixed mass and formation redshift. They
also exhibit assembly bias like effects, even when the steps in the walks
themselves are uncorrelated. Finally, we show how the predictions are modified
if we add the requirement that halos form around peaks: these depend on whether
the peaks constraint is applied to a combination of the overdensity and the
other variable, or to the overdensity alone. Our results demonstrate the power
of requiring models to reproduce not just halo counts but the distribution of
overdensities at fixed protohalo mass as well.Comment: 9 pages, 5 figures, submitted to MNRA
On the spatial distribution of neutral hydrogen in the Universe: bias and shot-noise of the HI Power Spectrum
The spatial distribution of neutral hydrogen (HI) in the Universe contains a
wealth of cosmological information. The 21 cm emission line can be used to map
the HI up to very high redshift and therefore reveal us something about the
evolution of the large scale structures in the Universe. However little is
known about the abundance and clustering properties of the HI over cosmic time.
Motivated by this, we build an analytic framework where the relevant parameters
that govern how the HI is distributed among dark matter halos can be fixed
using observations. At the same time we provide tools to study the column
density distribution function of the HI absorbers together with their
clustering properties. Our formalism is the first one able to account for all
observations at a single redshift, . The linear bias of the HI and the
mean number density of HI sources, two main ingredients in the calculation of
the signal-to-noise ratio of a cosmological survey, are then discussed in
detail, also extrapolating the results to low and high redshift. We find that
HI bias is relatively higher than the value reported in similar studies, but
the shot noise level is always sub dominant, making the HI Power Spectrum
always a high signal-to-noise measurements up to in the limit of no
instrumental noise and foreground contamination.Comment: 10 pages, 9 figure
Measuring dark matter-neutrino relative velocity on cosmological scales
We present a new method to measure neutrino masses using the dark
matter-neutrino relative velocity. The relative motion between dark matter and
neutrinos results in a parity-odd bispectrum which can be measured from
cross-correlation of different cosmic fields. This new method is not affected
by most systematics which are parity even and not limited by the knowledge of
optical depth to the cosmic microwave background. We estimate the detectability
of the relative velocity effect and find that the minimal sum of neutrino
masses could be detected at high significance with upcoming surveys.Comment: 6 pages, 2 figures, 1 table, published versio
Local Primordial Non-Gaussianities and Super-Sample Variance
Fluctuations with wavelengths larger than the volume of a galaxy survey
affect the measurement of the galaxy power spectrum within the survey itself.
In the presence of local Primordial Non- Gaussianities (PNG), in addition to
the super-sample matter density and tidal fluctuations, the large-scale
gravitational potential also induces a modulation of the observed power
spectrum. In this work we investigate this modulation by computing for the
first time the response of the redshift-space galaxy power spectrum to the
presence of a long wavelength gravitational potential, fully accounting for the
stochastic contributions. For biased tracers new response functions arise due
to couplings between the small-scale fluctuations in the density, velocity and
gravitational fields, the latter through scale dependent bias operators, and
the large-scale gravitational potential. We study the impact of the
super-sample modes on the measurement of the amplitude of the primordial
bispectrum of the local-shape, , accounting for
modulations of both the signal and the covariance of the galaxy power spectrum
by the long modes. Considering DESI-like survey specifications, we show that in
most cases super-sample modes cause little or no degradation of the
constraints, and could actually reduce the errorbars on
by (10 - 30)\%, if external information on the bias parameters is available.Comment: 15 pages, 4 figure
Massive neutrinos and the Large Scale Structure of the Universe
This thesis deals with the phenomenology of large scale structures in cosmolo-
gies with massive neutrinos. Cosmology has the power to constraint the value
of neutrino masses down to very high accuracy, but to achieve this target
a careful description of the effect neutrinos could induce on cosmological
observables is needed.
With the help on numerical N-body simulations that include a massive
neutrino component we provide results for clustering beyond the linear level
of both cold dark matter and neutrinos, comparing the measurements with
analytical predictions derived in higher order perturbative approaches and
with existing fitting formulae.
We also discuss the abundance in mass of tracers of the cold dark matter
like halos, identifying the right variable, the variance of the cold dark matter
field, that describe the counts measured in the simulations. We highlight the
systematics effects introduced by a wrong parametrization of the halo mass
function, that can bias the inferred cosmological parameters. We present
results for the spatial distribution of halos, focusing on the relation with
the underlying cold dark matter distribution. To this end we computed the
power spectrum of halos in the simulations, finding that the same variable
describing the halo mass function provides a consistent picture of spatial
clustering of the halos.
The analysis is repeated in redshift space and with higher order correlation
functions, the bispectrum in our case, leading to the same conclusions and
reinforcing our results
The Gaussian streaming model and Lagrangian effective field theory
We update the ingredients of the Gaussian streaming model (GSM) for the
redshift-space clustering of biased tracers using the techniques of Lagrangian
perturbation theory, effective field theory (EFT) and a generalized Lagrangian
bias expansion. After relating the GSM to the cumulant expansion, we present
new results for the real-space correlation function, mean pairwise velocity and
pairwise velocity dispersion including counter terms from EFT and bias terms
through third order in the linear density, its leading derivatives and its
shear up to second order. We discuss the connection to the Gaussian peaks
formalism. We compare the ingredients of the GSM to a suite of large N-body
simulations, and show the performance of the theory on the low order multipoles
of the redshift-space correlation function and power spectrum. We highlight the
importance of a general biasing scheme, which we find to be as important as
higher-order corrections due to non-linear evolution for the halos we consider
on the scales of interest to us.Comment: 28 pages, 5 figures. Revised to match version accepted by journa
Halo bias in Lagrangian Space: Estimators and theoretical predictions
We present several methods to accurately estimate Lagrangian bias parameters
and substantiate them using simulations. In particular, we focus on the
quadratic terms, both the local and the non local ones, and show the first
clear evidence for the latter in the simulations. Using Fourier space
correlations, we also show for the first time, the scale dependence of the
quadratic and non-local bias coefficients. For the linear bias, we fit for the
scale dependence and demonstrate the validity of a consistency relation between
linear bias parameters. Furthermore we employ real space estimators, using both
cross-correlations and the Peak-Background Split argument. This is the first
time the latter is used to measure anisotropic bias coefficients. We find good
agreement for all the parameters among these different methods, and also good
agreement for local bias with ESP theory predictions. We also try to
exploit possible relations among the different bias parameters. Finally, we
show how including higher order bias reduces the magnitude and scale dependence
of stochasticity of the halo field.Comment: 13 pages, 12 figure
Intensity mapping with neutral hydrogen and the Hidden Valley simulations
This paper introduces the Hidden Valley simulations, a set of
trillion-particle N-body simulations in gigaparsec volumes aimed at intensity
mapping science. We present details of the simulations and their convergence,
then specialize to the study of 21-cm fluctuations between redshifts 2 and 6.
Neutral hydrogen is assigned to halos using three prescriptions, and we
investigate the clustering in real and redshift-space at the 2-point level. In
common with earlier work we find the bias of HI increases from near 2 at z = 2
to 4 at z = 6, becoming more scale dependent at high z. The level of
scale-dependence and decorrelation with the matter field are as predicted by
perturbation theory. Due to the low mass of the hosting halos, the impact of
fingers of god is small on the range relevant for proposed 21-cm instruments.
We show that baryon acoustic oscillations and redshift-space distortions could
be well measured by such instruments. Taking advantage of the large simulation
volume, we assess the impact of fluctuations in the ultraviolet background,
which change HI clustering primarily at large scales.Comment: 36 pages, 21 figures. Simulations available at
http://cyril.astro.berkeley.edu/HiddenValley/ Minor changes in HI
normalization described in footnote of section
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