461 research outputs found
A New Statistic for Analyzing Baryon Acoustic Oscillations
We introduce a new statistic omega_l for measuring and analyzing large-scale
structure and particularly the baryon acoustic oscillations. omega_l is a
band-filtered, configuration space statistic that is easily implemented and has
advantages over the traditional power spectrum and correlation function
estimators. Unlike these estimators, omega_l can localize most of the acoustic
information into a single dip at the acoustic scale while also avoiding
sensitivity to the poorly constrained large scale power (i.e., the integral
constraint) through the use of a localized and compensated filter. It is also
sensitive to anisotropic clustering through pair counting and does not require
any binning. We measure the shift in the acoustic peak due to nonlinear effects
using the monopole omega_0 derived from subsampled dark matter catalogues as
well as from mock galaxy catalogues created via halo occupation distribution
(HOD) modeling. All of these are drawn from 44 realizations of 1024^3 particle
dark matter simulations in a 1h^{-1}Gpc box at z=1. We compare these shifts
with those obtained from the power spectrum and conclude that the results
agree. This indicates that any distance measurements obtained from omega_0 and
P(k) will be consistent with each other. We also show that it is possible to
extract the same amount of acoustic information using either omega_0 or P(k)
from equal volume surveys.Comment: 12 pages, 7 figures. ApJ accepted. Edit: Now updated with final
accepted versio
Improving Cosmological Distance Measurements by Reconstruction of the Baryon Acoustic Peak
The baryon acoustic oscillations are a promising route to the precision
measure of the cosmological distance scale and hence the measurement of the
time evolution of dark energy. We show that the non-linear degradation of the
acoustic signature in the correlations of low-redshift galaxies is a
correctable process. By suitable reconstruction of the linear density field,
one can sharpen the acoustic peak in the correlation function or, equivalently,
restore the higher harmonics of the oscillations in the power spectrum. With
this, one can achieve better measurements of the acoustic scale for a given
survey volume. Reconstruction is particularly effective at low redshift, where
the non-linearities are worse but where the dark energy density is highest. At
z=0.3, we find that one can reduce the sample variance error bar on the
acoustic scale by at least a factor of 2 and in principle by nearly a factor of
4. We discuss the significant implications our results have for the design of
galaxy surveys aimed at measuring the distance scale through the acoustic peak.Comment: 5 pages, LaTeX. Submitted to the Astrophysical Journa
Flowing with Time: a New Approach to Nonlinear Cosmological Perturbations
Nonlinear effects are crucial in order to compute the cosmological matter
power spectrum to the accuracy required by future generation surveys. Here, a
new approach is presented, in which the power spectrum, the bispectrum and
higher order correlations, are obtained -- at any redshift and for any momentum
scale -- by integrating a system of differential equations. The method is
similar to the familiar BBGKY hierarchy. Truncating at the level of the
trispectrum, the solution of the equations corresponds to the summation of an
infinite class of perturbative corrections. Compared to other resummation
frameworks, the scheme discussed here is particularly suited to cosmologies
other than LambdaCDM, such as those based on modifications of gravity and those
containing massive neutrinos. As a first application, we compute the Baryonic
Acoustic Oscillation feature of the power spectrum, and compare the results
with perturbation theory, the halo model, and N-body simulations. The
density-velocity and velocity-velocity power spectra are also computed, showing
that they are much less contaminated by nonlinearities than the density-density
one. The approach can be seen as a particular formulation of the
renormalization group, in which time is the flow parameter.Comment: 20 pages, 7 figures. Matches version published on JCA
Statistical methods in cosmology
The advent of large data-set in cosmology has meant that in the past 10 or 20
years our knowledge and understanding of the Universe has changed not only
quantitatively but also, and most importantly, qualitatively. Cosmologists rely
on data where a host of useful information is enclosed, but is encoded in a
non-trivial way. The challenges in extracting this information must be overcome
to make the most of a large experimental effort. Even after having converged to
a standard cosmological model (the LCDM model) we should keep in mind that this
model is described by 10 or more physical parameters and if we want to study
deviations from it, the number of parameters is even larger. Dealing with such
a high dimensional parameter space and finding parameters constraints is a
challenge on itself. Cosmologists want to be able to compare and combine
different data sets both for testing for possible disagreements (which could
indicate new physics) and for improving parameter determinations. Finally,
cosmologists in many cases want to find out, before actually doing the
experiment, how much one would be able to learn from it. For all these reasons,
sophisiticated statistical techniques are being employed in cosmology, and it
has become crucial to know some statistical background to understand recent
literature in the field. I will introduce some statistical tools that any
cosmologist should know about in order to be able to understand recently
published results from the analysis of cosmological data sets. I will not
present a complete and rigorous introduction to statistics as there are several
good books which are reported in the references. The reader should refer to
those.Comment: 31, pages, 6 figures, notes from 2nd Trans-Regio Winter school in
Passo del Tonale. To appear in Lectures Notes in Physics, "Lectures on
cosmology: Accelerated expansion of the universe" Feb 201
The sensitivity of BAO Dark Energy Constraints to General Isocurvature Perturbations
Baryon Acoustic Oscillation (BAO) surveys will be a leading method for
addressing the dark energy challenge in the next decade. We explore in detail
the effect of allowing for small amplitude admixtures of general isocurvature
perturbations in addition to the dominant adiabatic mode. We find that
non-adiabatic initial conditions leave the sound speed unchanged but instead
excite different harmonics. These harmonics couple differently to Silk damping,
altering the form and evolution of acoustic waves in the baryon-photon fluid
prior to decoupling. This modifies not only the scale on which the sound waves
imprint onto the baryon distribution, which is used as the standard ruler in
BAO surveys, but also the shape, width and height of the BAO peak. We discuss
these effects in detail and show how more general initial conditions impact our
interpretation of cosmological data in dark energy studies. We find that the
inclusion of these additional isocurvature modes leads to an increase in the
Dark Energy Task Force Figure of merit by 140% and 60% for the BOSS and ADEPT
experiments respectively when considered in conjunction with Planck data. We
also show that the incorrect assumption of adiabaticity has the potential to
bias our estimates of the dark energy parameters by () for a
single correlated isocurvature mode, and up to () for three
correlated isocurvature modes in the case of the BOSS (ADEPT) experiment. We
find that the use of the large scale structure data in conjunction with CMB
data improves our ability to measure the contributions of different modes to
the initial conditions by as much as 100% for certain modes in the fully
correlated case.Comment: 20 pages, 17 figure
Prospects in Constraining the Dark Energy Potential
We generalize to non-flat geometries the formalism of Simon et al. (2005) to
reconstruct the dark energy potential. This formalism makes use of quantities
similar to the Horizon-flow parameters in inflation, can, in principle, be made
non-parametric and is general enough to be applied outside the simple, single
scalar field quintessence. Since presently available and forthcoming data do
not allow a non-parametric and exact reconstruction of the potential, we
consider a general parametric description in term of Chebyshev polynomials. We
then consider present and future measurements of H(z), Baryon Acoustic
Oscillations surveys and Supernovae type 1A surveys, and investigate their
constraints on the dark energy potential. We find that, relaxing the flatness
assumption increases the errors on the reconstructed dark energy evolution but
does not open up significant degeneracies, provided that a modest prior on
geometry is imposed. Direct measurements of H(z), such as those provided by BAO
surveys, are crucially important to constrain the evolution of the dark energy
potential and the dark energy equation of state, especially for non-trivial
deviations from the standard LambdaCDM model.Comment: 22 pages, 7 figures. 2 references correcte
Baryon Acoustic Oscillations and Dynamical Dark Energy
We compute the impact of dark energy at last scattering on measurements of
baryon acoustic oscillations (BAOs). We show that an early dark energy
component can contribute a systematic uncertainty to BAO measurements of up to
2.5%. Whilst this effect turns out to only slightly affect current BAO surveys,
the results of future BAO surveys might become biased. We find that BAO surveys
alone appear unable to resolve this systematic uncertainty, so supplementary
measurements are necessary.Comment: 4 pages, added sections and references, matches published versio
Supersonic Relative Velocity Effect on the Baryonic Acoustic Oscillation Measurements
We investigate the effect of supersonic relative velocities between baryons
and dark matter, recently shown to arise generically at high redshift, on
baryonic acoustic oscillation (BAO) measurements at low redshift. The amplitude
of the relative velocity effect at low redshift is model-dependent, but can be
parameterized by using an unknown bias. We find that if unaccounted, the
relative velocity effect can shift the BAO peak position and bias estimates of
the dark energy equation-of-state due to its non-smooth, out-of-phase
oscillation structure around the BAO scale. Fortunately, the relative velocity
effect can be easily modeled in constraining cosmological parameters without
substantially inflating the error budget. We also demonstrate that the presence
of the relative velocity effect gives rise to a unique signature in the galaxy
bispectrum, which can be utilized to isolate this effect. Future dark energy
surveys can accurately measure the relative velocity effect and subtract it
from the power spectrum analysis to constrain dark energy models with high
precision.Comment: 17 pages, 6 figures, submitted to JCA
Next-to-leading resummation of cosmological perturbations via the Lagrangian picture: 2-loop correction in real and redshift spaces
We present an improved prediction of the nonlinear perturbation theory (PT)
via the Lagrangian picture, which was originally proposed by Matsubara (2008).
Based on the relations between the power spectrum in standard PT and that in
Lagrangian PT, we derive analytic expressions for the power spectrum in
Lagrangian PT up to 2-loop order in both real and redshift spaces. Comparing
the improved prediction of Lagrangian PT with -body simulations in real
space, we find that the 2-loop corrections can extend the valid range of wave
numbers where we can predict the power spectrum within 1% accuracy by a factor
of 1.0 (), 1.3 (1), 1.6 (2) and 1.8 (3) vied with 1-loop Lagrangian PT
results. On the other hand, in all redshift ranges, the higher-order
corrections are shown to be less significant on the two-point correlation
functions around the baryon acoustic peak, because the 1-loop Lagrangian PT is
already accurate enough to explain the nonlinearity on those scales in -body
simulations.Comment: 18pages, 4 figure
Observational signatures of Jordan-Brans-Dicke theories of gravity
We analyze the Jordan-Brans-Dicke model (JBD) of gravity, where deviations
from General Relativity (GR) are described by a scalar field non-minimally
coupled to gravity. The theory is characterized by a constant coupling
parameter, ; GR is recovered in the limit . In such theories, gravity modifications manifest at early times,
so that one cannot rely on the usual approach of looking for inconsistencies in
the expansion history and perturbations growth in order to discriminate between
JBD and GR. However, we show that a similar technique can be successfully
applied to early and late times observables instead. Cosmological parameters
inferred extrapolating early-time observations to the present will match those
recovered from direct late-time observations only if the correct gravity theory
is used. We use the primary CMB, as will be seen by the Planck satellite, as
the early-time observable; and forthcoming and planned Supernov{\ae}, Baryonic
Acoustic Oscillations and Weak Lensing experiments as late-time observables. We
find that detection of values of as large as 500 and 1000 is
within reach of the upcoming (2010) and next-generation (2020) experiments,
respectively.Comment: minor revision, references added, matching version published in JCA
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