150 research outputs found
Dark matter and halo bispectrum in redshift space: theory and applications
We present a phenomenological modification of the standard perturbation
theory prediction for the bispectrum in redshift space that allows us to extend
the model to mildly non-linear scales over a wide range of redshifts,
. We find that we can describe the bispectrum of dark matter
particles with accuracy for at ,
for at , for at and for at . We also
test that the fitting formula is able to describe with similar accuracy the
bispectrum of cosmologies with different , in the range , and consequently with different values of the
logarithmic grow rate at , . We apply
this new formula to recover the bias parameters, and , by
combining the redshift space power spectrum monopole and quadrupole with the
bispectrum monopole for both dark matter particles and haloes. We find that the
combination of these three statistics can break the degeneracy between ,
and . For dark matter particles the new model can be used to
recover and with accuracy. For dark matter haloes we
find that and present larger systematic shifts, . The
systematic offsets arise because of limitations in the modelling of the
interplay between bias and redshift space distortions, and represent a
limitation as the statistical errors of forthcoming surveys reach this level.
Conveniently, we find that these residual systematics are mitigated for
combinations of parameters. The improvement on the modelling of the bispectrum
presented in this paper will be useful for extracting information from current
and future galaxy surveys. [abridged]Comment: 37 pages, 17 figures, 8 tables. Published in JCA
A tale of many
The Hubble parameter , is not a univocally-defined quantity: it relates
redshifts to distances in the near Universe, but is also a key parameter of the
CDM standard cosmological model. As such, affects several
physical processes at different cosmic epochs, and multiple observables. We
have counted more than a dozen 's which are expected to agree if a) there
are no significant systematics in the data and their interpretation and b) the
adopted cosmological model is correct.
With few exceptions (proverbially confirming the rule) these determinations
do not agree at high statistical significance; their values cluster around two
camps: the low (68 km/s/Mpc) and high (73 km/s/Mpc) camp. It appears to be a
matter of anchors: the shape of the Universe expansion history agrees with the
model, it is the normalizations that disagree.
Beyond systematics in the data/analysis, if the model is incorrect there are
only two viable ways to "fix" it: by changing the early time ()
physics and thus the early time normalization, or by a global modification,
possibly touching the model's fundamental assumptions (e.g., homogeneity,
isotropy, gravity). None of these three options has the consensus of the
community.
The research community has been actively looking for deviations from
CDM for two decades; the one we might have found makes us wish we
could put the genie back in the bottle.Comment: To appear in Annual Reviews of Astronomy and Astrophysic
A tale of two (or more) 's
We use the large-scale structure galaxy data (LSS) from the BOSS and eBOSS
surveys, in combination with abundances information from Big Bang
Nucleosynthesis (BBN) to measure two values of the Hubble expansion rate,
, each of them based on
very different physical processes. One is a (traditional) late-time-background
measurement based on determining the BAO scale and using BBN abundances on
baryons for calibrating its absolute size (BAO+BBN). This method anchors
to the (standard) physics of the sound horizon scale at pre-recombination
times. The other is a newer early-time based measurement associated with the
broadband shape of the power spectrum. This second method anchors to the
physics of the matter-radiation equality scale, which also needs BBN
information for determining the suppression of baryons in the power spectrum
shape (shape+BBN). Within the CDM model, we find very good consistency
among these two 's: BAO+BBN (+growth) delivers
km sMpc , whereas the shape+BBN
(+growth) delivers km
s Mpc, where "growth" stands for information from the
late-time-perturbations captured by the growth of structure parameter. These
are the tightest sound-horizon free constraints from LSS data to date. As
a consequence to be viable, any CDM extension proposed to address the
so-called "Hubble tension" needs to modify consistently not only the sound
horizon scale physics, but also the matter-radiation equality scale, in such a
way that both late- and early-based 's return results mutually consistent
and consistent with the high value recovered by the standard cosmic
distance ladder (distance-redshift relation) determinations.Comment: 42 pages, 12 figures, 3 tables, to be submitted to JCAP. Comments
welcom
Optimal Redshift Weighting For Redshift Space Distortions
The low statistical errors on cosmological parameters promised by future
galaxy surveys will only be realised with the development of new, fast,
analysis methods that reduce potential systematic problems to low levels. We
present an efficient method for measuring the evolution of the growth of
structure using Redshift Space Distortions (RSD), that removes the need to make
measurements in redshift shells. We provide sets of galaxy-weights that cover a
wide range in redshift, but are optimised to provide differential information
about cosmological evolution. These are derived to optimally measure the
coefficients of a parameterisation of the redshift-dependent matter density,
which provides a framework to measure deviations from the concordance
CDM cosmology, allowing for deviations in both geometric and/or
growth. We test the robustness of the weights by comparing with alternative
schemes and investigate the impact of galaxy bias. We extend the results to
measure the combined anisotropic Baryon Acoustic Oscillation (BAO) and RSD
signals.Comment: 10 pages, 5 figures, submitted to MNRA
Matter trispectrum: theoretical modelling and comparison to N-body simulations
The power spectrum has long been the workhorse summary statistics for
large-scale structure cosmological analyses. However, gravitational non-linear
evolution moves precious cosmological information from the two-point statistics
(such as the power spectrum) to higher-order correlations. Moreover,
information about the primordial non-Gaussian signal lies also in higher-order
correlations. Without tapping into these, that information remains hidden.
While the three-point function (or the bispectrum), even if not extensively,
has been studied and applied to data, there has been only limited discussion
about the four point/trispectrum. This is because the high-dimensionality of
the statistics (in real space a skew-quadrilateral has 6 degrees of freedom),
and the high number of skew-quadrilaterals, make the trispectrum numerically
and algorithmically very challenging. Here we address this challenge by
introducing the i-trispectrum, an integrated trispectrum that only depends on
four -modes moduli. We model and measure the matter i-trispectrum from a set
of 5000 \textsc{Quijote} N-body simulations both in real and redshift space,
finding good agreement between simulations outputs and model up to mildly
non-linear scales. Using the power spectrum, bispectrum and i-trispectrum joint
data-vector covariance matrix estimated from the simulations, we begin to
quantify the added-value provided by the i-trispectrum. In particular, we
forecast the i-trispectrum improvements on constraints on the local primordial
non-Gaussianity amplitude parameters and . For
example, using the full joint data-vector, we forecast
constraints up to two times () smaller in real (redshift) space than
those obtained without i-trispectrum.Comment: accepted: 6th of November 2020, published: 11th of January 2021 , 64
pages (35 pages for the main text), 15 figure
Blind Observers of the Sky
The concept of blind analysis, a key procedure to remove the human-based
systematic error called confirmation bias, has long been an integral part of
data analysis in many research areas. In cosmology, blind analysis is recently
making its entrance, as the field progresses into a fully fledged
high-precision science. The credibility, reliability and robustness of results
from future sky-surveys will dramatically increase if the effect of
confirmation bias is kept under control by using an appropriate blinding
procedure. Here, we present a catalog-level blinding scheme for galaxy
clustering data apt to be used in future spectroscopic galaxy surveys. We shift
the individual galaxy positions along the line of sight based on 1) a geometric
shift mimicking the Alcock-Paczynski effect and 2) a perturbative shift akin to
redshift-space distortions. This procedure has several advantages. After
combining the two steps above, it is almost impossible to accidentally unblind.
The procedure induces a shift in cosmological parameters without changing the
galaxies' angular positions, hence without interfering with the effects of
angular systematics. Since the method is applied at catalog level, there is no
need to adopt several blinding schemes tuned to different summary statistics,
likelihood choices or types of analyses. By testing the method on mock catalogs
and the BOSS DR12 catalog we demonstrate its performance in blinding galaxy
clustering data for relevant cosmological parameters sensitive to the
background expansion rate and the growth rate of structures.Comment: 45 pages, 18 figures, 6 table
GEO-FPT: a model of the galaxy bispectrum at mildly non-linear scales
We present GEO-FPT (Geometric Fitted Perturbation Theory), a new model for
the galaxy bispectrum anisotropic signal in redshift space, with functional
form rooted in perturbation theory. It also models the dependence of the
bispectrum with the geometric properties of the triangles in Fourier space, and
has a broader regime of validity than state-of-the-art theoretical models based
on perturbation theory. We calibrate the free parameters of this model using
high-resolution dark matter simulations and perform stringent tests to show
that GEO-FPT describes the galaxy bispectrum accurately up to scales of
for different cosmological models, as well as for
biased tracers of the dark matter field, considering a survey volume of
(Gpc . In particular, a joint analysis of the power spectrum and
bispectrum anisotropic signals, taking into account their full covariance
matrix, reveals that the relevant physical quantities -- the BAO peak position
(along and across the line-of-sight), and the growth of structure parameters
times the amplitude of dark matter fluctuations, -- are recovered in
an unbiased way, with an accuracy better than and respectively
(which is our statistical limit of the systematic error estimate). In
addition, the bispectrum signal breaks the degeneracy without
detectable bias: and are recovered with better than and
accuracy respectively (which is our statistical limit of the
systematic error estimate).
GEO-FPT boosts the applicability of the bispectrum signal of galaxy surveys
beyond the current limitation of Mpc % and makes the
bispectrum a key statistic to unlock the information content from the mildly
non-linear regime in the on-going and forthcoming galaxy redshift surveys.Comment: 37 pages, 14 figures. To be submitted to JCAP, comments welcom
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