30 research outputs found
Precision cosmology from the clustering of large-scale structures
Cosmology is going through an exciting period characterized by increasingly large and diverse surveys of the Universe. The resulting datasets contain both cosmological and astrophysical information, and provide many different perspectives on the components of the Universe and its evolution. Despite this wealth of data, there are still many open questions and unknowns. These
vary from broad questions about the nature of dark energy and dark matter, to more concrete ones, such as how fast exactly is the Universe expanding. In this thesis, we will focus on spectroscopic surveys of the large-scale structure (LSS) in the Universe, and how they are used to study these
problems.
We begin in Chapters 1-4 with an introduction of modern cosmology, focusing on the topics relevant to LSS surveys and the analysis tools used to extract cosmological information from these datasets. After that, in Chapter 5 we use the latest baryon acoustic oscillation (BAO) measurements and baryon density constraints based on big bang nucleosynthesis (BBN) to measure the expansion rate of the Universe, through the Hubble constant. This is an independent measurement that contributes to the ongoing Hubble tension debate. In Chapter 6, we perform for the first time a Bayesian analysis of the Lyman-α (Lyα) forest correlation functions in order to measure BAO. Finally, in Chapter 7 we study the possibility of an analysis of the full shape of the Lyα forest correlation functions. This would expand on BAO analyses by providing cosmological information from a broader range of scales. The next generation of cosmological surveys is just starting with
the Dark Energy Spectroscopic Instrument (DESI). Throughout this thesis, we performed multiple studies that are relevant for future cosmological analyses with DESI. These analyses will advance our physical understanding of the Universe by precisely mapping its evolution to higher redshifts than ever before
Baryon Acoustic Oscillations and the Hubble Constant: Past, Present and Future
We investigate constraints on the Hubble constant () using Baryon
Acoustic Oscillations (BAO) and baryon density measurements from Big Bang
Nucleosynthesis (BBN). We start by investigating the tension between galaxy BAO
measurements and those using the Lyman- forest, within a Bayesian
framework. Using the latest results from eBOSS DR14 we find that the
probability of this tension being statistical is assuming flat
CDM. We measure km s Mpc, with a weak
dependence on the BBN prior used, in agreement with results from Planck Cosmic
Microwave Background (CMB) results and in strong tension with distance ladder
results. Finally, we forecast the future of BAO BBN measurements of ,
using the Dark Energy Spectroscopic Instrument (DESI). We find that the choice
of BBN prior will have a significant impact when considering future BAO
measurements from DESI.Comment: 14 pages, 3 figure
Optimal data compression for Lyman- forest cosmology
The Lyman- (Ly) three-dimensional correlation functions have
been widely used to perform cosmological inference using the baryon acoustic
oscillation (BAO) scale. While the traditional inference approach employs a
data vector with several thousand data points, we apply near-maximal score
compression down to tens of compressed data elements. We show that carefully
constructed additional data beyond those linked to each inferred model
parameter are required to preserve meaningful goodness-of-fit tests that guard
against unknown systematics, and to avoid information loss due to non-linear
parameter dependencies. We demonstrate, on suites of realistic mocks and DR16
data from the Extended Baryon Oscillation Spectroscopic Survey, that our
compression framework is lossless and unbiased, yielding a posterior that is
indistinguishable from that of the traditional analysis. As a showcase, we
investigate the impact of a covariance matrix estimated from a limited number
of mocks, which is only well-conditioned in compressed space.Comment: Submitted to MNRA
Direct cosmological inference from three-dimensional correlations of the Lyman- forest
When performing cosmological inference, standard analyses of the
Lyman- (Ly) three-dimensional correlation functions only
consider the information carried by the distinct peak produced by baryon
acoustic oscillations (BAO). In this work, we address whether this compression
is sufficient to capture all the relevant cosmological information carried by
these functions. We do this by performing a direct fit to the full shape,
including all physical scales without compression, of synthetic Ly
auto-correlation functions and cross-correlations with quasars at effective
redshift , assuming a DESI-like survey, and providing a
comparison to the classic method applied to the same dataset. Our approach
leads to a constraint on the matter density , which is
about three to four times better than what BAO alone can probe. The growth term
is constrained to the level, and the
spectral index to . We demonstrate that the extra
information resulting from our `direct fit' approach, except for the
constraint, can be traced back to the Alcock-Paczy\'nski effect
and redshift space distortion information.Comment: Submitted to MNRA
Constraints on the Cosmic Expansion Rate at Redshift 2.3 from the Lyman-α Forest
We determine the product of the expansion rate and angular-diameter distance at redshift z ÂŒ 2.3 from the anisotropy of Lyman-α (Lyα) forest correlations measured by the Sloan Digital Sky Survey (SDSS). Our result is the most precise from large-scale structure at z > 1. Using the flat Î cold dark matter model we determine the matter density to be Ωm ÂŒ 0.36ĂŸ0.03 â0.04 from Lyα alone. This is a factor of 2 tighter than baryon acoustic oscillation results from the same data due to our use of a wide range of scales
(25 <r< 180 hâ1 Mpc). Using a nucleosynthesis prior, we measure the Hubble constant to be H0 ÂŒ 63.2 2.5 km=s=Mpc. In combination with other SDSS tracers, we find H0 ÂŒ 67.2
0.9 km=s=Mpc and measure the dark energy equation-of-state parameter to be w ÂŒ â0.90 0.12. Our Letter opens a new avenue for constraining cosmology at high redshift
New constraints on the expansion rate at redshift 2.3 from the Lyman- forest
We measure the expansion rate of the Universe at redshift from the
anisotropy of Lyman- (Ly) forest correlations measured by the
Sloan Digital Sky Survey (SDSS). Our result is the most precise from
large-scale structure at . In flat CDM we determine the matter
density to be from Ly alone, a
factor of two tighter than baryon acoustic oscillation results from the same
data. Using a nucleosynthesis prior, we measure the Hubble constant to be
km/s/Mpc. In combination with other SDSS tracers, we find
km/s/Mpc and measure the dark energy equation-of-state
parameter to be . Our work opens a new avenue for constraining
cosmology at high redshift.Comment: Submitted to PR
The AlcockâPaczyĆski effect from Lyman-α forest correlations: analysis validation with synthetic data
The three-dimensional distribution of the Lyâα forest has been extensively used to constrain cosmology through measurements of the baryon acoustic oscillations (BAO) scale. However, more cosmological information could be extracted from the full shapes of the Ly α forest correlations through the AlcockâPaczyĆski (AP) effect. In this work, we prepare for a cosmological analysis of the full shape of the Lyâα forest correlations by studying synthetic data of the extended Baryon Oscillation Spectroscopic Survey (eBOSS). We use a set of 100 eBOSS synthetic data sets in order to validate such an analysis. These mocks undergo the same analysis process as the real data. We perform a full-shape analysis on the mean of the correlation functions measured from the 100 eBOSS realizations, and find that our model of the Lyâα correlations performs well on current data sets. We show that we are able to obtain an unbiased full-shape measurement of DM/DH(zeff), where DM is the transverse comoving distance, DH is the Hubble distance, and zeff is the effective redshift of the measurement. We test the fit over a range of scales, and decide to use a minimum separation of râᔹâ = 25 hâÂčMpc. We also study and discuss the impact of the main contaminants affecting Lyâα forest correlations, and give recommendations on how to perform such analysis with real data. While the final eBOSS Lyâα BAO analysis measured DM/DH(zeff = 2.33) with 4 perâcent statistical precision, a full-shape fit of the same correlations could provide an âŒ2 per cent
measurement
Mock data sets for the Eboss and DESI Lyman- forest surveys
{We present a publicly-available code to generate mock Lyman- (\lya)
forest data sets. The code is based on the Fluctuating Gunn-Peterson
Approximation (FGPA) applied to Gaussian random fields and on the use of fast
Fourier transforms (FFT). The output includes spectra of lya transmitted flux
fraction, , a quasar catalog, and a catalog of high-column-density systems.
While these three elements have realistic correlations, additional code is then
used to generate realistic quasar spectra, to add absorption by
high-column-density systems and metals, and to simulate instrumental effects.
Redshift space distortions (RSD) are implemented by including the large-scale
velocity-gradient field in the FGPA resulting in a correlation function of
that can be accurately predicted. One hundred realizations have been produced
over the 14,000 deg Dark Energy Spectroscopy Instrument (DESI) survey
footprint with 100 quasars per deg, and they are being used for the
Extended Baryon Oscillation Survey (eBOSS) and DESI surveys. The analysis of
these realizations shows that the correlation of follows the prediction
within the accuracy of eBOSS survey. The most time-consuming part of the
production occurs before application of the FGPA, and the existing pre-FGPA
forests can be used to easily produce new mock sets with modified
redshift-dependent bias parameters or observational conditions.Comment: to be submitted ot JCA
3D Correlations in the Lyman- Forest from Early DESI Data
We present the first measurements of Lyman- (Ly) forest
correlations using early data from the Dark Energy Spectroscopic Instrument
(DESI). We measure the auto-correlation of Ly absorption using 88,509
quasars at , and its cross-correlation with quasars using a further
147,899 tracer quasars at . Then, we fit these correlations using
a 13-parameter model based on linear perturbation theory and find that it
provides a good description of the data across a broad range of scales. We
detect the BAO peak with a signal-to-noise ratio of , and show that
our measurements of the auto- and cross-correlations are fully-consistent with
previous measurements by the Extended Baryon Oscillation Spectroscopic Survey
(eBOSS). Even though we only use here a small fraction of the final DESI
dataset, our uncertainties are only a factor of 1.7 larger than those from the
final eBOSS measurement. We validate the existing analysis methods of
Ly correlations in preparation for making a robust measurement of the
BAO scale with the first year of DESI data