Observational cosmology with multi-wavelength surveys

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics

Abstract

Cosmology studies the large-scale properties and evolution of the Universe. As such, cosmology is arguably one of the most complete branches of physics in that it must be able to describe the large-scale distribution and motion of matter, governed primarily by the gravitational forces, but also the intricate interactions between subatomic particles that dictated the rules during and after the Big Bang, as well as the violent physical processes that take place in galaxies and clusters of galaxies. However, the most characteristic feature of cosmology that separates it from other branches of physics, is the impossibility to replicate experiments. We only have one set of data: the Universe, and we cannot repeat it. This originally put cosmology in an awkward position, where due to the lack of experimental data, progress was mainly driven by theoretical work based on fundamental premises. Astonishingly, as astronomical observations improved, many of these theoretical predictions were actually found to be valid, and the last couple of decades have seen cosmology grow into a fully fleshed science driven by experimental observations.

Since there is only one Universe to observe, the cosmologist's quest is to observe as much of it as possible: to map out the distribution of matter and energy in the entire observable Universe. The aim of this endeavour is not merely cartographic. Due to the finiteness of the speed of light, we see distant structures the way they were at the time the photons we observe were emitted. This way the cosmologist is also able to travel in time, and therefore the cosmologist's ideal map describes not only the current state of the Universe, but also its evolution since the moment of the Big Bang. So far we have only been able to collect separate pieces of this map, covering the early stages in the evolution of the Universe from measurements of the cosmic microwave background (CMB) emitted shortly after the Big Bang, as well as the late-time steps in this evolution, in terms of observations of the distribution of galaxies around us. However, in the next decade, large steps will be taken towards the completion of the cosmologist's ideal map: at least half of the observable sky will be jointly mapped by different experiments in a wide range of the electromagnetic spectrum, and these observations will cover far larger volumes than have been accessible so far.

However, the cosmological information is encoded into these datasets in the form of an absorbing puzzle: different experiments cover different ranges of radial and angular scales, as well as different energy regimes, and certain sections of the data end up being dominated by non-cosmological sources and instrumental effects. The beautiful cosmologist's map must therefore be carefully disentangled from the raw experimental data, lest it be inevitably contaminated. This project focuses on identifying the regions and combinations of these datasets that are valuable to reconstruct this map, and that contain the most relevant cosmological information, making use of state-of-the-art statistical and computational tools. As an example, one of the main objectives of this project is the detection of primordial gravitational waves, the ripples in space-time originated during the Big-Bang, which could teach us a lot about the physical conditions in the early Universe. These waves leave an imprint in the polarisation of the CMB with an amplitude significantly smaller than the emission of our own galaxy, and therefore the latter must be carefully removed from the data before the former can be studied.

With cosmology soon entering the era of "big data", as most other branches of science are currently doing, many of the algorithms and methods developed for this project will be useful for a wide range of disciplines, from atmospheric physics to the social sciences, and the computing needs of cosmological studies will also act as a driver for technological development.

Publications

10 25 50
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Ade P (2019) The Simons Observatory: science goals and forecasts in Journal of Cosmology and Astroparticle Physics

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Aiola S (2020) The Atacama Cosmology Telescope: DR4 maps and cosmological parameters in Journal of Cosmology and Astroparticle Physics

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Alonso D (2019) A unified pseudo- C l framework in Monthly Notices of the Royal Astronomical Society

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Bellini E (2019) Sheer shear: weak lensing with one mode in The Open Journal of Astrophysics

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Chisari N (2019) Modelling baryonic feedback for survey cosmology in The Open Journal of Astrophysics

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Farr J (2020) LyaCoLoRe : synthetic datasets for current and future Lyman-a forest BAO surveys in Journal of Cosmology and Astroparticle Physics

Related Projects

Project Reference Relationship Related To Start End Award Value
ST/P004474/1 31/03/2018 31/03/2019 £477,401
ST/P004474/2 Transfer ST/P004474/1 01/04/2019 30/03/2023 £383,439
 
Description High-performance computing for observational cosmology
Amount £52,000 (GBP)
Funding ID 0007873 
Organisation Oxford University Press 
Sector Private
Country United Kingdom
Start 01/2020 
End 06/2020
 
Title Cosmic shear power spectra 
Description We developed accurate methods to measure the power spectrum of gravitational lensing cosmic shear datasets. These methods solve important numerical problems, such as accurately estimating the noise bias and covariance matrix of cosmic shear data. These present a challenge due to the large impact of mode coupling due to the complex survey geometry and noise properties of these data. Using these methods we produced accurate and validated measurements of two state-of-the-art lensing datasets (the Dark Energy Survey and the Hyper Suprime-Cam first-year datasets) and made them publicly available in a format that allows the community to use them straight away to produce cosmological constraints. 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
Impact The use of power spectrum techniques in weak lensing analysis has several advantages (accurate and fast covariances, robust and simple separation between reliable and unreliable scales) which have not been fully exploited by the community due to the numerical challenges stated above. These data show how these problems can be overcome. 
URL https://github.com/xC-ell/ShearCl
 
Description Atacama Cosmology Telescope - ACT 
Organisation Princeton University
Country United States 
Sector Academic/University 
PI Contribution I am a science team member of ACT. My contribution has mostly focused on foreground studies: quantifying the level of Galactic contamination in the maps, providing foreground-corrected legacy maps and foreground avoidance masks. I have also started working on the development of some of the pipeline analysis elements for power spectrum studies in the next season's analysis.
Collaborator Contribution ACT is a ground-based CMB observatory located in the Chilean Atacama plateau. The collaboration provides the combined expertise of different teams to cover all of the steps in the scientific process, data acquisition, reduction, analysis and final cosmological constraints. My work has focus on the intermediate stages, connecting raw data products with science-ready observables.
Impact ACT is one of the most powerful ground-based observatories, and has been at the forefront of CMB science after the Planck mission. Of particular relevance for my research is the fact that ACT will be able to provide state-of-the-art data on CMB B-modes, which will be invaluable to educate the analysis strategies used by the Simons Obsevatory in its quest to put constraints on the amplitude of primordial gravitational waves.
Start Year 2015
 
Description Large Synoptic Survey Telescope - LSST. Dark Energy Science Collaboration - DESC. 
Organisation LSST Corporation
Country United States 
Sector Charity/Non Profit 
PI Contribution I am co-convener of the Large-Scale Structure working group for DESC. As such, I am in charge of coordinating the work needed to provide one of the key data products needed to obtain robust constraints on Dark Energy: summary statistics describing the clustering of galaxies on large scales. As part of this work, I am the lead developer of the software in charge of providing two-point functions in harmonic space for both galaxy clustering and weak gravitational lensing data. I am also heavily involved with the Theory and Joint Probes working group, where I have co-led the efforts to provide a reliable cosmological theory library. Besides my scientific role, I am also a member of the DESC Collaboration Council, in charge of most policy issues for the collaboration.
Collaborator Contribution LSST is an 8.4-meter ground-based telescope that will carry out a wide and deep astronomical survey ~20,000 square degrees of the southern sky. LSST has the potential to provide ground-breaking constraints on the properties of the late-time accelerated expansion of the Universe. To achieve this goal, the DESC combines the skills of experts in the 5 main Dark Energy probes: large-scale structure, weak lensing, cluster science, supernovae and strong lenses. The members cover a wide range of skills: instrumentation, data management, data analysis and theory.
Impact LSST will dominate all cosmological constraints on low redshift phenomena from 2020. As part of the preparatory work carried out within the DESC, I have authored two papers since the award of this Fellowship (10.1093/mnras/stz093, one of them still under journal review).
Start Year 2015
 
Description Simons Observatory - SO 
Organisation Simons Observatory
Country Chile 
Sector Academic/University 
PI Contribution I am pipeline leader for the B-modes Analysis Working Group of SO. As such, I am in charge of delivering one of the key science cases for the collaboration: constraining the amplitude of primordial gravitational waves from the properties of large-scale CMB B-modes. I also contribute to the foregrounds, power spectrum and Sunyaev Zel'dovich working groups. Besides my scientific roles, I am also a member of the SO Publication Panel. In 2019 I secured institutional membership of SO for the University of Oxford, such that current and future postdocs and students in my group can join the collaboration.
Collaborator Contribution SO combines the resources and infrastructure of two existing CMB observatories: the Atacama Cosmology Telescope (which I am also a member of) and the Simons Array, both located in Chile. The collaboration combines the skills of about 100 experts from more than 40 institutions around the world, covering areas from instrumentation to theoretical predictions. The construction of the Observatory is funded by the Simons and Heising-Simons foundations and with contribution from the US lead institutions.
Impact SO will start taking data in 2021, and will then provide ground-breaking advances in cosmology, with primordial gravitational waves from B-modes being one of the main science drivers. Since joining the collaboration, I have co-authored two publications associated to SO (10.1088/1475-7516/2019/02/056, 10.3847/1538-4357/aac71f).
Start Year 2016
 
Description Square Kilometre Array - SKA 
Organisation SKA Square Kilometre Array
Country United Kingdom 
Sector Charity/Non Profit 
PI Contribution I am currently co-coordinator of the Cosmological Simulations focus group as part of the Cosmology Science Working Group. As a member of the SKA I have been mostly interested in the use of intensity mapping and continuum surveys for cosmology, as well as synergies with optical and CMB experiments.
Collaborator Contribution The Square Kilometre Array is a large multi radio telescope, the first phase of which is being built in Australia and in the South African Karoo desert. SKA is a multi-science facility targeting a wide range of objectives, such as pulsar science, galaxy evolution, epoch of reionization or cosmology. The members of SKA cover all areas of expertise needed for this experiment, from instrumentation to theory predictions. The focus of the Cosmology Science Working Groups covers mostly the last stages of the analysis process: data analysis and theoretical constraints.
Impact When SKA Phase 1 finishes construction, it will constitute a tremendous improvement in terms of statistical power with respect to existing facilities, and will therefore dominate any advances in the field.
Start Year 2015
 
Title Core Cosmology Library - CCL 
Description LSST DESC Core Cosmology Library (CCL) provides routines to compute basic cosmological observables with validated numerical accuracy. The library is written in C99 and all functionality is directly callable from C and C++ code. We also provide python bindings for higher-level functions. Although CCL has been developed within DESC, it aims to be of use for the cosmology community as a whole, and this has guided is design. 
Type Of Technology Software 
Year Produced 2018 
Open Source License? Yes  
Impact CCL is the official theory library for LSST DESC, and is now being integrated into all analysis pipelines. The Simons Observatory is also considering to add CCL to its likelihood software. I have co-led the development of CCL 
URL http://ccl.readthedocs.io/
 
Title NaMaster 
Description NaMaster is a software package to compute the power spectrum of any two-dimensional astronomical datasets. It provides generic tools to estimate power spectra in the presence of complex masks, systematic contamination from an arbitrary number of sources and E-B leakage in the case of spin-2 fields. It supports calculations in both curved and flat skies. The code has been designed so it implements generic methods that can be applied to any astronomical dataset, facilitating the joint analysis of a wide range of current and future datasets. 
Type Of Technology Software 
Year Produced 2018 
Open Source License? Yes  
Impact NaMaster has been released through a published paper outlining the novel methods implemented in it. The code has also already been used in a number of publications, and has become one of the official analysis tools for both LSST DESC and the Simons Observatory. 
URL http://namaster.readthedocs.io
 
Title schNell: Fast calculation of N_ell for GW anisotropies 
Description schNell is a very lightweight python module that can be used to compute basic map-level noise properties for generic networks of gravitational wave interferometers. This includes primarily the noise power spectrum "N_ell", but also other things, such as antenna patterns, overlap functions, inverse variance maps etc. 
Type Of Technology Software 
Year Produced 2020 
Open Source License? Yes  
Impact This software allows for a realistic calculation of the noise power spectrum of anisotropic stochastic gravitational wave searches. Given the shift of many cosmologists into gravitational wave science due to the recent developments in the area, this is a vital tool in order to produce quantitatively robust forecast for the detectability or otherwise of different stochastic gravitational wave contributions in the form of anisotropies. Before this code, these kind of forecasts were wildly inaccurate or used oversimplifying assumptions. 
URL https://github.com/damonge/schNell
 
Description Astrophysics outreach at O'Hanlon House 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact Outreach event organised at the O'Hanlon House homeless centre
Year(s) Of Engagement Activity 2019
 
Description COSMO2019 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Invited speaker at international conference on cosmology and astrophysics
Year(s) Of Engagement Activity 2019
URL https://indico.cern.ch/event/782784/
 
Description Cosmological Voids 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Invited speaker to international conference on the use of voids for cosmology, where I disseminated my work on using multi-wavelength measurements with voids.
Year(s) Of Engagement Activity 2018
URL https://www.simonsfoundation.org/event/workshop-on-cosmological-voids/
 
Description Invited seminars at Cardiff University, Johns Hopkins University, University of Geneva, University of Cambridge 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Invited scientific seminars and colloquia at different universities or research institutions aimed at other researchers, including graduate and undergraduate students, in cosmology and astrophysics.
Year(s) Of Engagement Activity 2019
 
Description Invited seminars at Cardiff University, Rutgers University, Princeton University, University of Bristol, Laboratoire de L'Accélérateur Linéaire 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Invited scientific seminars and colloquia at different universities or research institutions aimed at other researchers, including graduate and undergraduate students, in cosmology and astrophysics.
Year(s) Of Engagement Activity 2018,2019
 
Description Invited seminars at Ohio State University and Fermilab 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Invited scientific seminars and colloquia at different universities or research institutions aimed at other researchers, including graduate and undergraduate students, in cosmology and astrophysics.
Year(s) Of Engagement Activity 2020,2021
 
Description Lecture at IOP undergraduate event 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Undergraduate students
Results and Impact Talk on cosmology and astrophysics at an event organized by the Institute of Physics for undergraduates.
Year(s) Of Engagement Activity 2020
 
Description North West Science Network Launch Event 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact Outreach talk on astrophysics and cosmology for secondary school students
Year(s) Of Engagement Activity 2019
 
Description UNIQ summer school 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact Summer school for disadvantaged students hosted by Oxford
Year(s) Of Engagement Activity 2019
URL http://www.uniq.ox.ac.uk/