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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - Halo modelling (Using the halo model to maximise the information gain from forthcoming weak-lensing surveys.)

Teaser

Summary

As the Universe expands, tiny perturbations that are imprinted upon the universe during inflation eventually collapse under their own weight. These then provide the formation sites for the rich structure of the cosmos that is observed today. The details of this collapse process are governed by the constituent parts of the universe: If there is more dark matter perturbations weigh more and collapse faster; if there is more dark energy the universe expands faster and this retards structure formation. It follows that by measuring the progress of this collapse process over the history of the cosmos one can learn about dark matter and dark energy. The collapse can be measured indirectly via weak gravitational lensing, whereby light from distant sources is bent in a spatially coherent way by the spatial coherence of the intervening structure.

This project aims to address one of the main challenges in inference from contemporary cosmological data using weak lensing: On very large scales, the collapse of cosmological structure is driven by gravity and the only property of a particle that would be important for the dynamics would be the particle mass. However, on smaller scales, other processes influence the collapse process, particularly processes that originate on the scale of galaxies. These processes are electromagnetic in origin, and the most important of these processes are the heating of galactic gas by both accreting super-massive black holes in the centres of the galaxies and by the supernovae explosions of massive stars. The influence of these complicated electromagnetic processes make ab-initio theoretical calculations of structure formation very difficult. It becomes necessary to run computationally-intensive hydrodynamic simulations to understand how these electromagnetic processes redistribute gas, and therefore matter, in the cosmos. This project attempts to expand the knowledge of humanity with regards to how gas dynamics affects the distribution of structure in the Universe.

Work performed

The central project that the researcher has been working on during the fellowship has been to develop a new theoretical model of how dark matter, gas and starts are distributed in the universe and how these combine to determine the overall distribution of matter. Current models make no distinction between these three physically-distinct components of the matter and instead lump them all together. Considering the three components separately has the advantage of being able to use the model to make predictions for a number of different observables, particularly for two-point correlation functions of pairs of observables. This work is being undertaken in collaboration with researchers at UBC and UB. Currently the code for the new method, HMx, has been written and parameters of the new model are being tuned to reproduce the results of hydrodynamic simulations.

In addition to this, the researcher has continued to keep his public Halo-Model code up to date, particularly working on its inclusion with the Code for Anisotropies in the Microwave Background software package. This involves collaboration with Professor Anthony Lewis of the University of Sussex. Due to this, HMcode is now the de-facto standard for calculating the non-linear cosmological power spectrum in any cosmological data analysis. This includes all current galaxy weak-gravitational lensing analyses as well as the most recent Planck cosmic-microwave background analyses. By the conclusion of this fellowship HMcode should be replaced by the more sophisticated HMx that is being developed during this fellowship.

The researcher has become a member of the Large Synoptic Survey Telescope (LSST; https://www.lsst.org/) collaboration. This survey aims to have first light in the early 2020s and will provide the most detailed images of the cosmos ever over an unprecedented area of the sky. This will make the LSST dataset the data of choice for weak gravitational lensing science in the 2020s, at least until the next generation of space-based telescopes start flying. As a result of this, the researcher has been involved with developing some of the computing pipeline infrastructure that will eventually be used in processing of the data. The researcher has also continued his involvement with the Kilo Degree Survey (KiDS; http://kids.strw.leidenuniv.nl/) that currently provides some of the best constraints on the standard cosmological model that have been obtained using the technique of weak gravitational lensing.

In addition to the main projects listed above, the researcher has become involved with several other associated lines of research while at UBC. Highlights are as follows:

The researcher is currently trying to improve the halo model of structure formation by working on a theoretically-consistent method to incorporate non-linear halo bias within the model. When complete, this has the potential to impact upon a very wide range of cosmological problems.

Together with researchers at the University of Edinburgh, the researcher has worked on helping to develop a technique that the researcher originally proposed in 2017 based on the theoretical \'response\' of the halo model for different forms of dark energy. Matteo lead work that extended the original technique of the researcher to different modified-gravity scenarios. This is useful for disentangling effects that can obfuscate cosmological observations at small scales.

In collaboration with researchers at UBC, the fellow has been making direct measurements of the distribution of gas in the Universe using the Sunyaev-Zeldovich effect. This is done in an average sense using a stacking technique, such that the average distribution of gas around galaxies of different masses can been seen. This stacking has been done on individual galaxies, as well as physically close pairs of galaxies. By using pairs it was possible to see for the first time the \'bridge\' of gas that is thought to connect all galaxies in the cosmos, and to estimate i

Final results

So far the project has produced several novel results, detailed above. It is expected that by the conclusion of the fellowship a new theoretical tool will have been developed that will revolutionise cosmological data analyses that use weak-gravitational lensing.