## Cosmology and Gravitational Lensing

### Dr. Thomas Kitching

### 1. Dealing with Unknown Unknowns in Cosmological Analysis

Photons from galaxy’s are gravitationally lensed by the large-scale structure of the Universe and the amount of lensing that occurs depends on the geometry of the Universe and the growth of structure along the lines of sight. By measuring the lensing from galaxies we hope that information on dark energy can be extracted, and at MSSL we play a leading role in the new ESA cosmology satellite called Euclid that will launch in 2020 to measure this effect. However there are many systematic effects in the lensing signal that need to be accounted for: some of these can be modelled accurately, and therefore simultaneously estimated in the analysis along with the cosmological parameters; but for others even the functional form is unknown and methods to account for these are lacking. In this PhD we will apply and adapt path integral methods from quantum theory, Integrated Nested Laplace Approximation (INLA) methods from epidemiology, and machine learning to develop methods that can account for unknown functional behaviour in cosmological parameter estimation (see e.g. here) Initially this project will develop the theoretical framework for this work, apply this to a simpler case of supernovae dark energy studies, then apply this to simulations of galaxy weak lensing information, and finally apply this to the state-of-the-art galaxy weak lensing data from the ESO KiDS survey and the Dark Energy Survey.

### 2. Measuring the Geometry of the Universe with Clusters of Galaxies

Determining the geometry of the Universe is an important test of the standard cosmological model; that contains to unknown components, dark energy and dark matter, that account for 96% of the mass-energy budget but whose nature is entirely unknown. Dark energy is the phenomenon causing a change in the rate of expansion (an acceleration), hence any method that probes the rate of change cosmic environment or the expansion history will be sensitive to the exact nature of dark energy.

When galaxies are observed behind galaxy clusters we see their light distorted by the warping of spacetime caused by the mass of the cluster, an effect known as gravitational lensing. The amount of distortion depends on properties of the cluster and also on the geometry of the Universe. This PhD will develop a well-understood method that takes ratios of the gravitational lensing signal, which can remove the contaminating effects of the details of the cluster to isolate the signal cause by the geometry of the Universe.

One of the objectives of this PhD will be to develop these methods to the point that they will be able to test many competing theories for dark energy. As an example one such explanation is that gravity deviates from General Relativity on large scales – if the gravitational force becomes weaker or even repulsive on cosmic scales this could cause an accelerating expansion.

This PhD will apply these new developments to the state-of-the-art gravitational lensing data sets: the 154 square degree CFHTLenS survey which is already available and to galaxy clusters observed using the Hubble Space Telescope, and also to the 1500 square degree ESO KiDS survey that is observing data now. Finally, this project will build the analysis tools expected to be used in the upcoming ESA Euclid mission in which MSSL has a leading role. In order to ensure a successful PhD this project contains theoretical, simulation and data analysis elements that are flexible such that they can fit with the students skills and expertise.

* A massive galaxy cluster Abell 1689 (copyright: NASA), the gravitational
lensing effect caused by the warping of spacetime by the clusters mass, can be
seen as highly distorted arcs but in fact every galaxy in the image is at least
weakly distorted and this distortion depends on the geometry of the Universe *

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