## Cosmology and Gravitational Lensing

### Dr. Thomas Kitching

### 1. Measuring Dark Energy Properties with 3D Weak Lensing

We face a turning point in our understanding of the Universe. It is now known that it consists of two components named, to partly reflect our ignorance of them, dark matter and dark energy. Dark matter accounts for approximately 20% of the content of the Universe, so called because it does not emit or absorb light, it is thought to be a new type of particle beyond the standard model of particle physics. Much more mysterious though is dark energy, a phenomenon that is causing the expansion rate of the Universe to accelerate. The presence of dark energy could be explained by a “vacuum energy” however the density of dark energy inferred from current observations is 1060 times larger than that expected from particle physics, this is the largest discrepancy between theory and observation ever encountered in physics, a factor which cannot be reconciled without a fundamental re-evaluation of our understanding of physics.

To address the most important questions in cosmology requires the best and most comprehensive data analysis methods. Gravitational lensing, the effect whereby photons are deflected from a straight-line path by the presence of a massive object distorting spacetime locally, is widely accepted to be such a method. This phenomenon, predicted by Einstein, led to the acceptance of General Relativity as our canonical theory gravity in 1919. Now, almost 100 years after this discovery this same phenomenon applied to the lensing of light by large-scale structures of the Universe, as a function of distance or look-back time, enables us to map the 3D dark matter structure of the Universe as well as its expansion history. This combination of lensing information and distance is known as 3D weak lensing this PhD project will build on this foundation, and apply the best analysis methods to the best data available.

Development of 3D weak lensing is required for several reasons 1) new theoretical models need to be included, in particular parameterizations of modifications of gravity that extend General relativity to include a potential mechanism to explain dark energy 2) systematic effects, in particular those caused by the alignment of galaxies, that can mimic the weak lensing signal, need to be included. Without these improvements 3D weak lensing cannot reach its full potential.

This PhD will apply these new developments the state-of-the-art gravitational lensing data sets: the 154 square degree CFHTLenS survey which is already available and the 1500 square degree ESO KiDS survey that is observing data now. In addition 3D weak lensing–CMB cross correlation statistics will be investigated with an aim to apply this to KiDS and Planck. 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.

*Left (copyright: Columbi & Mellier) the large-scale cosmic web distorts **the
image of background galaxies in a measurable way that can help in determining
the nature of dark energy. Right (copyright: NASA) the Hubble Space Telescope
Ultra Depp Field, the Euclid ESA mission will observe tens of thousands times
more data with a similar fidelity with an objective to use gravitational
lensing to determine dark matter and dark energy properties.*

### 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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