Cosmology and Gravitational Lensing

Dr. Thomas Kitching

1. Cosmic Microwave Background Weak Lensing  

The cosmic microwave background (CMB) is the remnant radiation from 380,000 years after the big bang. By measuring the anisotropic fluctuations in the temperature and polarisation of this light cosmological parameters can be inferred to high accuracy. However the CMB photons do not follow a straight path to our telescopes, but instead their paths are distorted by the warping of spacetime caused by dark matter structures along the line of sight – an effect known as weak lensing. This has been measured with very high significance from the recent Planck satellite. As well as CMB photons being weakly lensed the visible light photons from galaxies are also weak lensed. By measuring the weak 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. This PhD project will develop the tools to combine Planck data with Euclid data in an optimal way. Because both data sets contain weak lensing information this is a task that will result detailed understanding of both galaxies and the early Universe. Initially this project will develop the theoretical framework for this work (building upon the study reported here, then apply this to simulations of the galaxy and CMB information, and finally apply this to the state-of-the-art galaxy and CMB weak lensing data from the ESO KiDS survey and ACT experiments. 


Depicts: Gravitational lensing of the Cosmic Microwave Background Copyright: ESA and the Planck Collaboration

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.  

Abell 1689

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