### Philip Robinson Cosmology PhD Studentship

### Available PhD projects

*Click on each topic to learn more with links to supervisors' contact details*

– The cosmic web as a laboratory for fundamental physics

– Testing gravity on cosmological scales

– Calculating the Wavefunction of the Universe

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#### The cosmic web as a laboratory for fundamental physics

The 21st century has transformed cosmology from a speculative field into a precision science driven by theoretical methods, numerical simulations and galaxy survey data. Measurements of galaxy clustering and weak gravitational lensing will map the 3d matter distribution over the past 10 billion years and answer fundamental questions in physics: What are the properties of the early universe? What is the nature of dark energy? What are the characteristics of dark matter?

*A slice through the Euclid Flagship mock catalog of 2.6 billion galaxies displaying the growth of structure over 10 billion years from early (green/right) to late times (red/left).*

Unravelling these mysteries is difficult because the information is hidden in the galaxy distribution that has been shaped by nonlinear clustering and is characterised by complex non-Gaussian statistics. As a PhD student working on this project, you will develop and apply state-of-the art statistical, analytical and computational techniques to extract the maximal information on fundamental physics from the late-time matter distribution. You will be part of a local research team in Newcastle and have the option to join the Euclid Consortium to contribute to the ESA space mission Euclid which will launch a dedicated satellite in 2022 to map the dark universe across one third of the sky.

**Supervisor**: Dr Cora Uhlemann

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#### Testing gravity on cosmological scales

The fundamental mystery of modern cosmology is the accelerated expansion of the Universe. In our standard model of ΛCDM, this is attributable to a constant dark energy, but the theoretical prediction for this energy density is orders of magnitude off from what we measure observationally. Furthermore, recent discrepancies in the value of the Hubble constant as measured in the early and late Universe have pointed to possible cracks in the universality of the ΛCDM model. It is therefore more crucial than ever that we explore other ideas. One exciting possibility is that General Relativity may be superseded by a different theory of gravity on cosmological scales.

Current and upcoming surveys such as the Dark Energy Survey and the Rubin Observatory Legacy Survey of Space and Time (LSST) offer an unprecedented opportunity to pin down the cosmological nature of gravity, with exquisitely precise measurements of gravitational lensing and large-scale structure from tens of billions of galaxies. However, with this great statistical power comes great responsibility to ensure that theoretical models are accurate to the same level. Only with both these pieces in place can we make robust statements about the nature of gravity on the largest scales. Currently, we are forced to jettison huge portions of our data at smaller cosmological scales from tests of gravity because of a lack of accurate theoretical modelling. This is particularly frustrating as these smaller scales are expected to hold a disproportionate fraction of information about the nature of gravity. In this PhD project, you will develop and apply methods to rigorously incorporate smaller-scale cosmological data into tests of gravity, and in doing take an important step forward in pinning down the cause of the accelerated expansion of the Universe.

**Supervisor**: Dr Danielle Leonard

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#### Cosmic Shear Cosmology

Weak gravitational lensing is now established as one of the most compelling probes of cosmology, allowing to map out the dark matter distribution on the sky while providing some of the best measurements of its clumpiness and abundance. Existing cosmic shear data such as that from the Kilo Degree Survey (KiDS) are currently being analysed and novel methods are being deployed in order to maximize their scientific outcome.

Among these, simulation-based approaches are receiving an increasing level of attention for their potential at better capturing the information contained in the data. In this project, the PhD student will specialize in some of these novel analysis techniques, validate the methods on simulations at first, then apply the findings on the KiDS legacy data, and subsequently on the first data release of the LSST and Euclid surveys.

**Supervisor**: Dr Joachim Harnois-Deraps

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#### Calculating the Wavefunction of the Universe

The early universe is thought to have undergone an intense burst of accelerated expansion known as inflation. During this time, quantum fluctuations were stretched out to macroscopic scales seeding the formation of structure in today’s universe. This project will investigate new tools for computing the wavefunction for the universe, based on recent advances in conformal field theory. These tools enable inflation to be described holographically in terms of a three-dimensional quantum theory with scale symmetry that lives on the future boundary of spacetime. A key goal is to understand how the unitarity of cosmological time evolution constrains the wavefunction of the universe, and how these constraints can be leveraged to place bounds on cosmological observables, such as the power spectrum and non-Gaussianities of the cosmic microwave background.

**Supervisor**: Dr Paul McFadden

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