Research

As a cosmologist, I’m interested in how the Universe works on the largest scales. Two of the biggest unsolved problems in our field are what exactly are dark energy and dark matter.

Dark energy is the name given to the substance that drives the late-time accelerating expansion of the Universe. Dark matter is the name given to all the mass in the Universe which does not interact with light — we only know of its presence due to its gravitational effects on visible matter. There is much more dark matter than normal matter in the Universe. There are many theories and models that try to describe dark energy and dark matter in a way that fits with observational data.

The gold standard model is called the ΛCDM model, where dark energy is represented by the cosmological constant, Λ. This is the energy due to the quantum excitations that occur naturally and randomly in any empty space. The other dark component of the Universe is CDM, or Cold Dark Matter. This says that dark matter is made of massive particles, like normal matter, and they move slowly in space.

cosmic pie
The cosmic pie: approximately 95% of our Universe is made up of dark energy and dark matter. Image credit: ESA/ Planck

The ΛCDM model is the simplest explanation for the observations that we make, but the model has a number of flaws that drive theorists to look for alternative explanations. A large competing class of dark energy models are called dynamical dark energy models, where, unlike in the cosmological constant model, the dark energy does not have a constant value.

My PhD thesis mainly focused on studying interacting vacuum dark energy models, where dark energy is modelled by a dynamical vacuum energy and dark matter can decay into the vacuum. I used the numerical codes CAMB, CosmoMC and Cobaya to analyse these models and make predictions which I tested against observational data.

Towards the end of my PhD, I also worked on forecasting constraints on the distance duality relation, which tells us how luminosity and angular diameter distances are related. A violation of the relation could be a signature of as-yet-unknown physics, such as photons converting into a “dark” particle like the hypothetical axion. I worked on producing forecasts for distance duality violation using mock datasets of merging neutron stars (also known as standard sirens) and supernovae whose light had been gravitationally lensed.

You can see the talk I recorded for Cosmology from Home 2021 here, where I present the work on constraining violations of distance duality using standard sirens.

I have also recently worked on exploring the possibility that dark matter is made of primordial black holes, a type of black hole which was proposed by Stephen Hawking and others to have formed in the very early Universe. My collaborators and I made a forecast of whether the next-generation gravitational wave detector known as the Einstein Telescope will be able to detect primordial black holes; you can read the paper here: https://arxiv.org/abs/2205.02639.

I also work on the use of strong gravitational lensing, the phenomenon of light being bent by massive objects like galaxies, to study the distribution of dark matter in the Universe. The presence of dark matter clumps subtly distorts strong lensing images, and if this distortion — known as cosmic shear — can be measured, it would likely allow for better constraints to be placed on the growth of structure due to dark matter than is possible using cosmic shear alone. You can read my first paper on this topic here: https://arxiv.org/abs/2210.07210.

You can see an up-to-date list of my publications by clicking here.