Research

Understanding the Universe as we know it

Dark energy, gravity and the large-scale structure of the cosmos

I study the evolution of the Universe as we know it: how galaxies and the cosmic web grew out of tiny early fluctuations, and why the expansion of the Universe is accelerating today. Most of my work sits where theory meets observation, asking what large galaxy surveys actually measure and what those measurements can tell us about dark energy, dark matter and gravity itself.

What I'm working on now

At present I am thinking about whether the latest galaxy-survey data paint a consistent picture of how the Universe has expanded, about gravitational waves from merging compact objects, and about the nature of dark matter in the very early Universe.

Themes in my work so far

Dark energy and the Hubble tension

Measurements of today's expansion rate from the nearby Universe disagree with values inferred from the early Universe. I have explored whether dynamical dark energy within modified gravity theories can ease this tension, and whether baryon acoustic oscillation (BAO) measurements from different surveys agree with each other, using model-independent reconstructions of the expansion history.

Reconstructed deceleration parameter q(z) from SDSS and DESI BAO data compared with LCDM
The deceleration parameter q(z), reconstructed without assuming a dark energy model from SDSS (left) and DESI (right) BAO data, compared with ΛCDM.
H(z)/(1+z) for Horndeski gravity models compared with data
Expansion history in Horndeski gravity models that can raise the local Hubble constant, confronted with local, BAO and quasar measurements.

Galaxy number counts and relativistic effects

We see galaxies not where they are, but where their light makes them appear. Gravitational lensing, redshift-space distortions and other relativistic effects all shift what we count. My PhD work developed full-sky descriptions of these observables, studied how lensing alters galaxy counts, and checked whether observables change under conformal and disformal transformations of the metric.

Testing gravity on cosmological scales

The Eg statistic combines lensing and galaxy clustering to test general relativity on cosmic scales. I worked on formulating it in terms of quantities surveys actually observe, and on modelling observables on smaller, non-linear scales.

Weak lensing and intrinsic alignments

Weak lensing measures the tiny, coherent distortions of galaxy shapes caused by intervening matter. But galaxies are also intrinsically aligned by the tidal fields they form in, which can mimic or mask the lensing signal. I have worked on shape and size correlations, higher-order lensing distortions (flexions), a unified alignment model for elliptical and disc galaxies, and whether lensing and alignments can break the distance-duality relation.

Stellar orbits in the disc plane perturbed by tidal shearA stellar orbit tilted out of the disc plane by a tidal field
Circular stellar orbits (blue) perturbed by the surrounding tidal field (green), within the disc plane (left) and tilted out of it (right): the seed of intrinsic alignments in disc galaxies.

The papers behind each of these themes are listed on the Publications page.