Cosmology and theories of gravity

Theoretical Cosmology

Hongsheng Zhao

Numerous observations from radio to X-ray imply the presence in galaxies, galaxy groups and clusters of much more dark matter than directly detectable luminous material. What is the nature of this ubiquitous dark matter? How is it distributed? These are the two theme questions addressed in my research. They are important for understanding structure formation, which is driven by the common gravitational field of the dark and luminous matter.
Several lines of attack are used here, combining constraints from galaxy dynamics and gravitational lensing. First, microlensing of LMC, SMC and Galactic bulge/bar stars can be used to constrain MACHOs in the halo, disk and bar of the Milky Way. Second the Galaxy’s gravitational potential can be mapped out or at least constrained by studying the motion, orbital decay and tidal distortion of its satellites (e.g., LMC, SMC, Sagittarius and Ursa Minor dwarfs.) The figure shows a simulation of the encounter between the Sagittarius dwarf and the Magellanic Clouds.
Gravitational lensing provides a powerful technique for probing the dark matter halo in distant galaxies. By modelling the images and time delays of lenses (e.g., the quadruple-imaged lens PG1115+080), I hope to gain greater understanding of the dark matter density, especially the central cusp, in these systems. The technique will also be able to check whether the lensing potentials are consistent with the popular Cold Dark Matter cosmology.


  • Observational Cosmology – Rita Tojeiro

  • Alternative gravity – Keith Horne


Dark Matter

Anne-Marie Weijmans

According to the standard model of cosmology, galaxies are embedded in massive haloes of dark matter. Although we cannot observe the dark matter directly (it’s called dark for a reason!), we can infer its presence from the motions of stars and gas in the galaxies. Using integral-field data from e.g. the MaNGA survey we measure the kinematics of stars and gas, and build dynamical models to map their dark matter content. We then investigate what role dark matter plays in galaxy evolution processes, by comparing with simulations and galaxy formation theories.