Astronomy Centre

Jon Loveday

Constraining modified gravity and galaxy formation models

Collaborators: David Seery, Robert Smith, Galaxy and Mass Assembly (GAMA) team (Europe, Australia)

 Start date: September 2017

Funding: STFC quota studentship or other

Recent cosmological observations appear to show that the Universe is not only expanding, but that it is doing so at an accelerating rate. Two explanations have been proposed to understand this: (i) the accelerated expansion is caused by some form of 'dark energy' (including a cosmological constant), or (ii) Einstein's law of gravity (GR) needs to be modified on cosmological scales.  Many surveys have been attempting to measure the equation of state, w, of dark energy; all results so far are consistent with w = -1, as expected for a cosmological constant.  Rather than attempting to measure w to higher precision, this project explores other signatures of modified gravity (MG), based on galaxy dynamics.  You will be using observational data from the Galaxy and Mass Assembly (GAMA, survey and the Kilo-Degree Survey (KiDS,


Years 1-2: The first signature of MG you will explore is the galaxypairwise velocity dispersion (PVD): the random pairwise peculiar motions of galaxies.  Hellwing et al. (2014, PRL, 22, 221102) have shown that the PVD provides one of the most sensitive diagnostics of modified gravity models, with some models predicting dispersions about 30% larger or smaller than GR.  Loveday et al. (in prep) have made preliminary measurements of the PVD from GAMA based on the anisotropy of redshift-space galaxy clustering.  You will refine and extend these estimates, and, in collaboration with David Seery and Robert Smith, make theoretical predictions with which to compare the observations.

At the same time, you will place constraints on semi-analytic and halo occupation distribution (HOD) models of galaxy formation by comparing with the model predictions.


Years 2-4: The second MG signature to be explored is the dynamical infall of galaxies into dark matter halos, e.g. Zu et al. (2014, MNRAS, 445, 1885).  In GR, the same potential is responsible for the gravitational deflection of light and the acceleration of mass.  In MG theories, these potentials may differ, and hence one obtains inconsistent mass estimates from gravitational lensing (via the KiDS survey) and galaxy motions (from GAMA data).  You will probe the infall of galaxies into overdense regions (clusters or groups) via the anisotropy of the redshift-space cluster–galaxy (or group–galaxy) cross-correlation function, and compare with the expectation from weak lensing measurements made by the KiDS team.

 For more information/to apply for this project, please contact J.Loveday[AT]