In general relativity generic averages of the Einstein field equations for a cosmological fluid constrained by observations of the cosmic microwave background radiation lead to differences from the standard Friedmann–Lemaître–Robertson–Walker (FLRW) metric as cosmic structures grow at late epochs. The difference yields models with backreaction of inhomogeneities, amongst which different phenomenological interpretations are possible. The most well developed phenomenology is that of the timescape model, which addresses foundational questions concerning the nature of quasilocal gravitational energy.
This project will address the evolution of the angular scale of the Baryon Acoustic Oscillation (BAO) seen in galaxy clustering statistics. This is conventionally treated by perturbation theory about the FLRW model. However, with new methods the evolution of perturbations for models with backreaction of inhomogeneities may now be investigated [1]. Phenomenologically, this involves a recalibration of the ratio of dark matter to ordinary baryonic matter, which determines the sound speed in the primordial plasma. The project will combine theory and phenomenology to constrain predictions for the timescape cosmology.
[1] Galoppo, M. and Mourier, P., 2025, Eprint 2511.17160 [gr-qc], "Cosmological perturbations on an averaged background"
Supervisors
Primary Supervisor: David L Wiltshire
Does the project come with funding
Yes – an external Marsden grant will provide a $22,000 stipend, plus fees is available for a 1-year MSc Part II thesis (PHYS690).
Final date for receiving applications
Thursday, 12 February
How to apply
By email to primary supervisor.
Keywords
General relativity, theoretical cosmology, backreaction of inhomogeneities, timescape.
