Wednesday seminar: Iain Brown: Gauge Issues in Modified Gravity

Modified theories of gravity have been popular in general relativity and cosmology for decades and have experienced a resurgence in interest due to the dark energy problem in cosmology, which can also be solved by introducing exotic species of matter. Differentiating between theories of modified gravity and exotic matter is impossible at the background level since models can be tuned to any expansion history, and we must consider the nature of perturbations on the background. Doing so is easier in the so-called "Einstein frame" where the modified gravity is written as standard general relativity non-minimally coupled to a scalar field. The study of perturbations in relativity introduces the gauge issue - the coordinates chosen to describe the perturbations. We show that transforming into the Einstein frame tangles the choice of gauge and that, if this is not properly considered, the results of calculations in the Einstein frame could be seriously compromised.