Gauge-fixing cosmological perturbations in massive bigravity.

Abstract: Lately, an industry of modified gravities has been developing, as General Relativity, even though accurate around our local universe, is not exempt of problems. In this industry, theoretical consistency checks and observational tests are essential.

In this talk, I will focus on cosmological tests. In the first part, I will discuss the study of the evolution of cosmological perturbations, and the technical issues related to gauge symmetries. I will explain a general gauge-fixing technique for addressing this issue, as an alternative to the standard procedure of using gauge-invariant variables. This method, along with greatly simplifying calculations, eliminates redundancies and makes physical degrees of freedom explicit. Therefore, it is particularly well suited to modified gravity scenarios, which generally introduce extra fields, and so complicate calculations.

In the second part, I will introduce Massive bigravity, a particular modified gravity characterised by propagating one massive and one massless spin-2 particle. I will discuss the cosmology of this theory and the use of the gauge-fixing method in this context. Features such as self-accelerated and bouncing universes make this model attractive. Finally, I will comment on our ongoing and future work.