### Adam Solomon

University of Cambridge

Are Two Metrics Better Than One?: The Cosmology of Massive (Bi)gravity

In recent years, massive gravity - which modifies general relativity by giving the graviton a mass - has garnered increasing attention as an alternative to ΛCDM which may not need to be fine-tuned against large quantum corrections. The ingredients of massive gravity are two metric-like tensors; the theory is often studied in its bimetric form, where both metrics are dynamical. It is this version of the theory which leads to viable FRW cosmologies.

I will discuss recent work on the cosmology of both massive gravity and bigravity, focusing on three aspects: 1) their stability, 2) their predictions for structure formation in the subhorizon regime, and 3) the possibility of extending the theory to couple matter to both metrics. Our findings on the first two of these picks out a particular submodel, infinite-branch bigravity (IBB), which is stable at all times and agrees with present observational constraints, yet presents unique signatures for structure formation. These should render massive bigravity testable in the near future by Euclid and SKA. The doubly-coupled theory is nascent; I will talk about some of its salient cosmological properties, and discuss the potential for nontrivial matter couplings to avoid the no-go theorem on FRW cosmologies in single-metric massive gravity.