# Upcoming seminars

28/01/2021 - 4pm, ** Paolo Benincasa **(IFT Madrid, Spain), Towards a reformulation of QFT in expanding universes.

04/02/2021 - 4pm, ** Giulia Cusin **(Universite de Geneve, Switzerland), TBA

18/02/2021 - 4pm, ** François Larrouturou **(IAP Paris, France), Refocus on the essentials: introducing the Minimal Theory of Bigravity

04/03/2021 - 4pm, ** Ali Seraj ** (Universite Libre de Bruxelles, Belgium), TBA

11/03/2021 - 4pm, ** Raissa Mendes ** (Universidade Federal Fluminenense, Brazil), TBA

18/03/2021 - 4pm, ** Kazuya Koyama ** (Portsmouth U., ICG, UK), TBA

25/03/2021 - 4pm, ** Johannes Noller ** (Cambridge University - DAMTP, UK), TBA

15/04/2021 - 4pm, ** Famaey Benoit** (Observatoire astronomique de Strasbourg, France), TBA

29/04/2021 - 4pm, ** Bayram Tekin ** (Middle East Tech. U., Ankara, Turkey), TBA

**28/01/2021 -- Paolo Benincasa (IFT Madrid, Spain) **

Place: Zoom seminar

QFT in nearly dS space-times and, more generally, in FRW backgrounds allows us to describe correlations at the end of inflation. However, how to extract fundamental physics out of them is still a challange: we do not even know how fundamental pillars such as causality and unitarity of the time evolution constrain them. In this talk I will present a recent program which aims to construct the wavefunction of the universe, which generates these correlations, directly from first principles without making any reference to time evolution: these observables naturally live at the boundary of the nearly dS/FRW space-times and the time evolution is integrated out. I will discuss two approaches: one approach aims to construct the wavefunction from the knowledge of its general analytic properties, in a similar fashion as scattering amplitudes in flat space-time (which can be formulated directly from on-shell data with no reference to fields whatsoever); the second approach aims to find new mathematical objects, with their own first principle definition, which has the very same properties we ascribe to the wavefunction of the universe, with all the basic physical principles such as causality and unitarity, emerging from their intrinsic definition.

**18/02/2021 -- François Larrouturou (IAP Paris, France) **

Place: Zoom seminar

In 2011, F. Hassan and R. Rosen achieved the construction of the first gosth-free theory of two interacting spin-2 fields. But despite its great elegance and interesting phenomenological implications, this theory of "bigravity" suffers from a gradient-type instability. Moreover, it postulates the existence of vectorial and scalar gravitational modes, the latter being severely constrained by observations of binary pulsars. This stimulated us to construct a "minimal" theory of bigravity, ie. a theory of two interacting spin-2 fields that propagates only four tensorial degrees of freedom. This talk will first introduce the motivations that led to this "minimal" theory of bigravity, and review its construction by a Hamiltonian procedure. I will then present its cosmological phenomenology, and show that it provides a stable nonlinear completion of the cosmology of the Hassan-Rosen bigravity. We will end by discussing interesting phenomenological features and possible ways to test the theory.