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We consider the spectral action as an action functional for modified gravity on a spacetime that exhibits a fractal structure modeled on an Apollonian packing of 3-spheres (packed swiss cheese) or on a fractal arrangements of dodecahedral spaces. The contributions in the asymptotic expansion of the spectral action, that arise from the real poles of the zeta function, include the Einstein-Hilbert action with cosmological term and conformal and Gauss-Bonnet gravity terms. We show that these contributions are affected by the presence of fractality, which modifies the corresponding effective gravitational and cosmological constants, while an additional term appears in the action, which is entirely due to fractality. This term is further affected by a contribution of oscillatory terms coming from the poles of the zeta function that are off the real line, which are also a property specific to fractals. We show that the shape of the slow-roll potential obtained by scalar perturbations of the Dirac operators is also affected by the presence of fractality. The talk is based on joint work with Adam Ball.

I will describe a proposal for a generalization of the BMS group in which the conformal isometries of the sphere (Lorentz group) are replaced by arbitrary sphere diffeomorphisms. I describe the computation of canonical charges and show that the associated Ward identities are equivalent to the Cachazo-Strominger subleading soft graviton formula. Based on joint work with Alok Laddha.

Scalar fields are a useful proxy for other complex interactions, but also an attractive extension of General Relativity and a possible dark matter component. I will discuss some aspects of the gravitational interaction of scalar fields, in particular (i) the formation and growth of self-gravitating structures and their interaction with compact stars. and (ii) superradiance around black holes and how it can be used to constraint particle masses.