We provide a systematic and updated discussion of a research line carried out by our group
over the last few years, in which gravity is modified at cosmological distances by the
introduction of nonlocal terms, assumed to emerge at an effective level from the infrared
behavior of the quantum theory. The requirement of producing a viable cosmology turns out
to be very stringent and basically selects a unique model, in which the nonlocal term
describes an effective mass for the conformal mode. We discuss how such a specific …