Final Activity Report Summary - EU-CosMO (Particle physics models of the early Universe and observational constraints from the cosmic background radiation) The aim of this project was to link astrophysical and cosmological observations of the early and present universe with theoretical models in the context of cosmology and particle physics, building a bridge between the data of modern cosmological experiments and the theoretical scenarios (such as string theory and loop quantum gravity) proposed in the attempt to understand the fundamental behaviour of Nature. High-energy physics can leave its imprint in the large-scale structure of the Universe during its primordial stage of expansion, and the microcosm affects the macrocosm in very nontrivial ways. The work performed was theoretical, as processed observative data were taken from the literature. Both analytic and numerical methods were employed.A feature which has still to be understood, known as the "cosmological constant problem", is the present acceleration of the universe. Most of the many models trying to explain this observation in a natural and convincing way involve either modifications of the long-range gravitational forces or particular matter constituents (scalar fields). We considered some of the better-motivated proposals reaching mainly negative yet important results: certain popular modifications of gravity (Gauss-Bonnet non-minimally coupled actions) and scalar fields ("old" Dirac-Born-Infeld tachyon) are not good triggers of cosmic acceleration, as their parameters should be tuned in an ad-hoc unnatural fashion in order to match observations.On the other hand, we concentrated on recent developments of "string field theory", which has been received considerable interest in the past couple of years and might provide a sensible framework within which to study the properties of the early, big-bang universe. As the definition of this theory and its main features (solutions, dynamics) are quite involved, a lot of energy has been spent on their formalisation and the systematic construction of solutions of immediate relevance to cosmological inflation and string theory. Incidentally, this led to the exploration of mathematical techniques interesting in their own right.