  The current description of fundamental interactions is based on two theories
with the status of standard models. The electromagnetic and nuclear
interactions are described at a quantum level by the Standard Model of particle
physics, using tools like gauge theories and spontaneous symmetry breaking by
the Higgs mechanism. The gravitational interaction is described on the other
hand by general relativity, based on a dynamical description of space-time at a
classical level.
  Although these models are verified to high precision in the solar system
experiments, they suffer from several theoretical weaknesses and a lack of
predictive power at the Planck scale as well as at cosmological scales; they
are thus not viewed anymore as fundamental theories. As its phenomenology
involves both these extreme scales, cosmology is then a good laboratory to
probe theories going beyond these standard models.
  The first part of this thesis focus on cosmic strings, topological defects
forming during the spontaneous symmetry breaking of grand unified theories in
the early universe. I show especially how to study these defects while taking
into account the complete structure of the particles physics models leading to
their formation, going beyond the standard descriptions in terms of simplified
toy-models. The second part is devoted to the construction and the examination
of different theories of modified gravity related to the Galileon model, a
model which tries in particular to explain the dark energy phenomenology.
