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Abstract

In this thesis two aspects of Standard Model extensions are discussed. Firstly, a Standard Model extension with a strongly coupled hidden sector is investigated. For suitable parameters, such a hidden sector is expected to undergo a first order phase transition,
consequently resulting in the production of gravitational waves. Due to their
strongly coupled nature, effective low energy models have to be used to determine the
phase transition dynamics and calculate the predicted gravitational wave signals. It is
shown that different effective models in general predict similar but by no means equal
gravitational wave signals. Thus showing that calculations from first principles, like
lattice calculations, are needed. Secondly, scalar extensions are discussed, which can
result in a first order electroweak phase transition. In contrast to phase transitions in
strongly coupled sectors, the dynamics of these phase transitions is rather well known.
Consequently, the gravitational wave signals from the electroweak phase transition can
be predicted reasonably well. In this thesis, instead of phase transition dynamics, a different
aspect of scalar extensions will be discussed, namely their effect on leptogenesis
via oscillations. Our results show that a scalar extension in general reduces the produced
baryon asymmetry of the Universe. In the future these results, together with
possible proof of a first order phase transition, can be used to further constrain scalar
extensions.