Scalar fields: fluctuating and dissipating in the early Universe
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Date
26/11/2015Author
Bartrum, Sam John Richard
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Abstract
It is likely that the early Universe was pervaded by a whole host of scalar fields
which are ubiquitous in particle physics models and are employed everywhere from
driving periods of accelerated expansion to the spontaneous breaking of gauge
symmetries. Just as these scalar fields are important from a particle physics
point of view, they can also have serious implications for the evolution of the
Universe. In particular in extreme cases their dynamical evolution can lead to
the failure of the synthesis of light elements or to exceed the dark matter bound in
contrast to observation. These scalar fields are not however isolated systems and
interact with the degrees of freedom which comprise their environment. As such
two interrelated effects may arise; fluctuations and dissipation. These effects,
which are enhanced at finite temperature, give rise to energy transfer between
the scalar field and its environment and as such should be taken into account for
a complete description of their dynamical evolution. In this thesis we will look at
these effects within the inflationary era in a scenario termed warm inflation where
amongst other effects, thermal fluctuations can now act as a source of primordial
density perturbations. In particular we will show how a model of warm inflation
based on a simple quartic potential can be brought back into agreement with
Planck data through renormalizable interactions, whilst it is strongly disfavoured
in the absence of such effects. Moving beyond inflation, we will consider the
effect of fluctuation-dissipation dynamics on other cosmological scalar fields,
deriving dissipation coefficients within common particle physics models. We also
investigate how dissipation can affect cosmological phase transitions, potentially
leading to late time periods of accelerated expansion, as well as presenting a novel
model of dissipative leptogenesis.
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