PhD Thesis Defenses

Licentiate thesis defense: Effects of the Higgs field on inflation and reheating

The model of inflation – a period of accelerated expansion in the very early Universe – was
introduced to give solutions to a number of problems encountered in the Standard Big Bang
paradigm. Additionally, due to its quantum nature, inflation is able to generate the necessary
primordial inhomogeneity “seeds”, which eventually evolve into large-scale structures. The
particular primordial inhomogeneities are imprinted on the Cosmic Microwave Background
radiation (CMB) as very small deviations (temperature fluctuations) from a perfect blackbody
spectrum. If the Standard Model (SM) Higgs is a light spectator field during inflation, it can,
also, acquire quantum fluctuations and seed additional, potentially observable, fluctuations.
This takes place via an effective breaking of electroweak symmetry at very high energy
scales, which results in the reheating process being different in different regions of the
Universe. We develop methods for calculating the amplitude, as well as the non-Gaussianity,
of such Higgs-induced temperature fluctuations in the CMB. In the case of reheating via
resonant inflaton decays to Abelian gauge bosons, we show that the amplitude of the Higgsinduced
temperature fluctuations always exceeds the observed value and that, therefore, such
decays cannot be the main reheating channel. In the case of reheating via perturbative inflaton
decays to SM fermions, we place strong constraints on the relevant SM parameters, using the
amplitude of the Higgs temperature fluctuations. By additionally using the associated non-
Gaussianity, we are able to strengthen the particular constraints even further. Having made a
connection between cosmological observations and SM parameters, such as the Higgs selfcoupling,
we suggest a way to probe the SM Higgs potential at very high energy scales and
constrain New Physics.