AbstractsPhysics

Grounded in the increasingly accurate astronomical observations of the past few decades, the study of cosmology has produced a comprehensive account of the history of the universe. This account is contained in the Hot Big Bang cosmological model which describes the expansion of a hot and dense state to become the universe as we observe it today. While the Big Bang model has been extremely successful in being able to account for a wide array of cosmological data, it leaves unexplained the special initial conditions that are required in order to produce the universe we find ourselves in. Such initial conditions are, however, a natural consequence of a period of quasi-exponential expansion of the universe known as inflation. Such a period of expansion can be realized if the universe is dominated by a scalar field - the inflation - which is slowly rolling down the slope of its potential. Inflation also provides a natural mechanism for the production of primordial seeds of structure in the universe through the growth of the quantum fluctuations in the inflaton field to super-horizon scales. Together inflation and the subsequent Big Bang evolution form the back bone of modern cosmology. However, the transition between the inflationary epoch and the thermal state which characterizes the initial conditions of the Big Bang evolution is not well understood. This process - dubbed reheating - involves the decay of the inflaton field into the particles of the Standard Model of particle physics, and may be highly non-trivial, with non-perturbative resonant processes playing a major role. Spectator fields - light scalar fields which are subdominant during inflation - may also play an important role during this epoch. The aim of this thesis is to showcase aspects of non-perturbative decay of scalar fields after inflation, focusing in particular on the role of spectator fields. This includes the modulation of the non-perturbative decay of the inflaton by a spectator field, the non-perturbative decay of a spectator into the Standard Model Higgs, as well as the non-perturbative decay of the Higgs field itself. Nojaten viime vuosikymmenien tähtitieteellisiin havaintoihin, kosmologian tutkimus on tuottanut kattavan kuvauksen maailmankaikkeuden historiasta. Tämä kuvaus on kiteytetty kuuman alkuräjähdyksen mallissa, joka kuvaa maailmankaikkeuden laajenemista hyvin kuumasta ja tiheästä tilasta. Vaikka kuuman alkuräjähdyksen malli on ollut hyvin menestyksekäs ja on pystynyt selittämään suuren määrän kosmologisia havaintoja, se myös vaatii erityisiä alkuehtoja maailmankaikkeuden kehitykselle, joita on vaikea motivoida. Etenkin varhaisen maailmankaikkeuden olisi täytynyt olla hyvin laakea ja homogeeninen. Tällaiset alkuehdot ovat kuitenkin hyvin luonnollisia jos hyvin varhainen maailmankaikkeus koki kiihtyvän laajenemisen aikakauden. Tätä aikakautta nimitetään kosmologiseksi inflaatioksi ja inflaatioteoria on myöskin ollut menestyksekäs ja on tarjonnut paitsi selityksen näille alkuehdoille myös mekanismin rakenteiden synnylle. …