Pieter Braat, PhD at Nikhef, will defend his thesis Wednesday 18 February 2026 at 11.00 at the University of Amsterdam.
“Hiding in the Dark. Constraining Dark Sectors with Cosmological Observations ”
The fundamental properties of dark matter remain unknown nearly one hundred years after its discovery. Because this form of matter is more than five times as abundant as “ordinary” or Standard Model (SM) matter, it constitutes one of the biggest open questions in fundamental physics. In this thesis, we have sought to contribute to answering this question. In particular, we have focused on observations from the early universe; measurements of the present-day dark matter abundance (the relic density), Big Bang nucleosynthesis, and the cosmic microwave background radiation are especially well suited to constraining dark matter models, as we have demonstrated in this thesis. Specifically, we examined a model in which dark matter consists of dark pions, analogous to SM pions, and in which an additional dark photon is introduced. By choosing the photon mass to be nearly twice as large as the pion mass, the phenomenology of the dark sector is modified. In addition, we studied a model in which dark matter consists of dark atoms. Both models exhibit interesting effects in the early universe, which are described in this thesis.
The defense will take place in the Aula of the Lutherse kerk, Singel 411, 1012 WN Amsterdam.
More information and a link to the livestream on the website of the UvA.
Supervisors: prof. dr. A.P. Colijn and prof. dr. M.E.J. Postma
