Doctoral defence in Physics: Sebastian Bohr

On July 7, Sebastian Bohr will defend his doctoral thesis Dark Acoustic Oscillations in Structure Formation: The High Redshift Universe. The defence will take place at the UI Aula and starts at 09:00. The defence will be streamed live at

Dr. Céline Bœhm, Professor at the University of Sydney, Australia
Dr. Aurel Schneider, Assistant Professor at the University of Zurich, Switzerland

Advisor: Dr. Jesús Zavala Franco, Associate Professor at the Faculty of Physical Sciences, University of Iceland

Doctoral committee:
Dr. Páll Jakobsson, Professor at the Faculty of Physical Sciences, University of Iceland
Dr. Steen H. Hansen, Associate Professor at the University of Copenhagen, Denmark

Chair of Ceremony: Einar Örn Sveinbjörnsson, Professor and the Head of the Faculty of Physical Sciences at the University of Iceland

The nature of dark matter (DM) as a particle is still an unresolved mystery in Physics. Therefore, a vast amount of competing particle models have been proposed. A promising category among these models are those that include relevant collisional damping in the primordial power spectrum due to the interactions with relativistic particles in the early Universe. This damping is reflected in the DM distribution as dark acoustic oscillations (DAOs) before the onset of structure formation (analogous to the baryonic acoustic oscillations in the photon-baryon plasma, but at smaller, galactic scales) which are potentially observable.

In this Thesis, two effective parameters are proposed that fully describe DAO models based on their key features in the linear power spectrum: the amplitude/height (relative to the Cold Dark Matter expectations) and scale of their primary DAO peak (effectively setting the cut-off scale for structure formation). In the limit of a peak height of zero, this parametrization also includes warm dark matter (WDM), which has a very different particle origin with a collisionless damping and a featureless cut-off in the power spectrum. A large suite of tailored N-body zoom-in simulations is used to cover the DAO parameter space that is still unconstrained, but relevant for galaxy formation. A novel (scale-dependent) way to compare different structure formation is introduced that makes it possible to identify the regions of distinct non-linear structure formation at high redshifts based on statistical measures such as the non-linear power spectrum and the halo mass function. It is found that for a large part of the DAO parameter space, the non-linear power spectrum is actually indistinguishable from WDM models and only a small region of the models with the strongest DAOs has a distinct power spectrum. However, the halo mass function breaks this WDM-DAO degeneracy and even the weakest DAO models show a distinct slope in the halo mass function for low-mass haloes, as long as the DAO scale is large enough. With these results, the proposed parametrization offers a quick way to connect a specific DM particle model to its linear power spectrum and from there to the non-linear power spectrum and halo mass function. It is also shown that the properties of DAO haloes can be well described by the extended Press-Schechter (EPS) formalism using a smooth-k filter. On the other hand, the structure of haloes within the DAO cosmology is well described by the well-known Navarro-Frenk-White profile (widely used in Cold Dark Matter, CDM). Relative to CDM, low-mass haloes in DAO models have a a lower concentration, which is also well approximated by the concentration-mass relation predicted by the EPS model and a simple mass assembly model based on hierarchical structure formation.

These results can be used to perform inexpensive calculations of the (high-redshift) halo mass function and concentration-mass relation instead of computationally expensive N-body simulations for virtually all the DAO parameter space explored in this Thesis. Finally, we show that truly distinct strong DAO features can potentially survive in the 1D Flux power spectrum down to redshifts probed by the Lyman-a forest (z = 3-5.4) and upcoming 21-cm observations at the cosmic dawn (z = 10-25).

About the doctoral candidate:
Sebastian Bohr was born in Grevenbroich, Germany in 1993. He completed his BSc and MSc degrees in Physics at RWTH Aachen University in 2015 and 2017, respectively. In 2017, he moved to Iceland to pursue a PhD degree at the University of Iceland.

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