Cosmology and Structure Formation of Scalarfield Dark Matter

The nature of dark matter (DM) remains one of the most profound open problems in cosmology and physics. While cosmological observations have determined the present cosmic energy density in DM with high precision, its particle nature is still unknown. Standard DM candidates are weakly-interacting massive particles from extensions to the standard model of particle physics, which give rise to what has become the standard collisionless cold dark matter (CDM) model. While the CDM model has proved successful on large scales, its predictions on smaller galactic scales have not been confirmed by observations: CDM predicts an overabundance of satellite galaxies around hosts of Milky-Way size, in contrast to what is observed in the Local Group and beyond. CDM simulations predict a universal density run for DM halos, which has a central cusp. In contrast, DM-dominated galaxies have been observed to follow flat cores out to around 1 kpc. All this evidence against collisionless CDM models have spurred activity in the community to study alternatives for the DM to solve these problems. In this Elise Richter application, we propose to continue our study of the cosmology and structure formation of one of these alternatives, namely scalar-field dark matter (SFDM). SFDM is made up of ultralight bosons, which also arise in extensions to the particle standard model. Promising SFDM candidates to solve the above problems possess a large enough "Jeans scale", below which DM clustering is suppressed. We will perform a coherent analysis of the growth of structure in a universe with SFDM throughout its entire evolution. Especially, we will expand upon the existing literature by considering more general SFDM models, in which the scalar field can be complex (not only real), and in which boson self-interactions are included. In such models, the expansion history can be different from the standard cosmological model with CDM, and we will study its impact on structure and galaxy formation. To this end, we will accomplish analytic calculations, as well as numerical simulations during this project. Multiple observables will be used to compare data with the predictions of SFDM models in order to determine whether SFDM as the dark matter can reproduce observations of galactic and cosmological scales self-consistently.

The nature of dark matter (DM) in the Universe remains one of the most profound open problems in Astronomy and Physics. While astronomical observations have determined the cosmic energy density in DM with high precision, its particle nature is still unknown. Among the most prominent DM candidates are weakly-interacting massive particles from extensions to the standard model of particle physics, which give rise to what has become the standard collisionless cold dark matter (CDM) model. While the CDM model has proven successful on large cosmological scales, its predictions on "smaller" galactic scales have been continuously challenged by observations. In addition, CDM particle candidates have not yet been detected. Various alternative DM particles have thus been suggested in recent years, also from extensions to the particle standard model. This project focused on the study of the dynamics of one broad class of alternatives, namely scalar field dark matter (SFDM), which is made up of (ultra-)light bosons. We analyzed the growth of galactic structures - socalled DM halos - in SFDM universes during their entire cosmic evolution, by performing analytical calculations and numerical simulations. We compared our theoretical predictions to astronomical observations, in order to find constraints on SFDM models which are much stronger than those of previous literature. While SFDM could still be the dark matter in the Universe, our work helped to reduce significantly the available parameter space of SFDM. Some of the new insights gained are as follows: i) The amount of structure formation between CDM and SFDM models varies tremendously, depending upon the strength of the self-interaction between the bosons. Thus, it remains to be seen, if SFDM models can explain the observations of dwarf galaxies better than CDM. ii) In SFDM models without self-interaction, no vortices will form in the centers of galactic SFDM halos, even if angular momentum is high. However, vortices form in the outer envelope regions of halos. While simulations have previously found that this occurs, it is only with our analytical models that we can explain what happens. iii) The central cores of SFDM halos can undergo gravitational collapse, depending on model parameters, and thereby explain the formation of supermassive black holes that have been previously detected in the centers of galaxies. iv) We devised novel, "coarse-grained" hydrodynamical models to describe SFDM halos, which could prove useful for other systems in physics, especially for those with high degree of turbulence.