The Nature of Dark Matter and Structure Formation

The nature of the dark matter (DM) in the Universe remains one of the most profound open problems in cosmology and physics. While astronomical observations have determined the present 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 cosmic scales, its predictions on smaller galactic scales have been continuously challenged by observations: CDM predicts an overabundance of satellite galaxies around galaxies of Milky- Way size, in contrast to what is observed in our Local Group of galaxies and beyond. CDM simulations predict a universal density run for the DM in and around galaxies, which has a central cusp. In contrast, DM- dominated galaxies have been observed to have almost constant central densities out to around 1 kpc. Also, a common dynamical mass scale of dwarf galaxies around our Milky Way has been identified, despite a difference in their luminosities of up to four orders of magnitude. All this evidence points to a new characteristic scale for DM clustering, which is in contradiction with collisionless CDM models. Various alternatives for the DM to solve these problems have thus been suggested. This project aims to study and analyze structure formation of one of these alternatives, namely scalar-field dark matter (SFDM). SFDM is made up of ultralight bosons, which have condensed into their ground state early in the evolution of the Universe. SFDM arises also in extensions to the particle standard model. The QCD axion is the best known example, and has been one of the most favoured DM candidates ever since its inception. Promising SFDM candidates to solve the above problems have much smaller masses than the QCD axion, for then the scale below which DM clustering is prohibited can be in accordance with observations. In this project, we plan to carry out for the first time a coherent and complete analysis of the growth of linear and nonlinear perturbations, thus the growth of structure in a universe with SFDM throughout its entire evolution. We will pursue analytic calculations as well as numerical simulations, to compare with data in order to determine whether SFDM can reproduce observations of galactic and cosmological scales self-consistently, and whether it could indeed account for the DM in the Universe.

The nature of dark matter (DM) in the Universe remains one of the most profound open problems in cosmology 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 cosmic 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 standard model. This project focused on the study of one broad class of alternatives, namely scalar-field dark matter (SFDM), which is made up of ultralight bosons. We focused on the cosmological consequences of SFDM. We carried out analytical and numerical calculations to compare with data in order to determine whether SFDM can reproduce observations of galactic and cosmological scales self-consistently, and whether it could indeed account for the DM in the Universe. While SFDM could still account for the DM, we showed that the allowed parameter space of SFDM models is significantly constrained, thereby disproving expectations from the literature that SFDM and CDM models should result in more or less the same cosmological evolution. We used cosmological observables from Big Bang nucleosynthesis, the Cosmic Microwave Background Radiation and the stochastic gravitational wave background from inflation. We showed that the latter is enhanced in SFDM cosmologies. While the former two observables have been also used in the past by other researchers, we demonstrated in this project for the first time that gravitational wave measurements by LIGO/Virgo can already place a new kind of cosmological constraint on SFDM! A wider range of SFDM models will be accessible to future LIGO/Virgo runs, as well as to the future space experiment LISA, with the potential to detect this unique signature of the SFDM model, or to rule out more of the parameter space. Thus, gravitational wave detection experiments can help in our quest to uncover the nature of dark matter! Another aspect of the modified expansion history of SFDM models concerns its impact on further processes in the early Universe. We showed that the baryon asymmetry another major open issue of modern cosmology, can indeed be preserved during the electroweak phase transition (even though it is not of first order), because the expansion rate in SFDM cosmologies can be many orders of magnitudes higher than in the standard cosmological model. Altogether, SFDM may thus explain different, a priori unrelated riddles of modern physics.