From the stellar IMF to large-scale structure formation: How the ΛCDM model is incompatible with observations over all probed astrophysical scales

Abstract

This doctoral thesis investigates the current standard model of cosmology ($Lambda$ Cold Dark Matter – $Lambda$CDM – framework) by performing tests on different astrophysical scales which could indicate if the missing mass problem implies indeed the existence of cold dark matter (CDM) or rather emerges because of a breakdown of Newton's law of gravity. <br /> In the local Universe, we showed that the Magellanic Clouds-Milky Way system and morphological distribution of galaxies suggest that dynamical friction on galactic scales is much less efficient than in the $Lambda$CDM framework. State-of-the art $Lambda$CDM simulations overproduce thick galaxies while the majority of observed massive galaxies are thin spirals pointing to a failure of hierarchical merger- driven galaxy evolution in which galaxies grow efficiently through mergers due to dynamical friction on CDM haloes. <br /> On larger scales, we quantified that the observed Keenan-Barger-Cowie (KBC) void falsifies the $Lambda$CDM framework on co-moving radial distance scales of $approx300$ cMpc whereas the enhanced growth of structures in Milgromian dynamics (MOND) allows the formation of KBC-like voids from which induced large-scale matter bulk flows resolve the Hubble tension. The almost constant star formation history (SFH) of the majority of local galaxies challenges the traditional interpretation of the Lilly-Madau plot according to which the evolution of the star formation rate density over cosmic time represents the global SFH of the Universe. We argued that the peak of the Lilly-Madau plot at redshift $zapprox1.9$ may reflect the imprint of a large-scale overdensity rather than the cosmic noon. <br /> In the early Universe, we showed that the spectroscopically confirmation of galaxies with derived stellar masses of $gtrsim 10^{9},M_{odot}$ for an invariant canonical stellar initial mass function (IMF) at $z gtrsim 10$ by the James Webb Space Telescope (JWST) would imply that the observed stellar mass buildup is inconsistent with the $Lambda$CDM framework. The JWST observations illustrate that testing cosmological models requires assumptions about the IMF. Therefore, we developed the photometric GalIMF code as a package of the publicly available chemical galaxy evolution code GalIMF to compute the photometric properties of galaxies for an invariant canonical but also varying galaxy- wide IMF. <br /> In conclusion, the performed tests disprove the $Lambda$CDM framework over all probed scales ranging from galactic to large-scale structures. This established a consistent picture of the Universe in which dynamical friction on galactic scales is next to absent and structure formation is more enhanced than predicted by the $Lambda$CDM framework disfavoring the existence of CDM on galactic scales and pointing to a long-range correction of Newtonian gravity as provided by MOND. As the interpretation of photometric measurements depends on the properties of the IMF, galaxy evolution and cosmological models have to be considered in a more holistic picture by bridging stellar population of galaxies with cosmology.