Origin of Galaxies
Nikolaos Samaras
TL;DR
The origin of galaxies paper surveys the standard ΛCDM cosmology and its hydrodynamical simulations, noting the success of ΛCDM in explaining expansion history and large-scale structure but highlighting tensions such as the Hubble constant discrepancy. It then introduces νHDM, a Milgromian dynamics–based cosmology that replaces cold dark matter with a hot sterile neutrino and employs MOND gravity at late times, tested with high-resolution hydrodynamical simulations. The optimized opt‑νHDM variant, fit to Planck data, yields a heavier universe (Ωm≈0.5, H0≈55.6) and a later onset of structure formation, challenging JWST high‑redshift galaxy observations. The work demonstrates that MONDian cosmologies can reproduce many ΛCDM features at the background level but face significant challenges with early structure formation and high‑redshift constraints, underscoring the need for advanced MONDian N-body/hydrodynamical modeling and tighter observational tests.
Abstract
Milgromian Dynamics (MOND) has been particularly successful in predicting scaling relations for galactic systems, namely the baryonic Tully-Fisher for spirals, the Faber-Jackson for ellipticals and the Radial Acceleration Relation for late-type galaxies. Its essential tenet is the modification of the gravity law at low accelerations. Nevertheless, despite MOND's passed tests, the puzzle of the missing mass on galaxy clusters' scales still persists. One proposed scenario to complete this deficit and apply it further to cosmology is the so-called $ν$HDM model. It is composed of a sterile neutrino and the MOND gravity. In this Ph.D. thesis, I have put forward this cosmological idea, conducting hydrodynamical simulations to investigate the $ν$HDM structure formation. In the debut attempt, the $ν$HDM model was proven capable of reproducing the $Λ$CDM cosmology, imitating the same expansion history ($Ω_m \approx$ 0.3 and $H_0 \approx$ 67.6 km/s/Mpc), reproducing the CMB phenomenology, while the cosmic web is nicely formed. In the next step, I have optimized the $ν$HDM model using Bayesian statistics, re-branding it to opt-$ν$HDM. I managed to optimally fit the angular power spectrum of the CMB calculated by \textit{Planck}, except its fourth peak, but the opt-$ν$HDM cosmological parameters changed significantly with respect to its precursor, resulting in a much heavier cosmos, with $Ω_m \approx$ 0.5 and $H_0 \approx$ 55.6 km/s/Mpc. Next, I explore the opt-$ν$HDM expansion history, its structure formation and the resulting mass function. The appearance of the early-time structures, firstly in the $ν$HDM model, as well as in the opt-$ν$HDM model occurs quiet late, at $z\approx4$ and at $z\approx 5.5$, respectively. Although the resolution has improved from previous studies, the $ν$HDM variants cannot explain easily the high-redshift galaxies, observed by JWST.