arXiv subjects

Horst Foidl

Publications and source records attributed to Horst Foidl.

5 recordsLinked to original sources

arXiv2024

A $\Lambda$CDM Extension Explaining the Hubble Tension and the Spatial Curvature $\Omega_{k,0} = -0.012 \pm 0.010$ Measured by the Final PR4 of the Planck Mission

The measurements of the CMB have determined the cosmological parameters with high accuracy, and the observation of the flatness of space have contributed to the status of the concordance $\Lambda$CDM model. However, the cosmological constant $\Lambda$, necessary to close the model to critical density, remains an open conundrum. We explore the observed late-time accelerated expansion of the Universe, where we consider that the Friedmann equation describes the expansion history of FLRW universes in the local reference frame of freely falling comoving observers, which perceive flat, homogeneous and isotropic space in their local inertial system, where, as a consequence of the equivalence principle, special relativity applies. We use this fact to propose an extension to $\Lambda$CDM, incorporating the initial conditions of the background universe, comprising the initial energy densities as well as the initial post big bang expansion rate. The observed late-time accelerated expansion is then attributed to a kinematic effect akin to a dark energy component. Choosing the same $\Omega_{m,0} \simeq 0.3$ as $\Lambda$CDM, its equation of state $w_{de} \simeq -0.8$. Furthermore, we include the impact on the expansion history caused by the cosmic web of the late Universe, once voids dominate its volume, and find that the initially constant $w_{de}$ becomes time-dependent, evolving to a value of $w_{de} \simeq -0.9$ at the present. While this impact by voids is minor, it is sufficient to provide a solution to the Hubble tension problem. We use CLASS to calculate the expansion history and power spectra of our extension and compare our results to concordance $\Lambda$CDM and to observations. We find that our model agrees well with current data, in particular with the final data release PR4 of the Planck mission, where it explains the reported spatial curvature of $\Omega_{k,0} = - 0.012 \pm 0.010$.

astro-ph.CO↗

arXiv2023

A proposal to improve the accuracy of cosmological observables and address the Hubble tension problem

(abridged)Cosmological observational programs often compare their data not only with $\Lambda$CDM, but also with extensions applying dynamical models of dark energy (DDE), with a time-dependent equation of state (EoS) parameter $w$. We found a degeneracy in the customary computational procedure for the expansion history, once DDE models are applied. This degeneracy provides an infinite number of cosmological models reproducing the Planck-measured CMB spectrum. Moreover, this degeneracy biases the comparison of $\Lambda$CDM with DDE extensions. We present a complementary computational approach, which breaks this degeneracy: the ``fixed early densities (EDs) approach'' evolves cosmological models from the early Universe to the present, in contrast to the customary ``fixed $H_0$ approach'' which evolves cosmological models in reverse order. We find the EDs are accurately approximated by the $\Lambda$CDM model. We implement a refined procedure, appyling both approaches, in an amended version of the code CLASS, where we focus on representative DDE models using the CPL parametrization. Our results reveal that the CPL model $w(a)=-0.9 + 0.1(1-a)$ could provide a resolution to the Hubble tension problem. Moreover, we find that combining both approaches, while requesting to yield consistent results and being in agreement with observations across cosmic time, can serve as a kind of consistency check for cosmological models. Moreover, our proposed consistency check applied within current data analysis methods will increase the accuracy of inferred cosmological parameters significantly, in particular for $\Lambda$CDM extensions. We find characteristic signatures in the late expansion histories of cosmological models, allowing a phenomenological discrimination of DE candidates and a possible resolution of the Hubble tension, by ongoing and future observational programs.

astro-ph.CO↗

arXiv2023

Halo formation and evolution in scalar field dark matter and cold dark matter: New insights from the fluid approach

(abridged) We present simulations of halo formation and evolution in scalar field dark matter (SFDM) cosmologies in the Thomas-Fermi regime, aka ``SFDM-TF", where a strong repulsive 2-particle self-interaction (SI) is included, being a valuable alternative to CDM, with the potential to resolve its ``cusp-core" problem. In general, SFDM behaves like a quantum fluid. Previous literature has presented two fluid approximations for SFDM-TF, as well as simulations of halo formation. These results confirmed earlier expectations and are generally in mutual agreement, but discrepancies were also reported. Therefore, we perform dedicated 3D cosmological simulations for the SFDM-TF model, applying both fluid approximations, as well as for CDM. Our results are very well in accordance with previous works and extend upon them, in that we can explain the reported discrepancies as a result of different simulation setups. We find some interesting details: The evolution of both SFDM-TF and CDM halos follows a 2-stage process. In the early stage, the density profile in the center becomes close to a $(n=1.5)$-polytropic core, dominated by an "effective" velocity-dispersion pressure $P_{\sigma}$ which is common to both dark matter models. Consecutively, for CDM halos, the core transitions into a central cusp. In SFDM-TF halos, the additional pressure $P_\text{SI}$ due to SI determines the second stage of the evolution, where the central region follows closely a $(n=1)$-polytropic core, embedded in a nearly isothermal envelope, i.e. the outskirts are similar to CDM. We also encounter a new effect, namely a late-time expansion of both polytropic core plus envelope, because the size of the almost isothermal halo envelope is affected by the expansion of the background universe. So, an initial primordial core of $\sim 100$ pc can evolve into a larger core of $\gtrsim 1$ kpc, even without feedback from baryons.

astro-ph.GA↗

arXiv2022

Cosmological structure formation in complex scalar field dark matter versus real ultralight axions: a comparative study using CLASS

(abridged) We continue the study of $\Lambda$SFDM cosmologies, which differ from $\Lambda$CDM in that CDM is replaced by scalar field dark matter (SFDM) by calculating the evolution of the background Universe, as well as linear perturbations, focusing on scalar modes. We consider models with complex scalar field with a repulsive, quartic self-interaction (SI), and models without SI, referred to as fuzzy dark matter (FDM). To this end, we modify the Boltzmann code CLASS, to incorporate the physics of complex SFDM which has as one of its characteristics that its equation of state is maximally stiff in the very early Universe, dominating then over all the other cosmic components, even over radiation. We calculate CMB and matter power spectra as well as unconditional Press-Schechter halo mass functions for various models, expanding previous literature that were limited either to the background, or to a semi-analytical approach to SFDM density perturbations neglecting the early stiff phase. Comparing our results of each, SFDM and FDM, with real-field ultralight axion models (ULAs) without SI, we characterize the differences between the respective background evolution and linear structure growth. Our calculations confirm previous results of recent literature, implying that SFDM models with $\gtrsim$ kpc-size halo cores are disfavored, questioning their ability to explain the small-scale problems on dwarf-galactic scales. Also we find that the kinetic energy due to the phase of the complex field leads to marked differences between SFDM/FDM versus ULAs. The mild falloff in the SFDM power spectrum toward high k is similar to that of CDM but based on different effects, namely the rapidly shrinking Jeans mass for SFDM as opposed to the Meszaros effect for CDM. In addition, we find that the sharp cutoff in the ULA power spectrum is also followed by a mild falloff, albeit at very small power.

astro-ph.CO↗

arXiv2020

Lyman-αabsorption beyond the disk of simulated spiral galaxies

We present an analysis of the origin and properties of the circum-galactic medium (CGM) in a suite of 11 cosmological zoom simulations resembling present day spiral galaxies. On average the galaxies retain about 50\% of the cosmic fraction in baryons, almost equally divided into disc (interstellar medium) gas, cool CGM gas and warm-hot CGM gas. At radii smaller than 50 kpc the CGM is dominated by recycled warm-hot gas injected from the central galaxy, while at larger radii it is dominated by cool gas accreted onto the halo. The recycled gas typically accounts for one-third of the CGM mass. We introduce the novel publicly available analysis tool \textsc{pygad} to compute ion abundances and mock absorption spectra. For Lyman-$α$ absorption we find good agreement of the simulated equivalent width (EW) distribution and observations out to large radii. Disc galaxies with quiescent assembly histories show significantly more absorption along the disc major axis. By comparing the EW and HI column densities we find that CGM Lyman-$α$ absorbers are best represented by an effective line-width $b\approx 50 - 70$ km s$^{-1}$ that increases mildly with halo mass, larger than typically assumed.

astro-ph.GA↗