arXiv subjects

Nick Samaras

Publications and source records attributed to Nick Samaras.

5 recordsLinked to original sources

arXiv2025

On the initial conditions of the $\nu$HDM cosmological model

The $\nu$HDM is the only cosmological model based on Milgromian Dynamics (MOND) with available structure formation simulations. While MOND accounts for galaxies, with a priori predictions for spirals and ellipticals, a light sterile neutrino of 11 eV can assist in recovering scaling relations on the galaxy-cluster scales. In order to perform MONDian cosmological simulations in this theoretical approach, initial conditions derived from a fit to the angular power spectrum of Cosmic Microwave Background (CMB) fluctuations are required. In this work, we employ CosmoSIS to perform a Bayesian study of the $\nu$HDM model. Using the best-fit values of the posterior, the CMB power spectrum is reevaluated. The excess of power in the transfer function implies a distinct evolution scenario, which can be used further as an input for a set of hydro-dynamical calculations. The resulting values H0 $\approx$ 56 km/s/Mpc and ${\Omega}_{m_{0}} \approx 0.5$ are far from agreement with respect to the best fit ones in the canonical Cold Dark Matter model, but may be significant in MONDian cosmology. The assumed Planck CMB initial conditions are only valid for the $\Lambda$CDM cosmology. This work constitutes a first step in an iterative procedure needed to disentangle the model dependence of the derived initial density and velocity fields.

astro-ph.CO↗

arXiv2024

Challenges to a sharp change in $G$ as a solution to the Hubble tension

It has been proposed that the gravitational constant $G$ abruptly decreased around 130 Myr ago, making Type Ia supernovae (SNe) in the Hubble flow intrinsically brighter than those in host galaxies with Cepheid distances. This would make Hubble flow SNe more distant, causing redshifts to rise slower with distance, potentially solving the Hubble tension. We explore a wide range of unattractive consequences of this ``$G$ step model'' (GSM). We find that since the luminosities of Sun-like stars scale as approximately $G^{5.6}$, the Solar luminosity would have dropped substantially 130 Myr ago in this scenario, likely pushing Earth into a planetary glaciation. However, there was no Snowball Earth episode in the last 500 Myr. The GSM also implies that the length of a year would have abruptly increased by about 10%, but the number of days per year has evolved broadly continuously according to geochronometry and cyclostratigraphy. The GSM would considerably alter stellar evolution, causing the Sun to have exhausted about 2/3 of its fuel supply rather than 1/2. This would make the Sun's helioseismic age exceed that of the oldest meteorite samples, but these agree excellently in practice. The expected age of the Universe also agrees well with that of the oldest Galactic stars assuming constant $G$. The GSM however implies these stars are younger, creating a lack of stars from the first 3 Gyr of cosmic history. These arguments pose significant challenges to models seeking to resolve the Hubble tension through a transition in $G$.

astro-ph.CO↗

arXiv2024

Constraints on the Hubble and matter density parameters with and without modelling the CMB anisotropies

We consider constraints on the Hubble parameter $H_0$ and the matter density parameter $\Omega_{\mathrm{M}}$ from: (i) the age of the Universe based on old stars and stellar populations in the Galactic disc and halo (Cimatti & Moresco 2023); (ii) the turnover scale in the matter power spectrum, which tells us the cosmological horizon at the epoch of matter-radiation equality (Philcox et al. 2022); and (iii) the shape of the expansion history from supernovae (SNe) and baryon acoustic oscillations (BAOs) with no absolute calibration of either, a technique known as uncalibrated cosmic standards (UCS; Lin, Chen, & Mack 2021). A narrow region is consistent with all three constraints just outside their $1\sigma$ uncertainties. Although this region is defined by techniques unrelated to the physics of recombination and the sound horizon then, the standard $Planck$ fit to the CMB anisotropies falls precisely in this region. This concordance argues against early-time explanations for the anomalously high local estimate of $H_0$ (the 'Hubble tension'), which can only be reconciled with the age constraint at an implausibly low $\Omega_{\mathrm{M}}$. We suggest instead that outflow from the local KBC supervoid (Keenan, Barger, & Cowie 2013) inflates redshifts in the nearby universe and thus the apparent local $H_0$. Given the difficulties with solutions in the early universe, we argue that the most promising alternative to a local void is a modification to the expansion history at late times, perhaps due to a changing dark energy density.

astro-ph.CO↗

arXiv2023

The many tensions with dark-matter based models and implications on the nature of the Universe

(Abridged) Fundamental tensions between observations and dark-matter based cosmological models have emerged. This updated review has two purposes: to explore new tensions that have arisen in recent years, compounding the unresolved tensions from previous studies, and to use the shortcomings of the current theory to guide the development of a successful model. Tensions arise in view of the profusion of thin disk galaxies, the pronounced symmetrical structure of the Local Group of Galaxies, the common occurrence of planes of satellite systems, the El Gordo and Bullet galaxy clusters, significant matter inhomogeneities on scales much larger than 100 Mpc, and the observed rapid formation of galaxies and super-massive black holes at redshifts larger than 7. Given the nature of the tensions, the real Universe needs to be described by a model in which gravitation is effectively stronger than Einsteinian/Newtonian gravitation at accelerations below Milgrom's acceleration scale. The promising nuHDM model, anchored on Milgromian dynamics but keeping the standard expansion history with dark energy, solves many of the above tensions. However galaxy formation appears to occur too late in this model, model galaxy clusters reach too large masses, and the mass function of model galaxy clusters is too flat and thus top-heavy in comparison to the observed mass function. Classes of models that reassess inflation, dark energy and the role of the CMB should be explored.

astro-ph.CO↗

arXiv2023

Hydrodynamical structure formation in Milgromian cosmology

We present the first hydrodynamical cosmological simulations in the $\nu$HDM framework based on Milgromian dynamics (MOND) with light (11~eV) sterile neutrinos. $\nu$HDM can explain the expansion history, CMB anisotropies, and galaxy cluster dynamics similarly to standard cosmology while preserving MOND's successes on galaxy scales, making this the most conservative Milgromian framework. We generate initial conditions including sterile neutrinos using \textsc{camb} and \textsc{music} and modify the publicly available code \textsc{phantom of ramses} to run $\nu$HDM models. The simulations start at redshift $z_e=199$, when the gravitational fields are stronger than $a_{_0}$ provided this does not vary. We analyse the growth of structure and investigate the impact of resolution and box size, which is at most 600 comoving Mpc. Large density contrasts arise at late times, which may explain the KBC void and Hubble tension. We quantify the mass function of formed structures at different redshifts. We show that the sterile neutrino mass fraction in these structures is similar to the cosmic fraction at high masses (consistent with MOND dynamical analyses) but approaches zero at lower masses, as expected for galaxies. We also identify structures with a low peculiar velocity comparable to the Local Group, but these are rare. The onset of group/cluster scale structure formation at $z_e\approx4$ appears to be in tension with observations of high redshift galaxies, which we discuss in comparison to prior analytical work in a MONDian framework. The formation of a cosmic web of filaments and voids demonstrates that this is not unique to standard Einstein/Newton-based cosmology.

astro-ph.CO↗