arXiv subjects

Nikolaos Samaras

Publications and source records attributed to Nikolaos Samaras.

8 recordsLinked to original sources

arXiv2026

The formation of the first cosmic structures in hybrid Milgromian cosmology

Tackling cosmology with Milgromian Dynamics (MOND) has been a challenge ever since its theoretical inception. In a hybrid MOND scheme, a hypothesized light sterile neutrino of $\approx$ 11 eV/$c^2$ rest-mass is able to provide realistic initial conditions, fitting the third peak of the Planck CMB power spectrum as a Hot Dark Matter (HDM) component. At the same time, MOND governs the gravitational dynamics of density perturbations, while the background expansion follows the standard FLRW metric. We conduct and analyze hydrodynamical simulations of the fiducial $Λ$CDM model ($H_0 = 67.8$ km/s/Mpc, $Ω_m \approx 0.31$, $Ω_{\text{CDM}} \approx 0.26$), the progenitor $ν$HDM model ($H_0 = 67.64$ km/s/Mpc, $Ω_m = 0.3, Ω_ν\approx 0.26$), and its optimized version (opt-$ν$HDM model, $H_0 \approx 55.6$ km/s/Mpc, $Ω_m \approx 0.49$, $Ω_ν \approx 0.42$), utilizing the publicly available codes RAMSES and Phantom of RAMSES. We compare for the first time the total matter power spectrum $P(k)$ of the hydrodynamical simulations at various redshifts, examining the systematic differences of the three investigated cosmologies. The redshift evolution of the power spectrum of the $Λ$CDM model compared to the $ν$HDM variants' counterpart offers important insight about the onset of structure formation. The combination of a light sterile neutrino and MOND gravity exceeds the $Λ$CDM model's matter $P(k)$ over all collapsing scales at late times ($z\lesssim 1$). The density and velocity fields reveal a distinct difference between the underlying laws of gravity. Regardless of the modeled dependence in the jointly estimated data from Planck, DES Y1, SDSS, and eBOSS, the $ν$HDM hybrid models seem to produce an excess of power at associated resolution scales due to their cosmological design.

astro-ph.CO↗

arXiv2026

Gravitational Lensing of the Bullet Cluster in the CCC+TL Cosmology: A Controlled Proof-of-Concept Study

A proof-of-concept study of gravitational lensing in the Bullet Cluster under Covarying Coupling Constants plus Tired Light (CCC+TL) cosmology. Using a JWST spectroscopic catalogue of 219 cluster members and 135 gold multiple images in 49 systems from Rihtarsic et al. (2026), we construct convergence maps for CCC+TL and a simplified $Λ$CDM model using their respective angular-diameter distances and critical surface densities. The CCC+TL model adopts a two-parameter effective-density prescription anchored to the same virial-scale mass budget as $Λ$CDM, rather than fitted to multiple-image positions. The CCC+TL and $Λ$CDM mean-convergence bands in radial rings overlap within the propagated $\pm 30 \%$ virial-mass uncertainty across all rings containing multiple images ($θ\leq80\arcsec$). On a source-plane back-projection diagnostic ($α_{\mathrm{phys}}(z_s)=α_{\mathrm{dft}}\timesβ(z_s)/β(z_{s,\mathrm{ref}})$ with $z_{s,\mathrm{ref}}=2$), the unoptimised two-parameter CCC+TL pipeline yields $\mathrm{RMS}=5.22\arcsec$ ($4.63\arcsec$--$5.91\arcsec$), compared with $6.82\arcsec$ ($5.75\arcsec$--$8.09\arcsec$) for the $Λ$CDM reference. The optimised 32-parameter Lenstool reconstruction achieves an image-plane RMS of $\sim\!0.41\arcsec$, which is not directly comparable to these source-plane values. Within the $\pm 30\%$ $M_{200}$ uncertainty band, the source-plane scatter ranges partially overlap. The CCC+TL effective extra-gravity distribution follows the galaxy distribution rather than X-ray gas, reproducing the qualitative morphology of the Bullet Cluster, though this agreement is partly structural as profiles center on member galaxies by construction. For the same mass, Einstein-radius scaling differs by only 1.7% between cosmologies. These results are a necessary first step toward a full image-plane likelihood test of CCC+TL.

physics.gen-ph↗

arXiv2026

The relevance of the cosmic microwave background for cosmology

Within the Hot Big Bang picture, as usually interpreted in the standard LCDM framework and its hierarchical scenario for elliptical galaxy assembly, the cosmic microwave background (CMB) is associated with photon decoupling near z = 1100, and the Planck angular power spectra are taken to constrain primordial 10^{-5}-level fluctuations that seeded later structure formation. We show that, in addition to the LCDM model, two cosmological models based on Milgromian gravitation are also consistent with the Planck CMB power spectra, such that the spectra do not uniquely favor dark matter. We then argue that the published CMB spectra are not direct observables, but reconstructed quantities obtained after foreground subtraction and corrections for weak lensing, photon-electron scattering, and gravitational energy redshifts along the line of sight. Because these steps depend on the assumed growth history of structure, the resulting spectra depend on the cosmological model adopted in the data reduction. We also note that the reported correlation between the hemispherical CMB asymmetry and the excess on-sky distribution of elliptical galaxies may point to matter inhomogeneities spanning large portions of the observable universe. Motivated by observational tensions in the hierarchical assembly of elliptical galaxies, we review the consequences of elliptical-galaxy formation on a downsizing timescale, which implies a significant extragalactic foreground released at 15< z < 20. Under conservative assumptions, this foreground contributes at least 1.4 percent of the observed CMB energy density and has not been included in CMB analyses. It follows that the reported 10^{-5}-level fluctuations is introduced by overly aggressive foreground cleaning.

astro-ph.CO↗

arXiv2026

Supernova Time Dilation in Hybrid Expansion-Tired-Light Cosmologies

Dark Energy Survey has shown that the emission light curve widths $Δt_{\rm em}$ of supernovae increase to $Δt_{\rm obs}$, obeying $Δt_{\rm obs}/Δt_{\rm em}=(1+z)^b$ with $b=1.003\pm0.011$, excluding a nondilating redshift. We test hybrid models with $1+z=(1+z_p)(1+z_t)$ and the phenomenological stretch $(1+z_p)R(z)$, where $z_p$ is the redshift associated with the expanding parent universe cosmology, $z_t$ is a tired-light (TL) contribution, and $R$ is the hybrid-to-parent lookback-time ratio. At $z=1$, $R$ and $1+z_t$ differ by $1.3\%$ for CCC+TL and $0.5\%$ for $Λ$CDM+TL, and their 95\% bands overlap the observed relation. This compatibility shows that the observations do not exclude the hybrid models, only fully nonexpanding ones.

physics.gen-ph↗

arXiv2026

Testing Covarying Coupling Constants (CCC) against the full SPARC rotation-curve sample: a like-for-like comparison with MOND and NFW

The Covarying Coupling Constants (CCC) framework, developed to account for high-redshift JWST observations, contains a mechanism -- a covarying-constant effective mass field keyed to local density -- that modifies galactic dynamics without particle dark matter. We test it against the full Spitzer Photometry and Accurate Rotation Curves (SPARC) sample of 175 disc galaxies, extending an earlier study of a few objects. Working in an inverse formulation, in which each model predicts the baryonic rotation curve from the observed one, we compare CCC against Modified Newtonian Dynamics (MOND) and one- and two-parameter Navarro-Frenk-White (NFW) haloes on identical footing, using the reduced $χ^2_ν$. We show that the published sharp density turn-off in the earlier study is unphysical and replace it with a smooth transition -- the density-space analogue of the MOND interpolating function, introducing no new parameter. One-parameter smooth-CCC then performs comparably to galaxy-by-galaxy fitted MOND (the lower $χ^2_ν$ in 56 per cent of galaxies, mean $χ^2_ν$ of 2.58 versus 2.65; the paired difference is not significant), while two-parameter NFW shows a substantially broader fit-quality distribution and a larger tail of poor or boundary-limited fits (mean $χ^2_ν\approx 7$). The CCC turn-off density is not universal (scatter 0.82 dex) and correlates with galaxy size, qualitatively consistent with a spherical reconstruction applied to flattened disc systems. Recast as an acceleration, however, $a_t = V_{\rm flat}^2/R_t$ has scatter 0.33 dex (on the 91-galaxy resolved subset) -- matching the MOND scale $a_0$ (0.34 dex) -- and comparable magnitude of order $2 \times 10^{-10}$ m/s$^2$, with its size correlation removed. Though not designed for galactic dynamics, CCC describes rotation curves as well as galaxy-by-galaxy fitted MOND.

astro-ph.GA↗

arXiv2026

Big Bang Nucleosynthesis Constraints on the CCC+TL Cosmology

We investigate whether Big Bang nucleosynthesis (BBN) remains compatible with the Covarying Coupling Constants plus Tired Light (CCC+TL) cosmology. In this framework, only quantities with explicit length dimensionality covary through a universal scaling function $f \left( z \right)$, while dimensionless constants and dimensionless ratios remain invariant. At the redshifts $z$ relevant to BBN, $f \left( z \right )$ approaches a constant plateau $f_{\text{max}} \left( z \right) \simeq 3$, and the tired-light contribution is negligible, so the early-time dynamics reduce to a global rescaling of dimensioned quantities. In particular, the Hubble expansion rate $H$ at fixed temperature $T$ satisfies $H_{\text{CTL}} \left( T \right) = f^{-1}_{\text{max}} H_{Λ\text{CDM}}\left( T\right)$, implying a longer cooling time $Δt$ between weak freeze-out and the onset of nucleosynthesis by the same factor (CCC+TL labeled as $\textit{CTL}$). We find that BBN predictions are preserved provided the relevant interaction rates $Γ$ and decay rates governing the neutron lifetime $τ_n$ share the same plateau scaling as $H$, so that governing combinations such as $Γ\text{/}H$ and $\text{exp} \left( -Δt \text{/} τ_n \right)$ remain invariant. Implementing these plateau rescalings in the Kawano/NUC123 network (via a single control parameter $\texttt{fctl} \equiv f_{\text{max}}$) yields identical light-element abundances for $\texttt{fctl}= 1$ ($Λ$CDM) and $\texttt{fctl} = 3\left( \text{CCC+TL} \right)$ to within $10^{-3} - 10^{-4}$ level, consistent with numerical rounding. We also illustrate that adopting the lower late-time CCC+TL baryon density from the Pantheon+ data fit can reduce the ${}^7\text{Li}$ discrepancy but simultaneously increases D/H, implying that BBN alone does not select between the late-time baryon-density inferences considered here.

astro-ph.CO↗

arXiv2025

Origin of Galaxies

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.

astro-ph.CO↗

arXiv2025

Enhanced Outsourced and Secure Inference for Tall Sparse Decision Trees

A decision tree is an easy-to-understand tool that has been widely used for classification tasks. On the one hand, due to privacy concerns, there has been an urgent need to create privacy-preserving classifiers that conceal the user's input from the classifier. On the other hand, with the rise of cloud computing, data owners are keen to reduce risk by outsourcing their model, but want security guarantees that third parties cannot steal their decision tree model. To address these issues, Joye and Salehi introduced a theoretical protocol that efficiently evaluates decision trees while maintaining privacy by leveraging their comparison protocol that is resistant to timing attacks. However, their approach was not only inefficient but also prone to side-channel attacks. Therefore, in this paper, we propose a new decision tree inference protocol in which the model is shared and evaluated among multiple entities. We partition our decision tree model by each level to be stored in a new entity we refer to as a "level-site." Utilizing this approach, we were able to gain improved average run time for classifier evaluation for a non-complete tree, while also having strong mitigations against side-channel attacks.

cs.CR↗