arXiv subjects

A. G. Nikiforov

Publications and source records attributed to A. G. Nikiforov.

3 recordsLinked to original sources

arXiv2026

A semi-analytical approach to cosmic void evolution

We present a theoretical study of the non-linear evolution of cosmic voids -- underdense regions that occupy a large fraction of the observable Universe. We model a void as an isolated homogeneous spheroidal (axisymmetric) ellipsoid embedded in a homogeneous $Λ$CDM universe. Starting from a small initial density contrast at redshift $z=500$, we numerically integrate the equations of motion for the ellipsoid semi-axes and follow their evolution to the present epoch. We examine the anisotropic expansion of the void and the corresponding change in its shape, characterised by the eccentricity $e$. We find that the void non-sphericity always decreases, but rather slowly: the eccentricity drops from $e\approx0.87$ at $z=500$ to $e\approx0.81$ at $z=0$. Thus the void becomes rounder but remains aspherical throughout the evolution. The evolution and final value of the void underdensity are virtually independent of the void's eccentricity. The nonlinearity of void evolution becomes apparent very early: a ten percent deviation from the linear regime occurs already at $z\simeq8$, when $\varepsilon = Δρ/ρ\sim10\%$. Notably, our calculations show that the majority of voids are not strongly underdense and contain a significant amount of matter, $μ>0.5$.

astro-ph.CO↗

arXiv2026

Modeling of the magnetic stellar wind braking of the ssrAp 33 Lib (HD137949)

Using the ssrAp star 33 Lib (HD137949) as an example, we show that magnetic braking caused by the interaction of a strong magnetic field with a stellar wind can play a key role in slowing the rotation of ssrAp stars. Numerical modeling of stellar rotation spin-down in the MESA package, taking into account the evolution of magnetic fields and the stellar wind, shows that stars with rotation periods of up to 80 years and longer can form. Moreover, braking by a magnetized wind makes it possible to estimate the mass-loss rate for stars of moderate mass ($1.25 M_\odot < M < 2 M_\odot$), which is difficult to do by other methods. We introduce the dimensionless parameter $Ξ$, which reflects the spin-down time and the stellar lifetime. Thus, when $Ξ\gg 1$, braking is important, whereas when $Ξ\ll 1$, it is negligible.

astro-ph.SR↗

arXiv2025

The impact of the eccentricity on the collapse of an ellipsoid into a black hole

We consider the gravitational collapse of a homogeneous pressureless ellipsoid. We have shown that the minimal size $r$ that the ellipsoid can reach during collapse depends on its initial eccentricity $e_0$ as $r\propto e_0^ν$, where $ν\approx 15/8$, and this dependence is very universal. We have estimated the parameters (in particular, the initial eccentricity) of a homogeneous pressureless ellipsoid, whereat it collapses directly into a black hole.

gr-qc↗