Department of Relativistic Astrophysics and Cosmology
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1. Sebastian J. Szybka, Mieszko Rutkowski
Einstein clusters as models of inhomogeneous spacetimes
Journal, vol. xxx, pp. xxx-xxx (2019).
[abstract] [preprint] [journal]

We study the effect of small-scale inhomogeneities for Einstein clusters. We construct a spherically symmetric stationary spacetime with small-scale radial inhomogeneities and propose the Gedankenexperiment. An hypothetical observer at the center constructs, using limited observational knowledge, a simplified homogeneous model of the configuration. An idealization introduces tensions and side effects. The inhomogeneous spacetime and the effective homogeneous spacetime are given by exact solutions to Einstein equations. They provide a simple toy-model for studies of the effect of small-scale inhomogeneities in general relativity.

2. Piotr T. Chru¶ciel, Sebastian J. Szybka, Paul Tod
Towards a classification of vacuum near-horizons geometries
Class. Quantum Grav. 35 (2018) 015002, vol. 35, p. 015002 (2018).
[abstract] [preprint] [journal]

We prove uniqueness of the near-horizon geometries arising from degenerate Kerr black holes within the collection of nearby vacuum near-horizon geometries.

3. Sikora S., Bratek Ł., Jałocha J., Kutschera M.
The influence of the velocity dispersion on the velocity-density relation in the disk model of the galaxy
Acta Phys. Pol., B , vol. 48, p. 2071 (2017).
[abstract] [journal]

Taking the velocity dispersion term into account in the Jeans equation describing the disk model, counterintuitively allows to reduce the local mass-to-light ratio at the outskirts of flattened galaxies, and stop the unbound growth of mass function. This is possible thanks to a more intertwined relation between the gravitational potential and the rotation curve than for spheroidal systems. This effect is illustrated on the example of galaxy UGC 6446 by finding iteratively the column mass density from the rotation curve supplemented with an isotropic velocity dispersion profile in the disk plane. Along with galactic magnetic fields, this effect would allow to reduce the local mass-to-light ratio at the galactic outskirts.

4. Szymon Sikora, Krzysztof Głód
Example of an inhomogeneous cosmological model in the context of backreaction
Phys. Rev. D: Part. Fields , vol. 95, p. 063517 (2017).
[abstract] [preprint] [journal]

In this article, we present an example of an inhomogeneous cosmological model, which is inspired by the linear perturbation theory. The metric of this model can be described as the Einstein–de Sitter background with periodically distributed dust overdensities. The model construction enables application of the Green-Wald averaging scheme and the Buchert averaging technique simultaneously. We compare the angular diameter distance function of the considered model to the angular diameter distances corresponding to the average space-times given by the Green-Wald and the Buchert frameworks respectively.

5. Editor: Sebastian J. Szybka
The 3rd Conference of the Polish Society on Relativity
Acta Physica Polonica B (2017), PL ISSN 1899-2358 [journal]


6. Jałocha J., Bratek Ł., Pękala J., Sikora S., Kutschera M.
Magnetic flux density from the relative circular motion of stars and partially ionized gas in the Galaxy mid-plane vicinity
Astrophys J. , vol. 833, p. 174 (2016).
[abstract] [preprint] [journal]

Observations suggest a slower stellar rotation relative to gas rotation in the outer part of the Milky Way Galaxy. This difference could be attributed to an interaction with the interstellar magnetic field. In a simple model, fields of order 10 micro Gauss are then required, consistently with the observed values. This coincidence suggests a tool for estimating magnetic fields in spiral galaxies. A North-South asymmetry in the rotation of gas in the Galaxy could be of magnetic origin too.

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