7. Sebastian J. Szybka, Syed U. Naqvi
Freely falling bodies in a standing-wave spacetime
Phys. Rev. D: Part. Fields , vol. 103, p. 024011 (2021).
[abstract] [preprint] [journal] [pdf]

Abstract:
We study the motion of free masses subject to the influence of standing gravitational waves in the polarized Gowdy cosmology with a three-torus topology. We show that antinodes attract freely falling particles and we trace the velocity memory effect.

8. Szymon Sikora, Krzysztof Głód
Construction of the cosmological model with periodically distributed inhomogeneities with growing amplitude
The European Physical Journal C, vol. 81, p. 208 (2021).
[abstract] [preprint] [journal]

Abstract:
We construct an approximate solution to the cosmological perturbation theory around Einstein–de Sitter background up to the fourth-order perturbations. This could be done with the help of the specific symmetry condition imposed on the metric, from which follows that the model density forms an infinite, cubic lattice. To verify the convergence of the perturbative construction, we express the resulting metric as a polynomial in the perturbative parameter and calculate the exact Einstein tensor. In our model, it seems that physical quantities averaged over large scales overlap with the respective Einstein–de Sitter prediction, while local observables could differ significantly from their background counterparts. As an example, we analyze the behavior of the local measurements of the Hubble constant and compare them with the Hubble constant of the homogeneous background model. A difference between these quantities is important in the context of a current Hubble tension problem.

9. Sebastian J. Szybka
The little book about the large universe
Philosophical Problems in Science, vol. 71, pp. 203-209 (2021).
[abstract] [preprint] [journal] [pdf]

Abstract:
We live in extraordinary times for cosmologists. A vast amount of new astronomical data is pushing our model of the universe to its limits. An interest in cosmology is growing. "The Little Book of Cosmology" by Lyman Page offers a concise, up-to-date, and comprehensible introduction to the subject.

10. Krzysztof Głód
1+1+2 covariant formulation of light propagation in spacetime
Phys. Rev. D: Part. Fields , vol. 101, p. 024021 (2020).
[abstract] [preprint] [journal]

Abstract:
We present a covariant approach to the problem of light beam propagation in a spacetime. We develop our considerations within the framework of classical geometric optics in general relativity. Using the concept of a screen surface orthogonal to the observer velocity and to the bundle of geodesics, we introduce covariant four-dimensional definitions for Sachs and Jacobi optical fields and for the area distance. Then we give relationships between them and derive their propagation equations together with initial conditions for these equations. Ultimately, for practical use, we transform the resulting formulas into the redshiftdependent form.

11. Piotr T. Chruściel, Sebastian J. Szybka
A phenomenological algorithm for short-range predictions of the Covid-19 pandemic 2020
(2020).
[abstract] [preprint] [medRxiv]

Abstract:
We present an algorithm for dynamical fitting of a logistic curve to the Covid-19 epidemics data, with fit-parameters linearly evolving to the future. We show that the algorithm would have given reasonable short- and medium-range predictions for the mid-range evolution of the epidemics for several countries. We introduce the double-logistic curve, which provides a very good description of the epidemics data at any given time of the epidemics. We analyse the predictability properties of some naive models.

12. Piotr T. Chruściel, Sebastian J. Szybka
Universal properties of the dynamics of the Covid-19 pandemics
(2020).
[abstract] [preprint] [medRxiv]

Abstract:
We present evidence for existence of a universal lower bound for the initial growth rate of the epidemic curve of the SARS-CoV-2 coronavirus. This can be used to infer that, on average, an asymptomatic infected individual is infectious during 5.6 plus/minus 0.3 days. We further present evidence of an average time scale of 12 days for halving the number of new cases, or new deaths, during the extinction period of the first phase of the epidemic.