Department of Relativistic Astrophysics and Cosmology
 
 
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1. Krzysztof Głód, Szymon Sikora, Sebastian J. Szybka
An example of cross-polarized standing gravitational waves
(2021).
[abstract] [preprint] [download]

Abstract:
We use a cosmological counterpart of the cylindrical Halilsoy solution to illustrate properties of cross-polarized standing gravitational waves.

2. Piotr T. Chru¶ciel, Sebastian J. Szybka
On the lag between deaths and infections in the first phase of the Covid-19 pandemic
(2021).
[abstract] [preprint] [medRxiv]

Abstract:
One of the key issues in fighting the current pandemic, or the ones to come, is to obtain objective quantitative indicators of the effectiveness of the measures taken to contain the epidemic. The aim of this work is to point out that the lag between the daily number of infections and casualties provides one such indicator. For this we determined the lag during the first phase of the Covid-19 pandemic for a series of countries using the data available at the server of the John Hopkins University using three different methods. Somewhat surprisingly, we find a lag varying substantially between countries, taking negative values (thus the maximum daily number of casualties preceding the maximum daily number of new infections) in countries where no steps to contain the epidemic have been taken at the outset, with an average lag of $7\pm 0.3$ days. Our results can be useful to health authorities in a search for the best strategy to fight the epidemic.

3. Sebastian J. Szybka
Black Hole Flyby
Am. J. Phys., vol. 89, pp. 783-788 (2021).
[abstract] [preprint] [journal] [pdf]

Abstract:
We calculate the minimum distance at which one may approach a black hole in a free flyby. It corresponds to r=4m for the Schwarzschild black hole and a probe which was non-relativistic at infinity. The problem is formulated in a way that is useful for teaching introductory general relativity.

4. 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.

5. Piotr T. Chru¶ciel, 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.

6. Piotr T. Chru¶ciel, 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.

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