Seminars
Our seminars take place in the lecture room of the building at
Praha–Sporilov.
Next seminar:
19.02.2018 14:00
Michal Křížek
Institute of Mathematics, Czech Academy of Sciences, Prague
Local Hubble Expansion
Abstract
Michal Křížek
Local Hubble Expansion
Life on Earth has existed continually for at least 3.5 Gyr and this requires relatively stable conditions during this very long time period. However, since the luminosity of the Sun increases, the Earth should recede from the Sun. We present several examples indicating that the Solar System expands by a speed comparable to the Hubble constant. This guarantees that the Earth received almost constant solar flux during the last 3.5 Gyr. We give three independent arguments showing that the average Earth-Sun distance increases about 5 m/yr due to the finite speed of gravitational interaction. Such a large recession speed cannot be explained by solar wind, tidal forces, plasma outbursts from the Sun, or by the decrease of the Solar mass due to nuclear reactions. Models based on Newtonian mechanics can explain only a few cm per year.
The measured average increase in the Earth-Moon distance is 3.84 cm/yr, while Newtonian mechanics is able to explain only 2.1 cm/yr. We claim that this difference is also caused by the finite speed of gravitational interaction. Mars was much closer to the Sun as well, otherwise it could not have had rivers 3.5 Gyr ago, when the Sun’s luminosity was only 75 % of its present value, see [1] for details.
References
[1] M. Křížek, L. Somer, Manifestations of dark energy in the Solar system, Grav.
Cosmol. 21 (2015), 58–71.
20.02.2018 14:00
Christopher Jacobs
NASA JPL, Caltech, USA
VLBI and its application for building reference frames for spacecraft navigation
20.02.2018 14:30
Alet de Witt
The Hartebeesthoek Radio Astronomy Observatory (HartRAO), JAR
Imaging of extragalactic radio sources at K-band with VLBI
Previous seminar:
19.12.2017 14:00
Silke Britzen
Max-Planck-Institut für Radioastronomie, Bonn, Germany
High-resolution radio interferometry: from jets to the supermassive black hole
Abstract
Silke Britzen
High-resolution radio interferometry: from jets to the supermassive black hole
Very Long Baseline Interferometry (VLBI) observations provide the highest resolution information in Astronomy. They allow us to study the jets and the immediate vicinities of the active supermassive black holes in galaxies. In my talk, I will present our most recent results on two prototypical active black holes. OJ287 is the best candidate galaxy for hosting a supermassive binary black hole at very close separation. To our knowledge, this is the first time we can explain the historic light-curve variability and long-term jet kinematics in geometrical terms - by jet precession and jet rotation. The jet might be launched from the ergosphere of the active black hole. For the nearby, giant radio galaxy M87 with a supermassive black hole of 10^9 solar masses, we find strong indication for turbulent processes connecting the accretion disk and the jet. For both active galaxies we can provide insights into the long-standing problem of the origin of astrophysical jets.
If you would like to give a seminar in our group, please contact Petra Sukova or Vladimir Karas.
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