Seminar is held on Tuesdays at 13:10 pm in the lecture room of the Institute
on the 10th floor of the department building at Trója, V Holešovičkách 2, Prague 8
No seminars are planned for the near future.
An elementary introduction to scalar perturbations of the K-essence models. I will show how to obtain a late-time cosmic acceleration and indicate how to use the approach also for a>a0, which will be useful for modelling of the earlier stages of cosmic evolution.
I review some models that take into account dissipative effects of fluids in an expanding Universe, in the context of General Relativity. The effect of dissipation on the cosmic evolution is assessed by means of different methods, such as perturbation theory and dynamical systems analysis.
The package is availabe on the website www.xact.es. There you can find synopsis:
xAct is a suite of free packages for tensor computer algebra in Mathematica. xAct implements state-of-the-art algorithms for fast manipulations of indices and has been modelled on the current geometric approach to General Relativity. It is highly programmable and configurable. Since its first public release in March 2004, xAct has been intensively tested and has solved a number of hard problems in GR.
Below you can find notebooks prepared for the seminar:
Padmanabhan's horizon thermodynamics is an alternative proposal for seeking connections between black hole physics and thermodynamics. I will overview the original Padmanabhan's argument and point towards its weaknesses and how to amend them. Namely, I will show that by regarding the Einstein equations as equations of state, a full cohomogeneity horizon thermodynamics first law can be derived. In this approach both the entropy and the free energy are derived concepts, while the original (degenerate) horizon first law is recovered by a Legendre projection. These results readily generalize to higher curvature gravities and establish a way of how to formulate consistent black hole thermodynamics without conserved charges.
I will give an overview of the timescape cosmology. It is assumed that inhomogeneities - voids, walls and filaments - modify the average background geometry of the universe, which is no longer a simple solution of Einstein's equations with homogeneous dust. To obtain a viable phenomenology without dark energy, I provide a framework for interpreting Buchert's backreaction formalism, by revisiting fundamental issues relating to the definition of gravitational energy in a complex geometry. Cosmic acceleration is realized as an apparent effect due both to backreaction and the relative calibration of the asymptotic clocks of observers in gravitationally bound structures relative to the time parameter that best describes the average statistical evolution. The cosmic coincidence problem is solved directly in relation to the growth of the void fraction.
Predictions of the timescape phenomenology are very close to the standard cosmology, but with differences which can be tested. I will outline current observational constraints, future tests (e.g., with the Euclid satellite), and also theoretical challenges that need to be overcome for backreaction models to fully compete with the Lambda Cold Dark Matter cosmology.
Some 65 years ago (1951) Wolfgang Pauli noted that the zero-point energy density could be set to zero by a carefully fine-tuned cancellation between bosons and fermions. In this seminar I will argue in a slightly different direction: The zero-point energy density is only one component of the zero-point stress energy tensor, and it is this tensor quantity that is in many ways the more fundamental object of interest. I shall demonstrate that Lorentz invariance of the zero-point stress energy tensor implies finiteness of the zero-point stress energy tensor, and vice versa.
Jiří Bičák Oldřich Semerák