Seminář se koná v úterý ve 13:10 v posluchárně ÚTF MFF UK
v 10. patře katedrové budovy v Tróji, V Holešovičkách 2, Praha 8
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.
Loop quantum cosmology is a theory for the quantization of cosmological spacetimes that follows the ideas and techniques of loop quantum gravity. The detailed study of its quantum evolution for simple cosmological spacetimes has been shown that the classical singularity of the Big Bang is cured and replaced by a quantum bounce. In the recent years a lot of effort has been put in the study of cosmological perturbations in the context of loop quantum cosmology, mixing its techniques of quantization with more standard quantum field theory ones for the perturbations. In this talk I will focus on presenting numerical results for the primordial power spectra for cosmological perturbations using the effective equations of motion coming from loop quantum cosmology and different elections of vacuum for the perturbations.
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.
Investigation of Dark Energy remains one of the most compelling tasks for modern cosmology. It can be studied with several probes which are accessible through precise and deep surveys of the Universe. In the talk I will review the next generation of experiments to probe the Dark Energy with an emphasis on fully depleted silicon sensors with extended infrared sensitivity. Large Synoptic Survey Telescope which will start to take data in 2021 remains the front-runner in the sensitivity. It will precisely measure the positions and shapes of four billion galaxies along with estimates of their distances, 100,000 massive galaxy clusters, and 250,000 Type Ia Supernovae, providing an order-of-magnitude improvement relative to current experiments.
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.
Jiří Bičák Oldřich Semerák