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 a pedagogical review of Metric-Affine theories of Gravity (MAG), theories for which the metric and the affine connection are independent quantities (namely in the Palatini approach) and whose actions include covariant derivatives of the matter fields, with the covariant derivative naturally defined using the independent connection. MAG straightforwardly admit the presence of direct couplings involving matter and connection. I will summarize some physical consequences of such theories.
The mass of a black hole has traditionally been identified with its energy. We describe a new perspective on black hole thermodynamics, one that identifies the mass of a black hole with chemical enthalpy, and the cosmological constant as thermodynamic pressure. This leads to an understanding of black holes from the viewpoint of chemistry, in terms of concepts such as Van der Waals fluids, reentrant phase transitions, triple points, and isolated critical point. Both charged and rotating black holes exhibit novel chemical-type phase behaviour, hitherto unseen.
Deep inside the cell of uniformity and at late stages of the evolution, the Universe is ﬁlled with inhomogeneously distributed discrete structures (galaxies, groups and clusters of galaxies). Supposing that the Universe contains also the cosmological constant and a perfect ﬂuid with a negative constant equation of state parameter ω (e.g., quintessence, phantom or frustrated network of topological defects), we investigate scalar perturbations of the FRW metrics due to inhomogeneities. Our analysis shows that, to be compatible with the theory of scalar perturbations, this perfect ﬂuid, ﬁrst, should be clustered and, second, should have the equation of state parameter ω = −1/3 (in particular, this value corresponds to the frustrated network of cosmic strings). Therefore, the frustrated network of domain walls with ω = −2/3 is ruled out. A perfect ﬂuid with ω = −1/3 neither accelerates nor decelerates the Universe. We also obtain the equation for the nonrelativistic gravitational potential created by a system of inhomogeneities. Due to the perfect ﬂuid with ω = −1/3, the physically reasonable solutions take place for ﬂat, open and closed Universes. This perfect ﬂuid is concentrated around the inhomogeneities and results in screening of the gravitational potential.
I present a general method to reconstruct spherically symmetric metrics in GR. The method is based on definition of some special scalar variables in the framework of the 1+1+2 covariant approach proposed by Ehlers and Ellis. It provides a new way to explore the properties of spherically symmetric metrics in GR. After explaining the scheme I will give some examples and show that the new technique is also applicable to modified gravity.
We review our earlier work on the existence of periodic solutions of Einstein's equations and relate it to more recent results concerning the existence of spacetimes with helical symmetry in linearized gravity and electrodynamics. In addition we present a new calculation of Bondi mass for spacetimes with electromagnetic and scalar field sources.
It is generally believed that the black holes in active galactic nuclei (AGNs) and X-ray binaries (XRBs) work in a similar way. While XRBs evolve rapidly and several sources have undergone a few complete cycles from quiescence to an outburst and back, AGNs remain in the same state due to their larger characteristic time-scale, proportional to their size. However, the study of AGN spectral states is still possible with a large sample of sources. Multiwavelength observations are needed for this purpose since the AGN thermal disc-emission dominates in the ultraviolet energy range while the up-scattered hot-corona emission is detected in X-rays. Based on ROSAT All-Sky Survey and SDSS database, Koerding et al. (2006) constructed the disc-fraction/luminosity diagrams of AGNs that revealed the analogy with the state diagrams of XRBs. I would like to present our results that are based on the comparison of strictly simultaneous UV and X-ray measurements of AGNs obtained with the XMM-Newton satellite.
The topic of active galactic nuclei (AGN) is wide, complex and multi-disciplinary. From their inner core where general and special relativity rule the accretion and emission mechanisms, to their external boundaries where classical physics and dusty mixtures prevail, explaining the intrinsic composition and morphology of AGN remains a major challenge due to the variety of their spectroscopic, timing and polarimetric signatures. Describing AGN using a unique picture is a rather challenging task, but a unified model has slowly emerged. A so far very successful theory predicts that most of the differences can be explained by an orientation effect: AGN properties differ if the system is seen from a polar, an intermediate or an equatorial inclination, the difference being attributed to circumnuclear obscuration. But does this geometrical arrangement solve everything ? In this seminar, I will review the current status of the unified model of AGN and explore, throughout spectroscopy and polarization, the successes and failures of all the existing models.
Teleparallel gravity is a gauge theory for the translation group defined in the tangent bundle of a Riemannian spacetime, where the so-called tetrad plays the role of the dynamical field of the theory and is parallely transported (absolute parallelism condition) by assuming the Weitzenböck connection instead of the Levi-Civita connection, leading to a non-null torsion but a null scalar curvature. Whereas teleparallel gravity is equivalent to general relativity, its extensions, also known as f(T) gravities in analogy to f(R) gravity, give rise to new features and interesting properties but are not equivalent to f(R) gravity. In this talk, I will review some of the last analysis about f(T) gravity, including the violation of the local Lorentz invariance, the construction of conformal invariant actions or the non-existence of extra gravitational wave modes, among other issues.
In this talk, I will introduce a method for studying the perception of Hawking radiation by different observers outside a black hole, and for different vacuum states of the radiation field. The analysis is performed in terms of an effective-temperature function that varies along the trajectory of each observer. With this tool, I will show that not all observers crossing the horizon of a black hole in free-fall will fail to detect radiation, and that indeed it is not necessary to strictly form an horizon for obtaining Hawking radiation. Also, the radiation temperature perceived by a generic observer following an arbitrary radial trajectory outside the black hole (when it is possible to talk about a temperature) can be calculated directly from the local characteristics of its trajectory, in a way which has a clear physical interpretation. Finally, our results also point to a self-consistent buoyancy scenario near black holes, due to Hawking radiation.
Jiří Bičák Oldřich Semerák