Summer term 2016

(Link to the Physikalisches Kolloquium at the University of Bonn.)

Classical and quantum cosmology of Born-Infeld type models

Abstract: In this master colloquium we are going to explore the cosmology of Born-Infeld type models. We will first review the classical cosmology of these models and extend the discussion with a particular focus on the singularities occurring in the models with an inverse square potential. Afterwards we shall study the quantum cosmology in which we employ the framework of quantum geometrodynamics. The quantization of these models is quite challenging since the Hamiltonian constraints contain square roots over the phase space variables. We work around this problem by using canonical transformations. By doing so we derive the Wheeler-DeWitt equations for the constant and inverse square potential and analyze their solutions. For a general potential the Wheeler-DeWitt equation can be brought into the form of a difference equation. We derive these difference equations and discuss the form of their solutions.


First-order cosmological perturbations produced by point-like masses: all scales covered

Abstract: In the framework of the concordance cosmological model the first-order scalar and vector perturbations of the homogeneous background are derived without any supplementary approximations in addition to the weak gravitational field limit. The sources of these perturbations (inhomogeneities) are presented in the discrete form of a system of separate point-like gravitating masses. The obtained expressions for the metric corrections are valid at all (sub-horizon and super-horizon) scales and converge in all points except the locations of the sources, and their average values are zero (thus, first-order backreaction effects are absent). Both the Minkowski background limit and the Newtonian cosmological approximation are reached under certain well-defined conditions. An important feature of the velocity-independent part of the scalar perturbation is revealed: up to an additive constant it represents a sum of Yukawa potentials produced by inhomogeneities with the same finite time-dependent Yukawa interaction range. The suggesting itself connection between this range and the homogeneity scale is briefly discussed along with other possible physical implications.


The Pais-Uhlenbeck Oscillator and its Application in Quantum Cosmology

Abstract: The Pais-Uhlenbeck oscillator, a simple model for theories with higher order derivatives, is discussed classically and quantum-mechanically. Then, tensorial perturbations in Einstein plus Conformal Gravity for a flat Friedmann universe are investigated. The quantization scheme of the Pais-Uhlenbeck oscillator is applied and it is shown that the evolution of perturbations may be connected to the case of conformal coupling of scalar field to a closed Friedmann universe, which is dealt with before.


The classical harmonic chain: solution via Laplace transforms and continued fractions

Abstract: The harmonic chain is a classical many-particle system which can be solved exactly for arbitrary number of particles (at least in simple cases, such as equal masses and spring constants). A nice feature of the harmonic chain is that the final result for the displacements of the individual particles can be easily understood -- therefore, this example fits well into a course of classical mechanics for undergraduates. Here we show how to calculate the displacements by solving equations of motion for the Laplace transforms of the displacements. This leads to a continued fraction representation of the Laplace transforms which can be evaluated analytically. The inverse Laplace transform finally gives the displacements which generically have the form of Bessel functions. We also comment on the similarities between this approach and the Green function method for quantum many-particle systems.


Why is the world four-dimensional? Hermann Weyl's 1955 argument and the topology of causation

Abstract: Hermann Weyl asked the question of the dimensionality of the world in the manuscript of a lecture entitled "Why is the world four-dimensional?" (1955). In this talk, I shall spell out Weyl's argument and its main features. After rejecting any theological explanation of the dimensionality of the world, Weyl evaluates the fact that the Riemannian manifold is not bound to the special dimensionality 4, and as far as this metrical structure goes, any other dimension would do just as well. However, in Weyl's view, the question of the four-dimensionality of the world will rest unanswered if we do not introduce a possible explanation by appealing to what he calls a "topology of causation". I will clarify why Weyl thought that a solution to the question of the dimensionality of the world could come from topology and which kind of implications Weyl's approach have in recent history and philosophy of physics.


Construction of Asymptotically Free Theory of Gravity Holes

Abstract: I will discuss how to construct infinite derivative theory of Gravity which can be made Ghost Free and also Singularity Free (from Cosmological and Blackhole type singularities). I will also discuss quantum aspects of the problem and how one can obtain a finite theory in the ultraviolet, and discuss similar construction around deSitter and Anti-deSitter backgrounds.


Fluctuations and Transport Phenomena in Horizon-Fluids for Black Holes

Abstract: It is well-known that the Black Hole horizon obeys the equation of motion for a viscous fluid. A statistical model of this fluid can help us obtain some insight into the thorny problems of the microscopic Black Hole degrees of freedom and Black Hole entropy. In this talk, we shall take this fluid to be of a physical origin and construct a Theory of Fluctuations for it. In particular, I shall show that the Langevin equation governing the energy transported from outside into the horizon-fluid corresponds to the Raychaudhuri equation for the null congruences on the Black Hole horizon. We shall also briefly outline a method, that uses the Kubo formula to compute the coefficient of Bulk Viscosity of the Horizon fluid. We shall show that this comes out to be negative due to the teleological nat ure of the Black Hole event horizon.Finally, I shall briefly outline how it is possible to have a statistical mechanical understanding of the negative specific heat of Black Holes in a similar manner.


Conformal (Weyl rescaling) symmetry: scales vanish without a trace!

Abstract: In this pedagogical talk, I will demonstrate on an example of a non-minimally coupled scalar field that conformal (local Weyl) rescaling symmetry implies vanishing of the scale (i.e. 3-volume, determinant of the 3-metric, or 'intrinsic time') and the trace of the extrinsic curvature, making them arbitrary. This will be done in 3+1 decomposed variables, which are further decomposed to their scale and conformally invariant part. The decomposition turns out to be quite a useful tool, since it manifestly reveals conditions for conformal invariance, and simplifies the investigation, providing a deeper insight into the structure of the theory with respect to scale and conformal degrees of freedom. The decomposed variables are called 'unimodular-conformal' variables and may be used as canonical variables for more elegant and clear Hamiltonian formulations and quantization procedures in other theories as well.


Integrated Information Theory - Is the Universe Conscious?

Abstract: Integrated Information Theory (IIT) is an information-theoretic model of consciousness advocated by neuroscientists such as Guilio Tononi and Christof Koch which aims at a quantative understanding of the conditions which are necessary for consciousness to arise. The talk introduces the concept of IIT and outlines the problems faced when IIT is applied to quantum systems.


On 1/r gravitational potential in pure Lovelock gravity

Abstract: Gravitational potential for Einstein gravity goes as 1/r(d - 3) while for Lovelock gravity it goes as 1/rα where α = (d - 2N - 1)/N. For Einstein it goes as 1/r only in d = 4 in contrast for Lovelock it does so for in all dimensions, dN = 3N + 1. This means gravitational field is the same in dimensions, 4, 7, 10, ... corresponding to N = 1, 2, 3, ... . We will discuss some implications of this interesting Lovelock property.


Particle Sparsity in the Hawking Effect

Abstract: Although known since the 1970s, the sparsity of particles emitted in black hole evaporation remains under-appreciated. In this talk, I will present ways to quantify this aspect of black hole physics under a range of different assumptions: Different black hole configurations, different particle species and different approximations to the full effect. It will become apparent that super-radiation can and should be distinguished from the Hawking flux. Parts of the calculations will be demonstrated both semi-analytically and numerically, showing that this effect is essential to the Hawking process and is there to stay. Therefore, it leads to several interesting and important physical intricacies in the realm of black hole physics, of which I will discuss some and link others to future work.


Hybrid model of a black hole

Abstract: Hybrid theories describe systems with interacting classical and quantum subsystems. In this talk, I will consider a particular hybrid theory that is formulated in terms of ensembles on configuration space. The approach is very general and it satisfies a number of consistency requirements. After a brief overview, I will discuss the coupling of a classical gravitational field to a quantum field. This leads to a formalism that differs from the standard approach of semiclassical gravity. There are two ways of looking at the theory that results: as a consistent hybrid theory in its own right, or as an approximation to a fully quantized theory, one which may provide clues that can help in the search for a full quantum theory of gravity. I will focus on the application of the hybrid theory to black holes ? in particular, the example of a CGHS black hole interacting with a quantized scalar field.


What is dynamics in quantum gravity?

Abstract: I will discuss the reduced phase space quantization of canonical relativity. I will show how the choice of the so called internal clock determines the properties of the resultant quantum theory. It turns out that by changing a clock the quantum dynamics of a given system may be almost unboundedly altered. Nevertheless, there exist important statements about the dynamics of a given quantum system that are independent of the employed clock. For illustration I will use simple cosmological models. I will conclude with a discussion of how the obtained result could be reconciled with the usual quantum mechanics based on the deterministic evolution of wavefunctions.


Symplectic boundary degrees of freedom in Poincar\'e gauge theory of gravity

Abstract: The symplectic potential lies at heart of Hamiltonian mechanics since it induces a Poisson bracket on phase space in a geometric fashion. In this talk, we will consider a gravitational system whose spatial part is bipartite by means of a closed, spacelike hypersurface S. Gravitation is described using Poincar\'e gauge theory. We derive the most general symplectic potential and show that it is not gauge-invariant. The talk is closed by a motivation as to why symplectic boundary degrees of freedom can potentially ameliorate the spoiled gauge invariance; if time permits, analogies to the Yang--Mills theory and General Relativity in the metric formulation will be presented. This talk is based on work by Donnelly and Freidel [1]. [1] W. Donnelly and L. Freidel, ``Local subsystems in gauge theory and gravity,'' arXiv:1601.04744 [hep-th]


Date Time Speaker Topic Room
March 15 12:00 Nick Kwindzinski
Classical and quantum cosmology of Born-Infeld type models Konferenzraum 1 (Neubau)
March 22 12:00 Oleg Zaslavskii (Kharkov University)
High energy particle collisions near black holes and super-Penrose process Konferenzraum 1 (Neubau)
April 12 12:00 Maxim Eingorn
First-order cosmological perturbations produced by point-like masses: all scales covered Konferenzraum 1 (Neubau)
April 19 12:00 Jonnathan Onody
The Pais-Uhlenbeck Oscillator and its Application in Quantum Cosmology Konferenzraum 1 (Neubau)
April 19 12:45 Nick Kwidzinski
The classical har monic chain: solution via Laplace transforms and continued fractions Konferenzraum 1 (Neubau)
April 26 12:00 Silvia De Bianchi (Universitat Autònoma de Barcelona)
Why is the world four-dimensional? Hermann Weyl's 1955 argument and the topology of causation Konferenzraum 1 (Neubau)
May 3 12:00 Anupam Mazumdar (Lancaster University)
Construction of Asymptotically Free Theory of Gravity Konferenzraum 1 (Neubau)
May 10 12:00 Swastik Bhattacharya (Trivandrum, India)
Fluctuations and Transport Phenomena in Horizon-Fluids for Black Holes Konferenzraum 1 (Neubau)
May 24 12:00 Branislav Nikolic
Conformal (Weyl rescaling) symmetry: scales vanish without a trace! Konferenzraum 1 (Neubau)
May 31 12:00 Sinan Zeissner and Mathias Becker (Dortmund)
Integrated Information Theory - Is the Universe Conscious? Konferenzraum 1 (Neubau)
June 7 12:00 Naresh Dadhich (Pune, India)
On 1/r gravitational potential in pure Lovelock gravity Konferenzraum 1 (Neubau)
June 21 12:00 Sebastian Schuster (Wellington, New Zealand)
Particle Sparsity in the Hawking Effect Konferenzraum 1 (Neubau)
July 5 12:00 Marcel Reginatto (Braunschweig)
Hybrid model of a black hole Konferenzraum 1 (Neubau)
July 12 12:00 Przemyslaw Malkiewicz (Paris)
What is dynamics in quantum gravity? Konferenzraum 1 (Neubau)
July 12 17:30 Ratindranath Akhoury
Unitary S matrices with long range correlations: Gauge invariance, entanglement and black holes Konferenzraum 1 (Neubau)
July 15 14:00 Jens Boos
Symplectic boundary degrees of freedom in Poincar\'e gauge theory of gravity Konferenzraum 1 (Neubau)
July 19 12:30 Claus Kiefer
Conference reports Konferenzraum 1 (Neubau)
July 26 12:00 Tim Schmitz
Bachelor Colloquium: Closed timelike curves in the Gödel universe Konferenzraum 1 (Neubau)

Past seminars

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