Seminars

Winter term 2016/17


 

Particle creation in an expanding universe with a Big-Brake singularity

Abstract: As Leonard Parker has shown in his Ph.D. thesis, particles are created in an expanding universe if one applies Quantum Field Theory in Curved Spacetimes. In my Bachelor thesis, I investigated this mechanism for flat FLRW universes with a certain kind of future singularity which occur in the Big-Brake model. They are caused by a vanishing energy density at a finite time. Therefore it might be questioned if Quantum Field Theory in Curved Spacetimes offers a possibility to avoid this singularity by changing the energy density in the universe due to the created particles. After a short introduction of the mentioned mechanism, I will talk about the results of my numerical computations and there results on the evolution of the particle number while approaching the singularity.

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Discretization of Maxwell's Equations for Non-inertial Observers Using Space-Time Algebra

Abstract: We employ Maxwell's equations formulated in Space-Time Algebra to perform discretization of moving geometries directly in space-time. All the derivations are carried out without any non-relativistic assumptions, thus the application area of the scheme is not restricted to low velocities. The 4D mesh construction is based on a 3D mesh stemming from a conventional 3D mesh generator. The movement of the system is encoded in the 4D mesh geometry, enabling an easy extension of well-known 3D approaches to the space-time setting. As a research example, we study a manifestation of Sagnac's effect in a rotating ring resonator. In case of constant rotation, the space-time approach enhances the efficiency of the scheme, as the material matrices are constant for every time step, without abandoning the relativistic framework.

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Hints of quantum gravity from the horizon fluid

Abstract: For many years researchers have tried to glean hints about quantum gravity from black hole thermodynamics. However, black hole thermodynamics suffers from the problem of Universality --- at leading order, several approaches with different microscopic degrees of freedom lead to Bekenstein-Hawking entropy. We attempt to bypass this issue by using a minimal statistical mechanical model for the horizon fluid based on Damour-Navier-Stokes (DNS) equation. For asymptotically flat black hole spacetimes in General Relativity, we show explicitly that at equilibrium the entropy of the horizon fluid is the Bekenstein-Hawking entropy. Further we show that, for the bulk viscosity of the fluctuations of the horizon fluid to be identical to Damour, a confinement scale exists for these fluctuations, implying quantization of the horizon area. The implications and possible mechanisms from the fluid point of view are discussed.

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Gravitational waves and the interior structure of hypermassive neutron stars from binary mergers

Abstract: One hundred years after Albert Einstein developed the field equations of general relativity and predicted the existence of gravitational waves (GWs), these curious spacetime-ripples have been observed from a pair of merging black holes by the LIGO detectors. As GWs emitted from merging neutron star binaries are on the verge of their first detection, it is important to understand the main characteristics of the underlying merging system in order to predict the expected GW signal. Based on a large number of numerical-relativity simulations of merging neutron star binaries, the emitted gravitational wave and the interior structure of the generated hypermassive neutron stars (HMNS) have been analyzed in detail. This talk will focus on the internal HMNS properties (e.g. differential rotation profiles, structure of the space-time metric, particle composition) and their connection with the emitted GW signal.

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Measurement of the gravitational field in General Relativity

Abstract: In General Relativity, the comparison of test bodies moving along adjacent world lines is of direct operational significance. The observation of a suitably prepared set of test bodies allows for the determination of the components of the curvature. We present some recent results on generalized deviation equations for test bodies and the measurement of the gravitational field by means of these equations.

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Measurement of the gravitational field in General Relativity

Abstract: I will discuss how Casimir-Polder forces between two relativistic uniformly accelerated atoms exhibit a transition from the short distance thermal-like behavior predicted by the Unruh effect to a long distance nonthermal behavior. This phenomenology extends the Unruh thermal response detected by a single accelerated observer to an accelerated spatially extended system of two particles, and we identify the characteristic length scale for this crossover with the inverse of the proper acceleration of the two atoms. Furthermore, at the end of the talkI will discuss some preliminary results on analogue effects for the Casimir force of two atoms close to a black hole.

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The time warp: Changing the progression of time by using Alcubierre drive-like spacetime structures

Abstract: Since the beginning of space exploration, humanity has encountered a critical problem: the vast extend of space and the immense distances between objects seem to make probing a very time consuming activity. This fact is due to the limitation discovered by Einstein that no particle with mass can travel with the speed of light and nothing can exceed this speed. Therefore a journey to our closest neighbour galaxy, the Andromeda Galaxy, would take about 2.5 million years, even if we could travel at the speed of light. This has led to many searches for a way to exceed this speed limitation, proven by general relativity, without breaking the theory. Inspired by the science fiction way to solve this problem, Miguel Alcubierre developed a spacetime which acts like a "warp drive" and provides a theoretical solution to travel at superluminal velocities. In the first part of this work, we will give an introduction of the Alcubierre metric, followed by a short summary of discoveries which have been made while studying it (we don’t take quantum physics into account) and point out the problems that arise in the theory. This is followed by the analysis of the Alcubierre metric in Cartan calculus, which will give us its Einstein equations. In the second part we will use central structures of the Alcubierre metric to derive a "time warp". This being an region of space in which we can modify the progression of time compared to its outside. In the third part we will try to combine the aspects of both metricies i.e. a moving time warp structure which uses Alcubierres drive to move through space at arbitrary speeds.

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Possible doomsdays - what are observations and theory telling us at this regard?

Abstract: In this talk, we will present a brief review on dark energy singularities and abrupt events. We will start with a classical description of several models, including their behavior at the perturbative level. Then, we will contrast our results with observational data. On the second part of the talk, we will focus on the quantum analysis of these singularities/abrupt events. The analysis will encompass mainly modified theories of gravity. If time allows, we will as well show how phantom dark energy models might not at all imply dark energy singularities in Einstein-Cartan theory. Some references (on which the talk is based): arXiv:1611.00392, arXiv:1608.01679, arXiv:1611.03100, arXiv:1604.08365, arXiv:1609.00700

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Horndeski accelerating solutions as a result of an adjustment mechanism

Abstract: In the context of Horndeski cosmologies, we consider a dynamical adjustment mechanism able to screen any value of the vacuum energy of the matter fields leading to a fixed de Sitter geometry. Thus, we present the most general scalar-tensor cosmological models without higher than second order derivatives in the field equation that have a fixed spatially flat de Sitter critical point for any kind of material content or vacuum energy. These models allow us to understand the current accelerated expansion of the universe as the result of the evolution towards the critical point when it is an attractor.

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Classical and Quantum Cosmology of Kantowski-Sachs Models

Abstract: It is well known that by crossing the event horizon of an eternal Schwarzschild black hole the timelike Killing vector field tips over and becomes spacelike. Consequently staticity is traded for homogeneity. The interior Schwarzschild solution can therefore be regarded as belonging to a certain class of homogeneous but anisotropic cosmological models. These are the so called Kantowski-Sachs models which will be at the center of this talk. Special attention shall be given to the singularities occurring in these models and their resolution after quantization, which will be performed within the framework of quantum geometrodynamics.

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What can we learn from treating first order Lagrangians as higher derivative theories: The case of Einstein-Hilbert and non-minimally coupled scalar field theories.

Abstract: If a Lagrangian possesses a symmetry, then in its Hamiltonian formulation one ends up with first class constraints which reflect this symmetry. If this symmetry is not present, the corresponding constraints still present, but are now 'spoiled' and become second class. On an example of conformally coupled scalar field it is shown in the talk that in the standard Hamiltonian formulation of the theory there is no any first class constraints related to the conformal symmetry of this Lagrangian, even though the theory is conformally invariant. How can that be? Furthermore, standard Hamiltonian formulation of the Einstein-Hilbert theory (General Relativity) gives only 4 first class constriants reflecting the reparametrization invariance symmetry, but no second class constraints are found, in spite of the fact that conformal symmetry of GR is broken. It seems as if standard Hamiltonian formulation of GR and the scalar field does not give information about the conformal properties of these theories. Where is that information? The answer lies in treating the extrinsic curvature as an independent variable, and thus treating the mentoned theories as if they were higher derivative theories, ignoring their first order nature. The result is that the Hamiltonian theory now looks like it has additional degrees of freedom, but the missing information about conformal properties naturally reveals itself precisely in the additional first or second class constraints, and in this seminar we will try to understand why.

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Hawking radiation: Comparison of pure-state and thermal description

Abstract: We present the power spectral densities for a quantum field in the background of a Schwarzschild black hole and compare the cases of the field being in a pure state and in the usual thermal state. In the low-energy regime the densities strongly differ, while at high energies they practically coincide. We define a distance measure between the resulting two density operators and evaluate it between the pure and thermal states. We find that the distance is exponentially small with respect to the Hawking temperature. We finally discuss the operational meaning of the vanishing distance.

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Measuring Black Hole radiation in the lab

Abstract: Hawking's prediction that black holes should radiate was one of the biggest surprizes in physics of the late 20th century. It has dominated much of the study in the interface between gravity and Quantum Mechanics ever since. The calculation Hawking presented is problematic, and thus experimental confirmation would be good. Unfortunately, finding small black holes is hard. However, there turn out to be analog systems, systems with horizons which one can and is studying in the lab. I will present the arguments and the current experimental effort to see the radiation, and the arguments that some experiments have already seen it.

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Consequences of a Black Hole Firewall: Dynamics of a Dust Shell

Abstract:

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On Some Astounding Consequences of Klein-Noether Identities

Abstract: It is well-known that local symmetries of an action lead by Noether’s second theorem to identically fulfilled relations involving the Euler derivatives of the Lagrangian. Less well-known is the fact that these identities are a consequence of a larger set of identities discovered by Felix Klein in 1918. These Klein-Noether identities do have astounding consequences. (1) Assuming a specific local symmetry for the fields involved, this largely fixes the action of a theory. (2) The Klein-Noether identities - if expressed through phase-space variables - directly entail primary and secondary constraints. At this stage no Hamiltonian is needed. (3) A ”Noether charge” can be defined which turns out to be a specific linear combination of the first-class constraints. On the other hand, the first-class constraints can be read off from the charge - without referring to a Hamiltonian. (4) For Legendre projectable symmetry transformations this Noether charge is the generator of the transformations. After defining the Klein-Noether identities and discussing its generic consequences, the considerations are particularized for examples of Yang-Mills theories and generally covariant theories. The history-of-science story of the Klein-Noether identities and its relation to the field of constrained dynamics is presented as story of early discovery and later rediscovery.

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Hawking Radiation: Comparison of Pure-state and Thermal Description

Abstract: The cases of the Hawking radiation field being described by a pure state and by the usual thermal state are compared within the CGHS dilaton gravity model. The field-strength fluctuations of the Fourier modes are computed, showing a discrepancy in the low-energy regime, while coinciding at high energies. Then by defining a distance for density operators and evaluating it for the cases concerned, the difference between the pure and thermal descriptions is quantified and found to be exponentially small with respect to the Hawking temperature. Possible physical interpretations are discussed.

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Quantum properties of theories with anisotropic scaling

Abstract: We discuss the quantum properties of theories with an anisotropic scaling of space and time. Such theories break fundamental Lorentz invariance. New techniques have to be applied to analyze general renormalization properties and to perform explicit calculations – among them a non-local gauge fixing procedure. We present a method how calculations of quantum divergences in a non-covariant theory can be related to the well known heat kernel technique for covariant theories and discuss the application of this technique to the model of Hoˇrava-Lifshitz gravity.

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Date Time Speaker Topic Room
October 18 12:00 various speakers
Conference reports Konferenzraum 1 (Neubau)
October 18 Matthias Dahlmanns (Duisburg)
fo Particle creation in an expanding universe with a Big-Brake singularity Konferenzraum 1 (Neubau)
October 25 12:00 Mariusz Klimek (Darmstadt)
Discretization of Maxwell's Equations for Non-inertial Observers Using Space-Time Algebra Konferenzraum 1 (Neubau)
November 3 14:00 Bethan Cropp (IISER, Trivandrum)
Hints of quantum gravity from the horizon fluid Konferenzraum 1 (Neubau)
November 8 12:00 Matthias Hanauske (Frankfurt am Main)
Gravitational waves and the interior structure of hypermassive neutron stars from binary mergers Konferenzraum 1 (Neubau)
November 22 12:00 Dirk Puetzfeld (Bremen)
Measurement of the gravitational field in General Relativity Konferenzraum 1 (Neubau)
November 29 12:00 Jamir Marino (Cologne)
Thermal and non-thermal signatures of the Unruh effect in Casimir-Polder forces Konferenzraum 1 (Neubau)
December 6 12:00 Alexander Hermanns
The time warp: Changing the progression of time by using Alcubierre drive-like spacetime structures Konferenzraum 1 (Neubau)
December 16 10:00 Mariam Bouhmadi-Lopez (Bilbao/Covilha)
Possible doomsdays - what are observations and theory telling us at this regard? 0.03 (Neubau)
December 16 10:00 Prado Martin-Moruno (Madrid)
Horndeski accelerating solutions as a result of an adjustment mechanism 0.03 (Neubau)
December 16 10:00 Nick Kwidzinski
Classical and Quantum Cosmology of Kantowski-Sachs Models 0.03 (Neubau)
December 20 12:00 Branislav Nikolic
What can we learn from treating first order Lagrangians as higher derivative theories: The case of Einstein-Hilbert and non-minimally coupled scalar field theories. Konferenzraum 1 (Neubau)
January 17 12:00 YiFan Wang
Hawking radiation: Comparison of pure-state and thermal description Konferenzraum 1 (Neubau)
January 20 16:30 William G. Unruh (University of British Columbia, Vancouver)
Measuring Black Hole radiation in the lab TP seminar room 0.03 (Neubau)
January 23 10:15 Anton Krieger
Consequences of a Black Hole Firewall: Dynamics of a Dust Shell Bonn, Wegelerstr. 10, Konferenzraum 1 (W 0.027)
January 24 12:00 Kurt Sundermeyer (Berlin)
On Some Astounding Consequences of Klein-Noether Identities Konferenzraum 1 (Neubau)
February 1 11:00 YiFan Wang
Hawking Radiation: Comparison of Pure-state and Thermal Description Bonn, Wegelerstr. 10, Konferenzraum 1 (W 0.027)
February 9 14:00 Christian Steinwachs (Freiburg)
Quantum properties of theories with anisotropic scaling Konferenzraum 1 (Neubau)

Past seminars


Winter term 2014/15
Summer term 2014
Winter term 2013/14
Summer term 2013
Winter term 2012/13
Summer term 2012
Winter term 2011/12
Summer term 2011
Winter term 2010/11
Summer term 2010
Winter term 2009/10
Summer term 2009
Winter term 2008/09
Summer term 2008
Winter term 2007/08
Summer term 2007
Winter term 2006/07
Summer term 2006
Summer term 2005
Winter term 2004/05
Summer term 2004
Winter term 2003/04
Summer term 2003