Seminars
Winter term 2012/13
Date  Time  Speaker  Topic  Room 

October 9  12:00  Nassim Tanha
(Köln) 
Thermodynamics of Black Holes  R 215 
October 16  12:00  Juliane Behrend
(Utrecht) 
Einstein–Cartan Theory as an Averaged Theory of Gravity  R 215 
October 23  12:00  Gerhard Leßner
(Paderborn) 
Oscillating Universe  R 215 
October 30  12:00  Houri Ziaeepour
(MPE, München) 
Vacuum and dark energy  R 215 
November 6  12:00 

Tagungsberichte  R 215 
November 13  12:00  Mikel Fernández Méndez
(IEMCSIC, Madrid) 
Quantum cosmological perturbations in a Loop Quantum Cosmology background  R 215 
November 27  12:00  Silvia De Bianchi
(UCL, London) 
Kant’s cosmology and theory of matter: an answer to the 18th century astronomical riddles?  R 215 
December 4  12:00  Benjamin Eltzner
(Leipzig) 
Local Thermal Equilibrium in Quantum Field Theory on Curved Spacetimes  R 215 
December 18  12:00  Manuel Rabold
(Karlsruhe) 
Probing the Sensitivities of future Weak Lensing Surveys on cosmological Parameters  R 215 
January 8  12:00  Francesco Pessina
(Naples) 
An enhanced CMB power spectrum at large scale?  R 215 
January 29  12:00  Sebastian Schuster
(Köln) 
Strong coupling limit and LTB model in quantum gravity  R 215 
March 13  14:00  Alexander Voß
(Köln) 
Bachelorkolloquium  R 215 
Past seminars
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
Juliane Behrend (Utrecht)
Einstein–Cartan Theory as an Averaged Theory of Gravity
In this talk I will show that the construction of a macroscopic theory of gravity for a corpuscular medium along the lines of classical electromagnetism naturally leads to an EinsteinCartan theory. This theory is assumed to describe the universe on large scales.
Close
Einstein–Cartan Theory as an Averaged Theory of Gravity
In this talk I will show that the construction of a macroscopic theory of gravity for a corpuscular medium along the lines of classical electromagnetism naturally leads to an EinsteinCartan theory. This theory is assumed to describe the universe on large scales.
Close
Gerhard Leßner (Paderborn)
Oscillating Universe
An alternative model of the universe is presented. It comes into being from a geometrical phase transition of the Minkowski space and starts after the phase transition as antideSitter space. Matter and radiation are created in the contracting space from the gravitational energy over an extremely long period of about 38 billion years. About 46 billion years after the phase transition the universe runs through a nonsingular minimum and arrives about 44 billion years after the minimum at its maximal extension.
Close
Oscillating Universe
An alternative model of the universe is presented. It comes into being from a geometrical phase transition of the Minkowski space and starts after the phase transition as antideSitter space. Matter and radiation are created in the contracting space from the gravitational energy over an extremely long period of about 38 billion years. About 46 billion years after the phase transition the universe runs through a nonsingular minimum and arrives about 44 billion years after the minimum at its maximal extension.
Close
Houri Ziaeepour (MPE, München)
Vacuum and dark energy
Einstein’s Cosmological Constant is considered to be the energy density of vacuum. But what is the vacuum? In this talk I will first discuss the conflict between apparently a large vacuum energy density predicted by quantum field theory and the observed dark energy. I advocate a null vacuum energy and propose a new definition for vacuum state in quantum field theory as a frameindependent coherent state. This state is motivated by results from a detailed study of condensation of scalar fields in FLRW background. I briefly review this work and compare it to other quantum approaches to dark energy. Finally, I describe how we can distinguish between scalar field and alternative models such as modified gravity.
Close
Vacuum and dark energy
Einstein’s Cosmological Constant is considered to be the energy density of vacuum. But what is the vacuum? In this talk I will first discuss the conflict between apparently a large vacuum energy density predicted by quantum field theory and the observed dark energy. I advocate a null vacuum energy and propose a new definition for vacuum state in quantum field theory as a frameindependent coherent state. This state is motivated by results from a detailed study of condensation of scalar fields in FLRW background. I briefly review this work and compare it to other quantum approaches to dark energy. Finally, I describe how we can distinguish between scalar field and alternative models such as modified gravity.
Close
Mikel Fernández Méndez (IEMCSIC, Madrid)
Quantum cosmological perturbations in a Loop Quantum Cosmology background
We introduce scalar perturbations in a homogeneous and isotropic universe with compact spatial sections filled with a massive scalar field. The gauge freedom is then almost entirely removed by imposing two different sets of gaugefixing conditions, which lead to consistent results. We quantize completely the model by combining a polymer representation for the homogeneous degrees of freedom and a Fock representation for the perturbations. Criteria of symmetry and unitary dynamics determine the latter uniquely. We also present an equivalent quantization in terms of gaugeinvariant variables. Moreover, we propose a quantum implementation of the Hamiltonian constraint whose solutions are totally characterized by their initial value on the minimumvolume section.
Close
Quantum cosmological perturbations in a Loop Quantum Cosmology background
We introduce scalar perturbations in a homogeneous and isotropic universe with compact spatial sections filled with a massive scalar field. The gauge freedom is then almost entirely removed by imposing two different sets of gaugefixing conditions, which lead to consistent results. We quantize completely the model by combining a polymer representation for the homogeneous degrees of freedom and a Fock representation for the perturbations. Criteria of symmetry and unitary dynamics determine the latter uniquely. We also present an equivalent quantization in terms of gaugeinvariant variables. Moreover, we propose a quantum implementation of the Hamiltonian constraint whose solutions are totally characterized by their initial value on the minimumvolume section.
Close
Silvia De Bianchi (UCL, London)
Kant’s cosmology and theory of matter: an answer to the 18th century astronomical riddles?
By taking into account Kant’s scientific background in the preCritical writings, it is possible to show the impact of traditional paradoxes and dilemmas that grounded his cosmology and cosmogony. In the first part of my contribution, I shall expound the relevance of the Kepler and Halley’s riddle, better known as the starrydarksky paradox, which has been later discussed and formalized by Olbers in the 19th century. In the second part, I shall clarify which elements of Kant’s cosmology can be read as an answer to this riddle, involving a specific hierarchical structure of the universe, which is represented as an expanding sphere. In order to highlight this conception, I shall refer to the influence exerted by Sir William Herschel on Kant’s Critical writings on natural science and theory of matter, as well as to the development of Kant’s transcendental philosophy that provided useful tools in order to clarify and solve paradoxes and dilemmas of natural science.
Close
Kant’s cosmology and theory of matter: an answer to the 18th century astronomical riddles?
By taking into account Kant’s scientific background in the preCritical writings, it is possible to show the impact of traditional paradoxes and dilemmas that grounded his cosmology and cosmogony. In the first part of my contribution, I shall expound the relevance of the Kepler and Halley’s riddle, better known as the starrydarksky paradox, which has been later discussed and formalized by Olbers in the 19th century. In the second part, I shall clarify which elements of Kant’s cosmology can be read as an answer to this riddle, involving a specific hierarchical structure of the universe, which is represented as an expanding sphere. In order to highlight this conception, I shall refer to the influence exerted by Sir William Herschel on Kant’s Critical writings on natural science and theory of matter, as well as to the development of Kant’s transcendental philosophy that provided useful tools in order to clarify and solve paradoxes and dilemmas of natural science.
Close
Benjamin Eltzner (Leipzig)
Local Thermal Equilibrium in Quantum Field Theory on Curved Spacetimes
States that describe thermodynamic equilibrium in the thermodynamic limit of infinite volume cannot be described by density matrices in vacuum Fock space because of their everywhere finite energy density. In the framework of algebraic quantum field theory on Minkowski spacetime, such states can be readily described as expectation functionals with certain analyticity properties. In curved spacetimes, a globally defined temperature cannot generally be expected to exist, if the metric is not static. Therefore the description of thermal states is more difficult in this case. This talk explores the proposed concept of local thermal equilibrium states and discusses problems that arise even for the free scalar field in simple spacetime models.
Close
Local Thermal Equilibrium in Quantum Field Theory on Curved Spacetimes
States that describe thermodynamic equilibrium in the thermodynamic limit of infinite volume cannot be described by density matrices in vacuum Fock space because of their everywhere finite energy density. In the framework of algebraic quantum field theory on Minkowski spacetime, such states can be readily described as expectation functionals with certain analyticity properties. In curved spacetimes, a globally defined temperature cannot generally be expected to exist, if the metric is not static. Therefore the description of thermal states is more difficult in this case. This talk explores the proposed concept of local thermal equilibrium states and discusses problems that arise even for the free scalar field in simple spacetime models.
Close
Manuel Rabold (Karlsruhe)
Probing the Sensitivities of future Weak Lensing Surveys on cosmological Parameters
Weak gravitational lensing will be one of the key methods in the future of cosmology, for the determination of cosmological parameters. Its way of extracting statistical information from the formation of cosmic structures, poses a hybrid between the methods applied to the CMB and the ones applied to the matter density contrast, in the mathematical description. Since the deflection of light by mass through its gravity, does not distinguish between baryonic and dark matter, an increased sensitivity towards the latter one is expected, compared to the traditional sources of information, on a first point of view. Moreover through the application of tomographic binning in redshift space, it is also expected that weak lensing will offer profound insights into the late time evolution of cosmic structures, therefore allowing for a closer determination of parameters characterizing dark energy and neutrinos. At the moment several weak lensing surveys are planned. Among them the Large Synoptic Survey Telescope and the Euclid space probe. It is therefore important to a priori evaluate their sensitivities on the cosmological parameters of interest. A very nice way of doing this is through the Fisher matrix method.
Close
Probing the Sensitivities of future Weak Lensing Surveys on cosmological Parameters
Weak gravitational lensing will be one of the key methods in the future of cosmology, for the determination of cosmological parameters. Its way of extracting statistical information from the formation of cosmic structures, poses a hybrid between the methods applied to the CMB and the ones applied to the matter density contrast, in the mathematical description. Since the deflection of light by mass through its gravity, does not distinguish between baryonic and dark matter, an increased sensitivity towards the latter one is expected, compared to the traditional sources of information, on a first point of view. Moreover through the application of tomographic binning in redshift space, it is also expected that weak lensing will offer profound insights into the late time evolution of cosmic structures, therefore allowing for a closer determination of parameters characterizing dark energy and neutrinos. At the moment several weak lensing surveys are planned. Among them the Large Synoptic Survey Telescope and the Euclid space probe. It is therefore important to a priori evaluate their sensitivities on the cosmological parameters of interest. A very nice way of doing this is through the Fisher matrix method.
Close
Francesco Pessina (Naples)
An enhanced CMB power spectrum at large scale?
Recent work in the literature has investigated whether small quantum gravitational effects can be observed in the anisotropy spectrum of the cosmic microwave background radiation. For this purpose, a massive scalar field in a spatially flat Friedmann–Lemaitre–Robertson–Walker universe has been considered, and its fluctuations have been decomposed into Fourier modes. Eventually, a Jeffreys–Wentzel–Kramers–Brillouin approach leads to nonlinear differential equations for the evaluation of the various orders in the asymptotic expansion of the full wave function in powers of the Planck mass squared. Even at zeroth order in the Planck mass squared, new features are here found, i.e. the general integral of the nonlinear differential equation encountered at the beginning of the analysis is obtained in exact form in terms of amplitude and phase of a complex parameter. The resulting quantum correction to the CMB power spectrum is evaluated in detail, and it shows that there should be an enhancement of power at large scales. This holds both for an inflationary stage of finite duration and in the idealized case of eternal inflation. A detailed investigation of the unitarity violation issue is also performed.
Close
An enhanced CMB power spectrum at large scale?
Recent work in the literature has investigated whether small quantum gravitational effects can be observed in the anisotropy spectrum of the cosmic microwave background radiation. For this purpose, a massive scalar field in a spatially flat Friedmann–Lemaitre–Robertson–Walker universe has been considered, and its fluctuations have been decomposed into Fourier modes. Eventually, a Jeffreys–Wentzel–Kramers–Brillouin approach leads to nonlinear differential equations for the evaluation of the various orders in the asymptotic expansion of the full wave function in powers of the Planck mass squared. Even at zeroth order in the Planck mass squared, new features are here found, i.e. the general integral of the nonlinear differential equation encountered at the beginning of the analysis is obtained in exact form in terms of amplitude and phase of a complex parameter. The resulting quantum correction to the CMB power spectrum is evaluated in detail, and it shows that there should be an enhancement of power at large scales. This holds both for an inflationary stage of finite duration and in the idealized case of eternal inflation. A detailed investigation of the unitarity violation issue is also performed.
Close