I am a PhD student in the Theoretical Astrophysics group of Prof. Kostas D. Kokkotas at the University of Tübingen in Germany. Here we are working on our understanding of Albert Einstein's legacy and its fascinating implications for astrophysics and beyond. Since the recent groundbreaking detections of gravitational waves from binary black hole and neutron star mergers, we are entering a new era of physics and hopefully find new unexpected aspects of nature that wait to be explored. Outstanding problems like the mysterious dark energy that seems to fill the cosmos or a unified description of gravity and quantum mechanics could be addressed in a new way. The focus of my research is on the quasi-normal mode spectrum of compact relativistic objects, which describes the characteristic gravitational wave sound of their space-time. Understanding the quasi-normal mode spectrum allows to study the nature of compact objects and testing general relativity as prevailing theory of gravity. E-Mail
- 23 Aug
- Phys. Rev. Lett. accepted our paper on "Spectral lines of quantized, spinning black holes and their astrophysical relevance".
- 13 Aug
- Phys. Rev. D published our paper on "Scalar fields and parametrized spherically symmetric black holes: Can one hear the shape of space-time?".
- 24 July
- Report for the 69th Lindau Nobel Laureate Meeting in attempto online (German) and our website (English).
- 03 July
- Gave a talk at the Master Class of Brian P. Schmidt at the 69th Lindau Nobel Laureate Meeting in Lindau, Germany.
- 13 May
- Phys. Rev. D published our paper on "The Inverse Problem for Hawking Radiation".
- 03 May
- Class. Quantum Grav. published our paper "On the Inverse Spectrum Problem of Neutron Stars".
- 28 Feb
- Selected to participate in the 69th Lindau Nobel Laureate Meeting in Lindau, Germany.
- 23 Jan
- Gave a talk at Gravitational Waves, Black Holes and Fundamental Physics in Athens, Greece.
- 08 Oct
- Gave a talk at the 3rd HEL.A.S. SUMMER SCHOOL AND DAAD SCHOOL in Thessaloniki, Greece.
- 25 Sep
- Gave a talk at the Kepler Center PhD Workshop in Obermarchtal, Germany.
- 21 Sep
- Gave a talk at NEB2018 in Rhodes, Greece.
- 19 July
- Gave a talk at Domoschool 2018 in Domodossola, Italy.
- 28 May
- CQG+ published an invited blog article about our recent paper.
- 25 Apr
- Class. Quantum Grav. published our paper on "Wormhole Potentials and Throats from Quasi-Normal Modes".
- 21 Feb
- Presented my work at the Science Slam of the Doctoral Candidates’ Convention in Tübingen, Germany.
- 21 Feb
- J. Phys. Commun. published my paper on "Inverse Spectrum Problem for Quasi-Stationary States".
- 14 Feb
- My proposal for the PhD scholarship Landesgraduiertenförderung was successfull.
- 2017-Now, University of Tübingen, Germany
- 2015-2016, University of Tübingen, Germany
- 2011-2015, University of Tübingen, Germany
- A Semi-analytic Study of Axial Perturbations of Ultra Compact StarsSebastian H. Völkel and Kostas D. KokkotasClassical and Quantum Gravity 34, 125006, 2017Link to arxiv and journal
- Ultra Compact Stars: Reconstructing the Perturbation PotentialSebastian H. Völkel and Kostas D. KokkotasClassical and Quantum Gravity 34, 175015, 2017Link to arxiv and journal
- Parameter estimation of gravitational wave echoes from exotic compact objectsAndrea Maselli, Sebastian H. Völkel and Kostas D. KokkotasPhysical Review D 96, 064045, 2017Link to arxiv and journal
- Inverse spectrum problem for quasi-stationary statesSebastian H. VölkelJournal of Physics Communications 2, 025029, 2018Link to arxiv and journal
- Wormhole Potentials and Throats from Quasi-Normal ModesSebastian H. Völkel and Kostas D. KokkotasClassical and Quantum Gravity 35, 105018, 2018Link to arxiv and journal
- On the Inverse Spectrum Problem of Neutron StarsSebastian H. Völkel and Kostas D. KokkotasClassical and Quantum Gravity 36, 115002, 2019Link to arxiv and journal
- The Inverse Problem for Hawking RadiationSebastian H. Völkel, Roman Konoplya and Kostas D. KokkotasPhysical Review D 99, 104025, 2019Link to arxiv and journal
- Scalar fields and parametrized spherically symmetric black holes: Can one hear the shape of space-time?Sebastian H. Völkel and Kostas D. KokkotasPhys. Rev. D 100, 044026, 2019Link to arxiv and journal
- Spectral lines of quantized, spinning black holes and their astrophysical relevanceAndrew Coates, Sebastian H. Völkel and Kostas D. Kokkotasaccepted in Physical Review LettersLink to arxiv and journal
- Can one hear the shape of an ultra compact object?Sebastian H. Völkel and Kostas D. KokkotasCQG+, October 2, 2017Link to the blog
- The Sound of Exotic Astrophysical "Instruments"Sebastian H. Völkel and Kostas D. KokkotasCQG+, May 28, 2018Link to the blog
Talks and Posters
Talks (Conferences & Schools)
- Master Class of Brian P. SchmidtPresenting my field at the "69th Lindau Nobel Laureate Meeting"03. July 2019, Lindau, Germany
- Hearing the Nature of Compact ObjectsPresenting my research at Gravitational Waves, Black Holes and Fundamental Physics23. January 2019, Athens, Greece
- Probing the Nature of Compact Objects with QNMsOverview talk at the 3rd HEL.A.S. SUMMER SCHOOL AND DAAD SCHOOL08. October 2018, Thessaloniki, Greece
- The Sound of Relativistic ObjectsOverview talk at the Kepler Center PhD Workshop 201826 September 2018, Obermarchtal, Germany
- Probing the Nature of Compact Objects with QNMsPresenting my research at NEB1821 September 2018, Rhodes, Greece
- Hearing the Nature of Compact ObjectsPresenting my research at Domoschool 201819 July 2018, Domodossola, Italy
- From Compact Objects to Quasi-Normal Modes and BackPresentation of my current research at the X Black Holes Workshop18 December 2017, Aveiro, Portugal
- Exotic Compact ObjectsLecture for the DAAD Winter School: Neutron Stars in the Era of Gravitational Waves30 November 2017, Thessaloniki, Greece
- From Compact Objects to Quasi-Normal Modes and BackGeneral talk about my current research status at the University's Kepler PhD Workshop29 September 2017, Freudenstadt-Lauterbad, Germany
- From Compact Objects to Quasi-Normal Modes and BackPresentation of my research papers [1-3] at the 9th Aegean Summer School 22 September 2017, Sifnos, Greece
- Gravitational Waves from Compact ObjectsGeneral talk about my Master thesis at the University's Kepler PhD Workshop5 October 2016, Freudenstadt-Lauterbad, Germany
- The Sound of Black HolesMy contribution to the Science Slam at the Doctoral Candidates’ Convention21 February 2018, Tübingen, Germany
- Gravitationswellen von kompakten ObjektenGeneral poster about my research at the open day of the Institute for Astronomy and Astrophysics4 November 2017, Tübingen, Germany
- From Compact Objects to Quasi-Normal Modes and BackPresentation of my current status of research at the NewCompStar School 201714 September 2017, Sofia, Bulgaria
- From Compact Objects to Quasi-Normal Modes and BackPresentation of my current status of research25 April 2017, Bad Honnef, Germany
I gave lectures about:
- Gravitational Waves in CosmologyAdditional hours (2x90 minutes) for the Observational Cosmology lectureUniversity of Tübingen, Germany
I worked as tutor to discuss weekly exercises with a class of students for the following lectures:
- Theoretical Mechanics and Introduction to Quantum MechanicsUniversity of Tübingen, Germany
- ElectromagnetismUniversity of Tübingen, Germany
- Observational CosmologyUniversity of Tübingen, Germany
- Observational CosmologyUniversity of Tübingen, Germany
Work as RefereeI worked as referee for the following journals:
- Physical Review Letters
- Physical Review D
- The European Physical Journal C
- The European Physical Journal Plus
- General Relativity and Gravitation
The main focus of my research is on the spectrum of gravitational perturbations of compact relativistic objects. Different compact objects have different spectra and similar to atom spectroscopy or seismic studies of Earth, one can potentially learn a lot about the objects from the knowledge and correct interpretation of their spectrum.
 A Semi-analytic Study of Axial Perturbations of Ultra Compact Stars
Exotic ultra compact objects being more compact than neutron stars but horizonless have triggered much attention recently. Such objects, if they exist in nature, would change our understanding of what we think to know about gravity at its strongest fields. Calculating their characteristic quasi-normal mode spectrum is one interesting way to study them. In this paper we apply a generalized Bohr-Sommerfeld rule to the wave equation of axial perturbations from spherically symmetric and non-rotating ultra compact objects. We compare different applications of the Bohr-Sommerfeld rule with full numerical data and find good agreements. The mathematical form of the Bohr-Sommerfeld rule is potentially useful for similar problems, because it allows for a simpler, but approximate, calculation of the so-called trapped quasi-normal modes. Link to the paper on the arxiv and the journal.
 Ultra Compact Stars: Reconstructing the Perturbation Potential
Can one hear the shape of a drum? This famous mathematical physics question is closer to black holes and compact stars as one might expects. It is a beautiful example of how general concepts in physics often are. The answer might surprises. In the case of the drum it is known that a unique reconstruction of the shape is in general not possible. In the case of axial perturbations from spherically symmetric and non-rotating ultra compact objects, we generalize inverse methods for potential wells and barriers to reconstruct significant parts of the effective potential that appears in the perturbation equation. Making use of Birkhoff's theorem we are able to obtain unique, but approximate results for the reconstructed potential by using numerical data for the trapped quasi-normal mode spectrum as input. For a more general summary and introduction to this problem, one can find our invited blog article here. Link to the paper on the arxiv and the journal.
 Parameter estimation of gravitational wave echoes from exotic compact objects
In this work carried out in collaboration with Andrea Maselli, we are interested in the parameter estimation of so-called gravitational wave echoes from exotic compact objects. Although these type of problem has been addressed the first time almost two decades ago, as a more academic question, it has triggered a lot of attention recently. Claims that a signal of this type had been measured by LIGO in the first binary black hole merger event, resulted in an increasing interest in exotic compact objects. In this work we developed several phenomenological templates that should be able to describe the most important features of such echoes and tested if current and future gravitational wave detectors will be able to detect them. Although our approach can only be seen as a first step in a much more involved problem, we find that there is hope that in principle many of the parameters could be constrained with satisfying accuracy. Nevertheless, more work has to be done and it remains an interesting questions. Link to the paper on the arxiv and the journal.
 Inverse Spectrum Problem for Quasi-Stationary States
This paper gives a comprehensive and general presentation of the inverse spectrum method, which we developed and applied in  to the oscillations of ultra compact stars and gravastars. After demonstrating how the method is derived, we present it to analytic examples for the complex valued spectra of quasi-stationary states and investigate the corresponding WKB spectrum equivalent potentials by parameterizing them. Figure 4 shows a few of the parametrized subclasses for the reconstructed potentials for a given spectrum. We examine what type of potentials can be reconstructed from the spectrum and find some special cases. For example, although there are in principle infinitely many WKB spectrum equivalent potentials possible for a given spectrum, one of our studied spectra drastically reduces this non-uniqueness. Since the inverse method being presented is applicable to one-dimensional wave equations for a wide range of potentials that admits quasi-stationary states, we think it can in principle be of interest for a wide range of applications. Link to the paper on the arxiv and the journal.
 Wormhole Potentials and Throats from Quasi-Normal Modes
In this work we extend the inverse spectrum method, known from ultra compact stars , to the case of some symmetric wormhole models. Such models include the Damour-Solodukhin wormhole, whose perturbation potential is characterized by a symmetric double barrier potential. By using the so-called trapped quasi-normal mode spectrum, the inverse spectrum method allows to reconstruct approximatively the underlying potential. With the potential as intermediate result, it is further possible to recover the parameter of the model that describes the position of the wormhole throat. We also compared our results to the ones obtained for potential wells with only one potential barrier . Within the precision of the underlying method, we find that it can also be possible to reconstruct a potential of this kind, from the same spectrum. Thus it is not always possible to reconstruct only one type of potential, for which non-uniqueness is well known, but also for different types of potentials. In this situation one can not tell if the underlying object is a wormhole or some exotic ultra compact star. The situation can be resolved if more information is included. Ultra compact objects of both types are expected to emit so-called echoes, which describe the time-evolution of reflected perturbations. This can be used to distinguish both types of potentials from each other, because they differ in the exact shape of the potential barrier, which can be related to the first reflection. Link to the paper on the arxiv and the journal.
- Sebastian Völkel Theoretische Astrophysik Universität Tübingen Auf der Morgenstelle 10 D-72076 Tübingen