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The fate of black holes in high-energy physics -- exploring their dynamical instabilities

Periodic Reporting for period 1 - BHstabNL (The fate of black holes in high-energy physics -- exploring their dynamical instabilities)

Reporting period: 2016-09-29 to 2018-09-28

"This is the final report for the Marie-Curie (MC) grant ""BHstabNL"" that ran from 29/09/16 - 31/12/17. I had to terminate it prematurely because I received a prestigious Royal Society University Research Fellowship, a 5-year tenure-track type fellowship in the UK, that could not be postponed beyond 01/01/18.
My project focused on nonlinear dynamics of black holes (BHs), that are ideal ""test particles"" to explore gravitational phenomena in the strong field regime. The original proposal aimed at understanding their properties in higher dimensional gravity. However, by the time the fellowship started in late 2016 (proposal was submitted in 09/2014), these specific goals had already been addressed by other groups. Therefore, I slightly adjusted the project to guarantee an original and timely outcome, as agreed upon with my host at UB and with the EC contact person at the time, Y. Vacondios, in a personal discussion.
In 2015 LIGO directly detected gravitational waves originating from the collision of two BHs for the first time and, thus, opened an entire new way to unravel the mysteries of our universe. Because LIGO's data stream is dominated by noise, this Nobel-prize winning breakthrough has only been possible thanks to theoretical predictions of the expected gravitational wave signal, called waveforms, requiring a combination of analytic and numerical methods. My objectives have been concerned with the latter, i.e. studying nonlinear BH dynamics in extensions of GR that are well motivated by cosmological models or quantum gravity paradigms such as string theory. These predictions are crucial to probe gravity in its most violent regime and to confront them with gravitational wave observations. These goals are extremely timely and have the exciting potential to answer long standing open questions in cosmology, high-energy and gravitational physics in an entirely new fashion. Modelling the formation and collision of BHs in modified gravity have scarcely been attempted before, with the notable exception of standard scalar-tensor theories. Therefore, my MC project has set the stage to gain important new insight into fundamental physics and has supported the beginning of an exciting new research direction of numerical relativity beyond GR."
"The following papers were published in peer-reviewed international journals
- R. Benkel, T.P. Sotiriou and H. Witek, Class. Quant. Grav. 34, no. 6, 064001 (2017).
- R. Benkel, T.P. Sotiriou and H. Witek, Phys. Rev. D94, no. 12, 121503 (2016).
during my MC fellowship and I expect to complete
- G. Ficarra, P. Pani and H. Witek, ""Quasiadiabatic evolution of the black hole superradiant instability''
- H. Witek et al, ""Black hole binaries in Einstein-dilaton Gauss-Bonnet gravity''
- H. Witek and M. Zilhao, ""Proca fields around rotating black holes''
within the next few months. Because they are based on work largely carried out during the MC fellowship they will appropriately acknowledge the grant. I disseminated my results in conferences, e.g.
- ""Probing GR with gravitational waves'', ICCUB Winter Meeting, UB
- ""Challenging GR in the Gravitational Wave era'', IFAE Theory Seminar, Universitat Autonoma de Barcelona
- ""Black hole superradiance in the nonlinear regime'', Workshop, University of Nottingham
- Lectures on ""Strong gravity and numerical relativity'' and ""Probing strong-field gravity in the era of gravitational wave observations'', summer school, Benasque
- ""Black holes as search engines for axion dark matter and hidden photons'', Seminari Teorico INFN, La Sapienza Rome
- ""Testing gravity in the era of gravitational waves'', Workshop, Benasque
- ""Growing Black-Hole Hair -binaries in Einstein-dilaton Gauss-Bonnet gravity'', Workshop, Perimeter Institute
More direct deliverables include our newly developed CANUDA code, parts of which are publicly available at We are in the process of including it into the open-source EINSTEIN TOOLKIT (
My time at UB allowed for a high transfer of knowledge. E.g. I gave
- an official course on Numerical Relativity
- an introduction to the open-source EINSTEIN TOOLKIT and high-performance computing (HPC) to PhD students at UB
- a set of lectures on ""Strong gravity and numerical relativity'' at the summer school ""Gravitational waves for cosmology and astrophysics'' in the Science Center Benasque/ Spain
During the fellowship I secured additional funding, e.g.
- my Royal Society Fellowship UF160547 - BHbeyGR; 01/2018 - 12/2022; amount: £441,641.59
- HPC grants via the Partnership for advanced computing in Europe; PRACE grant Tier-0 PPFPWG; 10/2017 - 09/2018; amount: 15Mio CPU hours.
- HPC grants via the Barcelona Supercomputing Center (BSC/RES); AECT-2017-2-0011 and AECT-2017-3-0009; 07/2017 - 02/2018; amount 2.4Mio CPU hours.
To share my passion about science with the general public I engaged in outreach activities, e.g.
- development of the exhibition ""Unravelling the dark universe"" (see The online version is available in different languages (e.g. English, Spanish, Italian or Portuguese) to be usable throughout Europe.
- Lectures ""Abenteuer Universum'' and ""Das Konzert des Universums'' at the Rudolf-Hildebrand Gymnasium Stendal/Germany
- public talk ""Gravitational waves: the universe's symphony'' at the international Pint of Science series.
I also co-organized the event ""Women in Science: Celebrating the International Women's Day'' at UB to raise awareness of gender imbalance in STEM subjects."
"I adjusted the scope of the project towards studying the formation and dynamics of BHs in extensions of GR. After the first gravitational wave detections we are now able to look for observational signatures of more fundamental theories. To do so, however, theoretical predictions of the expected signal are crucial to identify it in the detectors' noise-dominated data stream. While the inspiral phase can be modeled analytically, the highly nonlinear merger regime requires full-fletched numerical simulations which were hardly attempted before. I addressed this problem for the first time in Einstein-dilaton Gauss-Bonnet (EdGB) gravity that appears naturally, e.g. in the low-energy limit of string theory or as special case of Horndeski theories that are popular in cosmology. This is just the beginning of a long-term project whose ultimate goal is the production of waveform templates to facilitate confrontation between theory and observation.
These important scientific advances have only been possible with the development of a new numerical infrastructure. Our CANUDA code includes this extension as well as couplings to ultralight fundamental fields that are popular dark matter candidates. The latter modules are already publicly available at
To kickstart this exciting new research area and to initiate new international collaborations I organize the workshop ""Numerical relativity beyond GR'' in June 2018; see It was met with great interest and internationally leading experts in the field already confirmed their attendance. Furthermore, E. Bentivegna and myself founded a new working group ""Cosmology and Particles'' within the EINSTEIN TOOLKIT community to extend numerical relativity towards novel applications in cosmology and high-energy physics; see"
Visiting the Marenostrum supercomputer in Barcelona