About Me

Hi! I am an Assistant Professor in Physics at Radboud University, where I do research in theoretical physics. In particular, I focus on foundational questions in general relativity and predictions for future gravitational-wave observatories such as the Einstein Telescope and LISA.

Short biography

Research

All my scientific publications are freely available online.

Some of my recent work includes:

A happy black hole with a Newtonian black hole mimicker

A black hole mimicker

Black holes have a very specific gravitational field, which one can characterize by its multipole moments. The multipole moments of black holes are all completely determined the mass and spin of the black hole. We asked the question: are there other objects that can mimick all the multipole moments of a rotating black hole? We constructed an explicit example of a Newtonian star with multipole moments identical to those of a rotating black hole, so the answer to this simple question is "yes".

The influence of a tidal resonance on the phase is observable across the parameter space

The importance of tidal resonances

A few years ago, my collaborators and I pointed out that tidal resonances may play an important role for Extreme Mass Ratio Inspirals. In this work, we have explored the entire parameter space and showed that these resonance leave an observable imprint on the waveform for most EMRI configurations. We also made the first steps needed in order to model these resonances efficiently. So more is to come!

Conformal diagram of decelerating FLRW spacetimes

BMS-like structures in cosmology

Symmetries play a key role in theoretical physics. However, most realistic systems posses no symmetries. Fortunately, if you go far away from a compact object with strong gravity such as black holes and neutron stars, the spacetime becomes approximately flat again. As a result, you do have "asymptotic symmetries". These asymptotic symmetries are described by the Bondi-Metzner-Sachs algebra (BMS) and they play an important role in gravitational science. However, if you take into account that the universe is expanding, the spacetime does not become asympotically flat. We carefully examined what happens if you have spacetimes that expand in a decelerating fashion and found that the asymptotic symmetry algebra of this large classs of spacetimes is like the BMS algebra. However, it is not exactly the same! See the paper for details :)

Melting of a neutron star due to resonance effects

Melting of neutron stars

When two neutron stars are in a binary system, they exert tidal forces on each other. These tidal forces increase as the two stars get closer during their inspiral. As a result, one of the natural modes of the stars -- called the interface mode -- can be resonantly excited. When this happens, the orbital energy is transferred to the star's crust, which can then melt. This all happens during the final phases of the inspiral and we predict that this is observable by future ground-based gravitational wave detectors. This would be cool as it provides more information about the properties of neutron stars and the fundamental properties of nuclear matter at high densities.

Different approaches to defining angular momentum in electromagnetism

Angular momentum radiated

This work is the final answer to a long list of papers about angular momentum radiated by electromagnetic and gravitational waves. It all started with a surprising fact about angular momentum radiated in electromagnetism: while one typically thinks of fluxes radiated to infinity to only depend on the radiative degrees of freedom, the total flux of angular momentum radiated also depends on the Coulombic degrees of freedom. These Coulombic degrees of freedom appear through an interaction term between the radiative degrees of freedom and the charge aspect. This occurs in realistic scenarios, for instance, all the angular momentum radiated by a charged spinning sphere with variable angular velocity is due to this interaction term. This is not the case for gravitational waves: the angular momentum radiated by gravitational waves is entirely encoded in the radiative degrees of freedom. This interesting difference between electromagnetism and gravity deserved further explorations, and that is exactly what we did in this paper!

An example of a 4:3 mean motion resonance

Mean motion resonances in black hole spacetimes

Mean motion resonance is a type of orbital resonance that occurs due to the gravitational interaction between two objects orbiting a central massive object. They are very common in planetary systems, for instance, the moons of Jupiter are in various mean motion resonances. We investigated whether mean motion resonance in the strong gravity regime around massive black holes. During this type of resonance, objects are "locked" and move in a synchronized way. As a result, mean motion resonance can create the perfect circumstances for tidal resonances to occur, which are likely observable by LISA. For a nice layman's description about tidal resonances, see this article in Inside the Perimeter.

News

Gravity+ track in our Master Programme

Since September 2020 the RU Science Faculty offers a Gravity+ synergy track to Master students enrolled in the Master's specialisations of Mathematics and Particle & Astrophysics. The courses reflect Radboud expertise in gravity, ranging from black-hole imaging (Event Horizon Telescope) and gravitational wave astronomy to quantum gravity and mathematical foundations of general relativity. In particular, it provides a solid preparation for research in my group. Click on the link for the short video clip for more information.

Special Issue "Gravitational Radiation in Cosmological Spacetimes"

The call for papers for a special issue in Universe about gravitational radition in cosmological spacetimes is open! This special issue aims to collect and act as a catalyzer for progress in our understanding of gravitational waves emitted by compact sources in cosmological spacetimes—both with and without a cosmological constant. Deadline for submission is 30 Nov 2021, but is extended to early 2022. Please send me an email if interested!

A special facebook live event about black holes!

For Dutchies only: De Nationale Wetenschapsagenda organiseerde een facebook live event op 7 december 2021 over zwarte gaten, waarin ik allerlei vragen beantwoord van de kijkers samen met Jaco de Swart. Voor een korte teaser, zie dit korte filmpje en het volledige vragen half-uurtje is hier terug te zien.

Teaching

Formal supervision

If you are a bachelor or master student looking for a thesis project, please send me an email. If you are doing a double bachelor (in particular, physics and mathematics), co-supervision with a supervisor from another department is possible.

Recent lectures

Seminars & Conferences

Upcoming seminars and conferences

  • TBD , Seminar at the University of Granada, 17 Jan 2022
  • Physics @ Veldhoven conference, 25-26 Jan 2022
  • TBD , Plenary talk during the Chennai Symposium on Gravitation and Cosmology (CSGC), 2-5 Feb 2022
  • Gravity with a positive cosmological constant , Conference Global Structure in Semi-Classical Gravity in Munich, 21-23 July 2022

Seminars and conferences in 2021

Seminars and conferences in 2020

Outreach

Recent face-to-face outreach

  • Lecture for 10-12 year olds at Sonnenborgh on 10 October 2021 about gravitational waves.
  • As part of the International Day of Women and Girls in Science on 11 February 2020, I gave a guest lecture about gravitational waves for 6VWO at high school Kamerlingh Onnes in Groningen.

Outreach online

This is a selection of some of my recent outreach that has found its way to the worldwide web:

  • I am a science advisor for this short 90-second explainer about black holes made by the Nationale Wetenschapsagenda (available only in Dutch).
  • I am a science advisor for Perimeter's updated eductional resource on black holes . It accurately describes what black holes are (using lightcone diagrams!) and covers the latest observational evidence for black holes.
  • I am a science advisor for Perimeter's new educational resource on teaching strategies .
  • I play a small role in the movie Secrets of the Universe, which is an IMAX movie about science at the LHC that is currently shown in science musea in the USA and Canada. The images are beautiful and you get to learn about a wide range of amazing scientists. Truly inspiring! Hopefully, it will make its way to Europe soon as well.
  • I am a science advisor for Perimeter's freely available educational resource Evidence for Climate Change.
  • I was a blogger for the fantastic website ParticleBites, which is an online particle physics journal club written by graduate students and postdocs.

For some movies in which I talk about physics, see:

Live Q&A session about black holes (in Dutch)
A live Q&A session about black holes organized by the Nationale Wetenschapsagenda (in Dutch).
Live panel discussion on the Event Horizon Telescope's first black hole image
Panel discussion on the day of the release of the first black hole image by the Event Horizon Telescope.
This educational movie delves into the concept of fields, which plays a central role in modern physics
This short clip about fields is part of the educational resource from the Perimeter Institute called Fields. The concept of fields plays a central role in modern physics.
This educational movie discusses model building and breaking.
This short clip about model building and breaking is part of the educational resource Deeper Understanding of Energy, a classroom resource from Perimeter Institute.

Contact

bbonga [at] science.ru.nl
@PhysicistBe

Visiting address

Huygens building - Room 02.822
Heyendaalseweg 135
6525 AJ Nijmegen
The Netherlands

Postal address

IMAPP, Faculty of Science
P.O. Box 9010
6500 GL Nijmegen
The Netherlands