In a paper published in Physical Review Letters this week, two brilliant young researchers – Gimmy Tomaselli (former PhD student in my group, now at IAS Princeton) and Thomas Spieksma (former MSc student, now MSc student at NBI Copenaghen) – and I showed that black hole mergers may unveil the existence of new particles.

IoP Press Release: In the same way that electrons can orbit a nucleus in an atom, a cloud of so far undiscovered ultralight particles may orbit pairs of black holes. Gravitational waves that are emitted by the merger of two black holes carry detailed information about the shape and evolution of the orbits of the components. A new study by physicists Giovanni Maria Tomaselli and Gianfranco Bertone from the University of Amsterdam (UvA), together with former UvA master student Thomas Spieksma, now at the Niels Bohr Institute in Copenhagen, suggests that a careful analysis of this information may reveal the existence of new particles in nature.

The mechanism that makes the detection of new particles possible is called black hole superradiance. When a black hole spins fast enough, it can shed some of its mass into a ‘cloud’ of particles around it. The black hole-cloud system is referred to as a ‘gravitational atom’, due to its similarity with the electron cloud around a proton. Since superradiance is only efficient if the particles are much lighter than the ones measured in experiments so far, this process provides the unique opportunity to probe the existence of new particles known as ultralight bosons, whose existence may resolve several puzzles in astrophysics, cosmology and particle physics.

The orbital evolution of binary black holes in the presence of ultralight boson clouds has been studied by UvA scientists in a series of influential papers over the past six years. One important new phenomenon that was discovered was that of resonant transitions, where the cloud ‘jumps’ from one state to another, similar to how an electron in an ordinary atom can jump between orbits. Another new phenomenon, again similar to the behaviour of ordinary atoms, is ionization, where part of the cloud is ejected. Both of these effects leave characteristic imprints on the emitted gravitational waves, but the details of such imprints depend on the – so far unknown – state of the particle cloud. In an effort to fill in these remaining details, the new study combines all the previous results, and follows the history of the system from the formation of the binary black hole to the black hole merger.

The main conclusions substantially improve our understanding of the binary gravitational atoms. The researchers found that there were two possible outcomes of the evolution of such a system, both equally interesting. If the black holes and the cloud initially rotate in opposite directions, then the cloud survives in the state originally produced by superradiance, and it becomes detectable through its ionization, which leaves a clear signature on the gravitational waves. In all other cases, resonant transitions destroy the cloud altogether, and the binary’s orbit acquires very specific values of eccentricity and inclination, which can be measured from the gravitational waves signal.

Thus, the new result provides a novel and solid search strategy for new particles, either via the detection of ionization effects in gravitational waveforms in one case, or in the other case via the observation of an anomalous excess of systems with the predicted values of eccentricity and inclination. For both cases, upcoming detailed gravitational wave observations will reveal very interesting information about the question whether new ultralight particles exist.

Link to Paper: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.133.121402

Link to Physics Viewpoint article: https://physics.aps.org/articles/v17/133

TeV Particle Astrophysics (TeVPA) is an international conference that covers the most recent advances in the field of Particle Astrophysics. This year, TeVPA returned to Chicago for the first time since its inaugural edition that I organised back in 2005 at Fermilab, together with a group of brilliant colleagues.

TeVPA 2024 featured a rich program, with morning plenary sessions, and afternoon parallels covering topics such as cosmic ray physics, gamma-ray astronomy, neutrino astronomy, cosmology, direct and indirect searches for dark matter, gravitational waves, and their connection to particle physics.

I was honored to deliver the final Summary Talk—a challenging yet rewarding task given the 25 plenary talks, 275 parallel session presentations, over 80 hours of scientific discussions, and more than 10,000 slides presented (not to mention the 15 hours of discussions over coffee, meals, and drinks!). During the talk, I reflected on the evolution of the field since 2005—what has changed and what has remained constant.

You can find the slides of my presentation here.

I was recently invited to contributed to an excellent outreach project called “The birth of an Idea”, led by my friend and colleague Vitor Cardoso, together with artist Ana Sousa Carvalho.

The aim of the project is “to build a catalogue of stories documenting the scientific process as it truly is: perplexing, difficult, painstaking, spontaneous, exciting and fun. Along the way, we also hope to tell a larger story: that of science itself, seen from the human side of the equations.“

I encourage you to check it out, as it contains very inspiring bits of wisdom, and an intimate look into the some of the most creative minds of our time.

https://birthofidea.tecnico.ulisboa.pt

Here’s my contribution:

Sailing in the Dark

<< Doing research in fundamental physics often feels like sailing in the dark. The course forward is rarely clear, and the lack of reference points to gauge your progress can be frustrating. In such uncertain waters, ideas become the lighthouses that guide your direction. Most of them dissipate like distant mirages, but others shine ever brighter, illuminating the route and steering your research, hopefully toward meaningful discoveries. The luckiest among us are those whose ideas become a lighthouse not just for their own research, but also for others navigating the same waters.

One of my fondest memories as a scientist is starting my first postdoc at Fermilab, near Chicago: I was fresh out of my PhD, in a new country, surrounded by brilliant colleagues, and I had the complete freedom to choose my research projects. One day, during the morning shower, it occurred to me that the mysterious dark matter particles I was studying could interact with stars, altering their properties (pro tip: always keep a notepad handy, activities like walking or showering stimulate creative ideas—look it up, it’s called “the shower effect”).

The thought that stars could serve as detectors for this mysterious form of matter was thrilling. That spark ignited my curiosity, and I took the plunge, immersing myself in the idea. I read everything that others had written on the subject. Self-gravitating clouds of dark matter inside stars and tiny black holes devouring stars from the inside became as real and vivid as the reality surrounding me. It is hard to describe the sense of power and freedom I felt in that period. I can only say it was akin to the feeling I experienced as a boy when the realization dawned on me that God may not exist—a blend of vertigo, fear, awe, and excitement. The jury is still out on whether dark matter influences stars, but since then I always try to align the research work of my team with the ideas that excite me most. By doing so, I aim to instill a sense of wonder and exploration in my team, hoping that together, our sparks will illuminate new routes to discovery.>>