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.>>

Sabbatical Abroad: Is It Worth It?

I have just returned from a truly stimulating three-month sabbatical in New York City, an experience that was both enriching and enlightening. I was based at Columbia University, hosted by the Italian Academy of Advanced Studies in America and the Department of Physics. It was a unique opportunity to immerse myself in a vibrant intellectual community, explore new scientific research directions, and engage with some of the brightest minds in my field and beyond. It also presented an extraordinary opportunity for my family, including my children aged 7 and 11, to be embedded in a different culture, and to live in one of the most exciting cities of the world.

On the other hand, logistical preparations were complex and required meticulous planning and support. The effort required to coordinate this move – identifying host institutions in the same city for my wife and me, finding adequate financial support, obtaining visas for the family, preparing mountains of paperwork, finding housing, enrolling the kids in school, organise non-academic life – was substantial and at times overwhelming. In this post, I briefly reflect on the overall experience to ask: was it all worth it? I hope this discussion will be useful to other academics (and beyond) considering a similar move.

Planning at home institution. The logistics of a sabbatical can vary significantly depending on your institution’s rules. My experience is that academic institutions are willing to let you take a sabbatical as long as you plan meticulously to ensure minimal disruption. This means arranging replacements for your teaching duties well in advance to avoid any negative impacts on your students’ education and your department’s curriculum. If your research involves group members, consider their ability to accompany you, which will depend on available funding. It’s also critical to prepare for all ongoing academic commitments. This might involve finding colleagues who can take over your responsibilities or setting up arrangements for remote participation in essential activities.

Establishing Connections and Securing Resources. Once obtained permission from your home institution, it’s crucial to establish connections with suitable host institutions. My experience is that it is very easy to find colleagues who are willing to host you in principle, but very difficult to find financial support for your visit, due to lack of dedicated budget lines. I was fortunate to receive a fellowship from the Italian Academy for Advanced Studies in America, which helped to cover my travel and staying expenses. If the host institution cannot provide support, look up the opportunities in your field of research, including bi-lateral agreements, national funding agencies, academic grants and fellowships (Fulbright Scholar Program, Guggenheim Foundation, etc.).

Visas and schools. Traveling with family adds another layer of complexity, especially concerning visas and schooling. To us, navigating the visa system felt like a step back in time, with its demand for mountains of documents (wedding certificate, birth certificates, certified translations, ..), the need to send physical passports via ordinary mail, enduring long waiting times, and facing high fees for applications. As for schools, the complexity of the organisation depends strongly on the country you visit. In the U.S., public schools are tied to your residential area, and registration typically can’t be completed until you are physically present. This was one of the most stressful aspects of the sabbatical for us, and although everything went fine in the end, I recommend taking into account this aspect carefully when considering a sabbatical.

Housing. Finding suitable housing that could accommodate a family and was within a reasonable commute to Columbia University involved navigating a competitive real estate market, which was a daunting task in itself. Most host institutions will not directly help you with finding housing, but they will provide a list of resources for you to explore, including websites like https://www.sabbaticalhomes.com/, https://www.vrbo.com/, https://www.homeexchange.com/, with the warning that they don’t take any responsibility in case of problems. Columbia U. and NYU have their own housing, but in our case it didn’t work out as the results of the application is given very close to the date of travel, and most apartments are very small for a family, and unfurnished (some colleagues ended up renting furniture – including bed and dining table and chairs, for several months).

Making it worth it. Once settled, it’s crucial to immerse yourself in the academic and cultural life of your host city. I made it a point to introduce myself and my work early on, giving presentations not only at Columbia but also at other institutions like the Center of Computational Astrophysics, MIT, Harvard, Stonybrook, and Yale. Engaging with the brilliant students, outstanding colleagues, and overall vibrant community at these places was a highlight of my stay. New York City itself is a bustling hub of culture and activity. From theaters and museums to sports events and concerts, the city offers an endless array of experiences. Participating in these activities not only enriched our personal life but also provided a broader context and inspiration for my work.

Final Thoughts. So, is a sabbatical abroad worth it? While it demands a significant investment of time, effort, and financial resources, for me the answer is a resounding yes—if planned carefully. The exposure to new academic environments and cultures is an unparalleled source of inspiration and ideas. The key is to be well-prepared and proactive in engaging with both the academic and local communities. A sabbatical abroad is not just an academic endeavor; it’s a life experience that can broaden your horizons and influence your personal and professional life in unimaginable ways. If you have the opportunity, I highly recommend seizing it.

During my three-month sabbatical in New York, hosted by the Italian Academy of Advanced Studies in America and the Center for Theoretical Physics at Columbia University, I had the privilege to visit several US institutions and to meet tens of outstanding colleagues, including many students and young researchers. It was also a great opportunity to catch up with friends and collaborators, and to present the research work that we carry out in my group in Amsterdam.

Here are the videos of two key presentations:

• Yale: This colloquium given at the Department of Physics at Yale University is representative of the ones I gave at Columbia, MIT, and Stony Brook.

• Harvard: This shorter presentation given at the Institute for Theory and Computation, Center for Astrophysics is similar to those I gave at the Center for Computational Astrophysics (CCA) and the BHs Conference at the Bahamas